astra.yaml

# ASTRA record of ShapePipe's scientific decisions: the choices embedded in
# the code and the committed workflow configs (workflow/config/cfis/), why
# they stand, and the alternatives. CLAUDE.md says when to amend it.
# workflow/config/cfis_image_sims/ is not yet in scope: its symlinks into
# cfis/ are governed there; its untagged overlays change input naming, the PSF
# model and some governed values (config_tile_Fe.ini COLNUM/EXP_PREFIX,
# final_cat.param columns).
#
# Conventions (`tests/helpers/decisions.py` implements them and documents the
# full value grammar; `tests/unit/test_decisions.py` enforces them):
#   * Put `@sc [decision:<id>]` at every code/config site that implements a
#     decision. Decision citations have no id or prose; the record is the source
#     of rationale. IDs are bare for top-level decisions and dotted for
#     sub-analysis decisions; repeat `decision:` metadata to cite several.
#     A local constraint adds a stable id and prose, with optional decision
#     metadata: `@sc [decision:<id>,label:<kind>] <contract-id>`.
#   * In Python, put tags in a def/class docstring or as a comment immediately
#     above a module statement. In Snakemake, put a comment immediately above
#     the rule/statement. In config, a tag governs the active settings in the
#     following paragraph, which ends at the next blank line or `@sc` line.
#     Every active key before that boundary is governed. A blank line between a
#     tag and its key is an error. To govern a whole section, put the tag
#     directly above its `[SECTION]` header; it governs active keys through the
#     next section header. A description comment may sit above the tag, and
#     `scope:file` governs the whole file.
#   * A rationale may end with `Values: <ref> = <value>; ... .`. Every ref must
#     resolve to exactly one site tagged for that decision and match its value.
#     Use a bare key when it selects one site; qualify it with a path suffix and
#     `#` or `::` when needed. `= absent` requires a same-decision tag somewhere
#     in the file (in that section or file-wide for INI/SETools) and asserts that
#     no active setting exists anywhere in the target scope. INI absence checks
#     include keys inherited from `[DEFAULT]`.
#   * The checker validates citations in both directions, unique local-contract
#     ids, literal values and test `decision` markers. The value grammar is
#     documented in the helper module's docstring.
#   * A decision's `default` is the option the committed code and configs
#     select; universes/committed.yaml pins it.
#   * `excluded: true` means considered and rejected. An option the code
#     does not implement says so in its description and is not excluded.
#   * [HARDCODED]: the committed choice is fixed in code, with no config key.
#     [LINT]: the code disagrees with itself or its own documentation.
#   * A prior insight repeated inside a sub-analysis carries a `_local`
#     suffix, because insight ids are scoped.

version: "0.0.14"
name: ShapePipe scientific decisions
description: >-
  Codebase-level decision record for the ShapePipe weak-lensing pipeline
  (UNIONS/CFIS) as orchestrated by workflow/Snakefile. Membership test: a
  different defensible choice would change which objects enter the shear
  catalogue, or the numbers attached to them.
tags: [shapepipe, weak-lensing, unions, codebase-record]
container: ghcr.io/cosmostat/shapepipe:develop

inputs:
  - id: tile_images
    type: data
    source: CFIS/UNIONS r-band MegaPipe tile stacks and single-exposure image/weight/flag triplets (workflow/config.yaml)
    description: >-
      Tiles and the exposures that built them; each exposure carries its
      instrument flag image.
  - id: healsparse_masks
    type: data
    source: UNIONS healsparse mask products (e.g. mask_ugriz_nside131072_n4.hsp)
    description: >-
      Sky-fixed mask maps, built outside ShapePipe and queried at object
      positions; how they are used is the masking sub-analysis.

outputs:
  - id: final_cat
    type: data
    format: fits
    description: >-
      Per-tile shear catalogue family, the terminal science product
      (make_cat_runner).
    inputs: [tile_images, healsparse_masks]
    decisions: [per_unit_completeness, postage_stamp_size, photometric_zeropoint]

decisions:

  # ── cross-cutting ────────────────────────────────────────────────────────

  per_unit_completeness:
    label: Per-unit completeness gate
    rationale: >-
      Every shapepipe_run rule checks its products against a per-runner
      nominal count (per chunk for tile_ngmix). The unit fails if a
      non-warning runner falls short or shapepipe_run exits nonzero, and a
      failed unit publishes no manifest for later rules to build on. A tile
      needs the exp_psf manifest of every exposure it lists, so one failed
      exposure keeps every tile it contributes to out of the catalogue; the
      tile does not run with fewer epochs. The one tolerated shortfall is the
      exposure-side psfex_interp VALIDATION output. A CCD whose model fails
      the acceptance gate (star_selection_psf.psf_acceptance_thresholds)
      produces nothing there, and the unit only warns. Science-path PSF
      rejection bypasses this table: psfex_interp drops that epoch per object
      inside the tile run. The MCCD table (one mask_query catalogue per CCD;
      one train and one test catalogue per exposure) warns on every runner,
      and the workflow refuses psf_model=mccd.
      Values:
      COMPLETENESS[exp_psf.psfex.psfex_interp_runner.warn] = True;
      COMPLETENESS[exp_psf.mccd.mask_query_runner.expect] = 40;
      COMPLETENESS[exp_psf.mccd.mccd_preprocessing_runner.expect] = 2.
    default: exact_counts
    options:
      exact_counts:
        label: Nominal count per runner; psfex_interp validation shortfall warns
        insights: [des_psf_blacklist, guinot22_star_floor_22]
      count_floor:
        label: Tolerate recorded attrition down to a per-runner floor
        excluded: true
        excluded_reason: >-
          The floors had no basis: across a 127-exposure, 64-tile campaign
          every non-warning runner produced exactly its nominal count, so a
          floor below it only admits failed units unremarked.
      no_gate:
        label: Accept whatever is produced
        excluded: true
        excluded_reason: >-
          A stage producing 2 of 40 CCDs would flow into the catalogue
          unremarked.

  postage_stamp_size:
    label: Postage-stamp size shared by vignets, ngmix stamps and PSF models
    rationale: >-
      Every stamp width is 51 px (about 9.5 arcsec): the tile VIGNET (cut
      by the tile SExtractor run), the multi-epoch vignetmaker stamps
      ngmix fits, the exposure SExtractor VIGNET PSFEx trains on, and the
      PSFEx model stamp. Only the first two are coupled: ngmix lays the tile
      VIGNET's -1e30 neighbour markers over each epoch stamp pixel for pixel,
      so they must be the same size. The stamp truncates the wings of large
      galaxies. No rationale for 51 px is recorded.
      Values:
      default_noimaflags.param#VIGNET = 51;
      default.param#VIGNET = 51;
      VIGNETMAKER_RUNNER_RUN_1.STAMP_SIZE = 51;
      VIGNETMAKER_RUNNER_RUN_2.STAMP_SIZE = 51;
      PSF_SIZE = 51.
    default: px_51
    options:
      px_51:
        label: 51x51 px everywhere
      larger_adaptive:
        label: Larger or size-adaptive stamps
        description: >-
          Per-object sizes are not implemented. A larger fixed size is a
          config change, provided the tile VIGNET and the multi-epoch stamp
          change together.

  photometric_zeropoint:
    label: Magnitude zero-point convention
    rationale: >-
      Exposures read the per-image header PHOTZP, so the PSF-star MAG_AUTO
      window is on each exposure's own scale. Epochs are multiplied by header
      FSCALE before the joint fit, which puts them on zero-point 30 only if
      FSCALE = 10^(-0.4 (PHOTZP - 30)). Nothing in the repo checks this, but it
      held to 0.02% on a sampled exposure (2114045p). ngmix magnitudes use
      MAG_ZP = 30. The tile SExtractor run uses a fixed zero-point of 30
      (ZP_FROM_HEADER=False), which assumes the MegaPipe stacks are
      calibrated to 30.
      Values:
      MAG_ZEROPOINT = 30.0;
      config_tile_Sx.ini#SEXTRACTOR_RUNNER.ZP_FROM_HEADER = False;
      NGMIX_RUNNER.MAG_ZP = 30.0;
      config_exp_psfex.ini#SEXTRACTOR_RUNNER.ZP_FROM_HEADER = True;
      SEXTRACTOR_RUNNER.ZP_KEY = PHOTZP.
    default: fixed_30_tiles_header_exposures
    options:
      fixed_30_tiles_header_exposures:
        label: Tiles fixed 30.0; exposures from header PHOTZP
      header_everywhere:
        label: Per-image header zero-points on tiles too
        description: >-
          ZP_FROM_HEADER=True for tile SExtractor; changes tile magnitudes
          only where a tile's header zero-point differs from 30. ngmix MAG_ZP
          stays 30.

prior_insights:
  des_psf_blacklist:
    claim: >-
      DES blacklists a CCD's PSF model rather than failing the exposure: in
      Y3, a CCD with fewer than 25 stars surviving outlier rejection is
      excluded downstream (~2% of data removed) and processing proceeds.
    created_at: "2026-07-16T00:00:00Z"
    evidence:
      - id: ev_jarvis_y3
        doi: "10.48550/arXiv.2011.03409"
        quote:
          exact: "we enter it into a"
          suffix: " “blacklist” and exclude this CCD"
        location: { page: 10 }
  guinot22_star_floor_22:
    claim: >-
      The published ShapePipe/UNIONS analysis applies a per-CCD quality floor
      rather than failing whole exposures: a CCD with fewer than 22 selected
      stars is discarded for PSF estimation and contributes no epoch to the
      shape measurement, while processing continues.
    created_at: "2022-04-01T00:00:00Z"
    evidence:
      - id: ev_guinot22_star_floor
        doi: "10.48550/arXiv.2204.04798"
        quote:
          exact: 'The dashed line represents the cut at 22 stars/CCD below which the CCD is discarded for the PSF estimation.'
        location: { page: 4 }

analyses:

  # ═════════════════════════════════════════════════════════════════════════
  masking:
    description: >-
      Which external masks reach the measurement, and where. The instrument
      flag image delivered with each exposure is the only one that reaches
      pixels. Sky-fixed masks (star halos and bodies, manual regions, missing
      bands) are healsparse maps built and designed outside ShapePipe, which
      only queries them at object positions into catalogue columns that no
      stage cuts on.
    inputs:
      - id: exposure_flags
        type: data
        source: per-CCD instrument flag images split from each exposure (exp_split family)
      - id: sky_masks
        type: data
        source: UNIONS healsparse mask maps
    outputs:
      - id: masked_measurement_inputs
        type: data
        format: fits
        description: >-
          Exposure detection catalogues carrying IMAFLAGS_ISO (and MASK_EXT when
          maps are configured), the flag stamps ngmix reads, and the final
          catalogue's optional MASK_<band> columns.
        inputs: [exposure_flags, sky_masks]
        decisions: [pixel_mask_source, psf_star_mask_veto, sky_mask_application]
    decisions:
      pixel_mask_source:
        label: >-
          Of the external masks, only the instrument flag image reaches pixels
        rationale: >-
          On exposures SExtractor reads the split flag image, producing
          IMAFLAGS_ISO, which the PSF star selection requires to be zero. The
          multi-epoch vignet run cuts flag stamps from the same image; ngmix
          gives flagged pixels weight 0 and drops epochs more than a third
          flagged (shape_measurement.defect_fill,
          shape_measurement.epoch_masked_fraction_cut). Tiles have no flag
          image, and ShapePipe's tile SExtractor run requests no IMAFLAGS_ISO
          (detection.detection_source_mode). Neighbour pixels are masked
          separately, from the -1e30 markers SExtractor writes into the tile
          VIGNET (shape_measurement.blend_handling). Sky-fixed masks never touch
          pixels: a star halo changes no stamp.
          Values:
          config_exp_psfex.ini#SEXTRACTOR_RUNNER.FLAG_IMAGE = True;
          VIGNETMAKER_RUNNER_RUN_2.ME_IMAGE_PATTERN = flag, image, weight,
          background, background_rms.
        default: instrument_flags_only
        options:
          instrument_flags_only:
            label: Instrument flags gate pixels; sky masks stay at catalogue level
          rasterised_sky_masks:
            label: Rasterise healsparse masks into the pixel flags
            insights: [farrens22_pipeline_masks]
            excluded: true
            excluded_reason: >-
              Sky-fixed masks say where an object sits, not that its pixels are
              corrupted, so acting on them is an analysis decision.
              Rasterising would bake one mask version into every shape.
      psf_star_mask_veto:
        label: PSF-star candidates rejected on instrument flags only
        rationale: >-
          Of the masks, star selection cuts only on IMAFLAGS_ISO. mask_query sits
          between SExtractor and setools in the exposure chain; when
          MASK_PATHS names maps it writes MASK_EXT (0 clean, nonzero flagged,
          off-coverage clean) onto each CCD's catalogue. MASK_PATHS ships
          commented out, so the module passes catalogues through without
          MASK_EXT. The intended map is the UNIONS star-body product (bit 2);
          halo bits 0 and 1 are left out because halos say nothing about
          whether a star is a good PSF sample.
          Values:
          MASK:star_selection.IMAFLAGS_ISO = "== 0";
          MASK:preselect.IMAFLAGS_ISO = "== 0";
          MASK:flag.IMAFLAGS_ISO = "== 0";
          MASK:star_selection.MASK_EXT = absent;
          MASK:preselect.MASK_EXT = absent;
          MASK:flag.MASK_EXT = absent.
        default: instrument_flags_only
        options:
          instrument_flags_only:
            label: IMAFLAGS_ISO == 0; no sky map queried
          star_body_veto:
            label: Also reject candidates on the star-body map (MASK_EXT == 0)
            description: >-
              Set MASK_PATHS to the star-body map and add MASK_EXT == 0 beside
              each IMAFLAGS_ISO cut in star_selection.setools (one line per
              mask block). Querying without the cut records MASK_EXT and
              changes no star.
          star_body_and_halo_veto:
            label: Also reject candidates inside star halos
            excluded: true
            excluded_reason: >-
              Halos flag objects for the final catalogue; a star inside another
              star's halo is not thereby a bad PSF sample.
      sky_mask_application:
        label: Object-level sky masking deferred downstream
        rationale: >-
          The final catalogue ships every detected object. When MASK_EXT_PATHS
          lists band:path pairs, make_cat queries each healsparse map at the
          object's windowed position and writes the map value verbatim into a
          MASK_<band> column; an object off coverage gets the map's sentinel
          (False for boolean maps, reading as unmasked; typically -1 for
          integer maps). The committed config sets no MASK_EXT_PATHS, so no
          mask column is written and all mask cuts happen downstream.
        default: deferred_downstream
        options:
          deferred_downstream:
            label: No mask columns; all objects shipped
          catalogue_columns:
            label: Per-band MASK_<band> columns from MASK_EXT_PATHS, no cut
          pipeline_cut:
            label: Drop masked objects inside the pipeline
            excluded: true
            excluded_reason: >-
              Location flags are analysis decisions; a pipeline cut would fix
              one mask version into the catalogue.
    prior_insights:
      farrens22_pipeline_masks:
        claim: >-
          The published ShapePipe pipeline generated its own masks, including
          Messier objects and CCD borders, and applied them to the images; the
          current pipeline applies no sky-fixed mask to pixels.
        created_at: "2022-06-01T00:00:00Z"
        evidence:
          - id: ev_farrens22_masks
            doi: "10.48550/arXiv.2206.14689"
            quote:
              exact: 'Messier objects, and border regions.'
            location: { page: 2 }

  # ═════════════════════════════════════════════════════════════════════════
  detection:
    description: >-
      Object detection on r-band tiles (the galaxy sample) and on
      single-exposure CCDs (PSF-star candidates), both by ShapePipe's own
      SExtractor runs. The tile run follows the MegaPipe parameters of Gwyn's
      UNIONS tile catalogue, and in committed data its detections are joined
      to that catalogue (tile_detection); exposures keep ShapePipe's stock
      values.
    inputs:
      - id: tile_stack
        type: data
        source: MegaPipe r-band tile stack + weight (uncompressed, merged headers)
    outputs:
      - id: tile_sexcat
        type: data
        format: fits
        description: Per-tile SExtractor LDAC catalogue with per-epoch CCD membership.
        inputs: [tile_stack]
        decisions:
          [tile_detection, detection_threshold_policy, deblending_policy,
           background_model,
           weight_map_usage, zero_weight_interpolation, detection_source_mode,
           epoch_membership_ccd_bounds, photometry_parameters,
           spurious_detection_cleaning, blend_photometry_mask_type,
           saturation_level]
    decisions:
      tile_detection:
        label: Object list and NUMBER of the tile galaxy sample
        rationale: >-
          Both options run ShapePipe's tile SExtractor, so data and image
          simulations share one detection and measurement path: windowed
          positions, VIGNET neighbour markers and every SExtractor column.
          In real data each detection is then paired with its mutual nearest
          neighbour within 1 px in Gwyn's UNIONS per-tile catalogue and takes
          its NUMBER, so shape, photometry and photo-z catalogues share one
          object list and one ID (TILE_UNIQUE_ID); unpaired detections leave
          the catalogue. The catalogue is MegaPipe's SExtractor run on the
          same DR6 image with the configuration of default_tile.sex
          (detection_threshold_policy), so pairs agree to 4e-5 px at the
          median. On tile 186.307 every one of the 41,201 detections pairs;
          over eight DR6 tiles spanning a rich cluster, a bright star's halo,
          low latitude and the survey edge, at least 99.7% of detections and
          99.2% of catalogue objects pair. The catalogue objects with no
          detection (0-0.7%, mostly fainter than MAG_AUTO 24.5) are
          deblended children that ShapePipe's SExtractor leaves merged,
          concentrated around very large objects; they have photometry but
          no shape. The join needs the tile image to be the catalogue's
          release: on the DR5 image of a tile only ~95% of detections pair.
          The run stops when the unpaired detections, or the unpaired
          catalogue objects, exceed both 2% of their side and 20 objects: a
          margin over deblending near large objects, a stop for other pixels. PSF stars come from the exposure-level
          SExtractor run in both options. Image simulations, which have no
          UNIONS catalogue, keep SExtractor's own NUMBER.
          Values:
          input_types.data.tile_detection = unions_catalogue;
          config_tile_Sx.ini#SEXTRACTOR_RUNNER.MATCH_RADIUS = 1.0;
          config_tile_Sx.ini#SEXTRACTOR_RUNNER.MATCH_MIN_FRACTION = 0.98;
          config_tile_Sx.ini#SEXTRACTOR_RUNNER.MATCH_TOLERATED_UNPAIRED = 20.
        default: unions_catalogue
        options:
          unions_catalogue:
            label: Tile SExtractor joined to Gwyn's UNIONS per-tile catalogue
          sextractor:
            label: Tile SExtractor with its own NUMBER
      detection_threshold_policy:
        label: Detection significance, minimum area, matched filter
        rationale: >-
          Tile SExtractor uses the MegaPipe tile-catalogue threshold, minimum
          area and filter, so that its galaxy sample reproduces the UNIONS
          catalogue whose NUMBER committed data adopts (tile_detection). Run
          on the DR6 image of tile 186.307, it reproduces the DR6 catalogue to 99.89% completeness
          and 100% purity with identical segmentation pixels; the 45 misses
          are faint deblended children of bright parents (#929). The pixel
          stack (MEMORY_PIXSTACK) holds the largest object on every tested
          tile, a bright star's halo included, so no object is truncated.
          Unexplained: on tiles with such a large object (a cD galaxy, a
          bright star's halo) MAG_AUTO and FLUX_RADIUS differ from the
          catalogue by more than 1% for 2-11% of objects, most of them past
          the large object in SExtractor's scan order, and on one
          low-latitude tile for 2% of objects spread over the tile. The
          catalogue's run must differ in a setting not recorded here
          (memory, version); shapes use ShapePipe's measurements and
          photometry the catalogue's, so for those objects the two differ. This
          validates the configuration on data pixels, not detection on
          simulated images. The 7x7 Gaussian of FWHM 3 px is
          near the CFIS average seeing of 0.65 arcsec (about 3.5 px at
          0.187 arcsec/px). Exposures only feed star selection and keep stock
          values with the 3x3 FWHM 2 px kernel. The tiles differ from Guinot+22
          in all three.
          Values:
          config_tile_Sx.ini#SEXTRACTOR_RUNNER.DOT_SEX_FILE =
          $SP_CONFIG/default_tile.sex;
          default_tile.sex#DETECT_THRESH = 1.0;
          default_tile.sex#ANALYSIS_THRESH = 1.0;
          default_tile.sex#DETECT_MINAREA = 3;
          default_tile.sex#FILTER = Y;
          default_tile.sex#MEMORY_PIXSTACK = 3000000;
          config_tile_Sx.ini#SEXTRACTOR_RUNNER.DOT_CONV_FILE =
          $SP_CONFIG/gauss_3.0_7x7.conv;
          config_exp_psfex.ini#SEXTRACTOR_RUNNER.DOT_SEX_FILE =
          $SP_CONFIG/default_exp.sex;
          default_exp.sex#DETECT_THRESH = 1.5;
          default_exp.sex#ANALYSIS_THRESH = 1.5;
          default_exp.sex#DETECT_MINAREA = 5;
          default_exp.sex#FILTER = Y;
          config_exp_psfex.ini#SEXTRACTOR_RUNNER.DOT_CONV_FILE =
          $SP_CONFIG/default.conv.
        default: megapipe_tiles
        options:
          megapipe_tiles:
            label: MegaPipe values on tiles; stock values on exposures
            insights: [guinot22_cfis_seeing]
          stock_tiles:
            label: Stock ShapePipe values on tiles (1.5 sigma, minarea 5, FWHM 2 px)
            insights: [guinot22_sextractor_params]
            excluded: true
            excluded_reason: >-
              Triggers spuriously on about 10% of grid-placed Sersic galaxies
              in image simulations, and does not match the MegaPipe tile
              catalogue.
      deblending_policy:
        label: Deblending contrast
        rationale: >-
          Tiles use the MegaPipe DEBLEND_MINCONT; exposures use a lower one,
          the value Guinot+22 lists. Both share DEBLEND_NTHRESH. Contrast sets
          object count, centroids, and blend contamination in shapes;
          matching image simulations must preserve the stage-specific settings.
          Values:
          default_tile.sex#DEBLEND_MINCONT = 0.002;
          default_exp.sex#DEBLEND_MINCONT = 0.001;
          default_tile.sex#DEBLEND_NTHRESH = 32;
          default_exp.sex#DEBLEND_NTHRESH = 32.
        default: megapipe_tiles
        options:
          megapipe_tiles:
            label: MINCONT 0.002 tiles / 0.001 exposures
            insights: [guinot22_sextractor_params]
          mincont_5em4_tiles:
            label: MINCONT 0.0005 on tiles
            excluded: true
            excluded_reason: >-
              Part of the stock tile parameter set rejected in favour of the
              MegaPipe values (see detection_threshold_policy).
      background_model:
        label: Background estimation and photometric background
        rationale: >-
          Both passes use SExtractor AUTO backgrounds: tiles with the MegaPipe
          mesh and filter sizes and a LOCAL photometric background, exposures
          with a finer mesh and a GLOBAL one. ngmix subtracts the exposure
          BACKGROUND map from each epoch and weights its pixels by
          BACKGROUND_RMS (shape_measurement.galaxy_pixel_weights). The header
          background path is off. Residual sky offsets propagate into
          thresholds, fluxes, completeness and shapes.
          Values:
          default_tile.sex#BACK_TYPE = AUTO;
          default_tile.sex#BACK_SIZE = 512;
          default_tile.sex#BACK_FILTERSIZE = 9;
          default_tile.sex#BACKPHOTO_TYPE = LOCAL;
          BACKPHOTO_THICK = 30;
          default_exp.sex#BACK_TYPE = AUTO;
          default_exp.sex#BACK_SIZE = 64;
          default_exp.sex#BACK_FILTERSIZE = 3;
          default_exp.sex#BACKPHOTO_TYPE = GLOBAL;
          config_exp_psfex.ini#SEXTRACTOR_RUNNER.BKG_FROM_HEADER = False;
          config_tile_Sx.ini#SEXTRACTOR_RUNNER.BKG_FROM_HEADER = False.
        default: auto_megapipe_tiles
        options:
          auto_megapipe_tiles:
            label: AUTO everywhere; MegaPipe mesh and LOCAL photometry on tiles
          manual_zero_tiles:
            label: Tile background fixed to 0, trusting the stack subtraction
            excluded: true
            excluded_reason: >-
              Part of the stock tile parameter set rejected in favour of the
              MegaPipe values (see detection_threshold_policy).
      weight_map_usage:
        label: Weight map as inverse variance for detection
        rationale: >-
          MAP_WEIGHT on both passes (SExtractor default NONE): the per-pixel
          variance sets the effective SNR and so the detection set.
          RESCALE_WEIGHTS and WEIGHT_GAIN keep their SExtractor defaults.
          Guinot+22 keeps every non-tabulated parameter at its default, which
          would mean no weight map.
          Values:
          default_tile.sex#WEIGHT_TYPE = MAP_WEIGHT;
          default_exp.sex#WEIGHT_TYPE = MAP_WEIGHT;
          default_tile.sex#RESCALE_WEIGHTS = Y;
          default_exp.sex#RESCALE_WEIGHTS = Y;
          default_tile.sex#WEIGHT_GAIN = Y;
          default_exp.sex#WEIGHT_GAIN = Y;
          config_tile_Sx.ini#SEXTRACTOR_RUNNER.WEIGHT_IMAGE = True;
          config_exp_psfex.ini#SEXTRACTOR_RUNNER.WEIGHT_IMAGE = True.
        default: map_weight
        options:
          map_weight:
            label: MAP_WEIGHT
          no_weight:
            label: No weight map (SExtractor default)
            insights: [guinot22_sextractor_params]
      zero_weight_interpolation:
        label: Interpolation across zero-weight pixels
        rationale: >-
          INTERP_TYPE ALL on both passes (SExtractor default NONE): SExtractor
          invents flux across zero-weight pixels, changing detections and
          photometry near masked regions. Matching image simulations must use
          the same interpolation operator; this is not a calibrated pixel repair.
          Values:
          default_tile.sex#INTERP_TYPE = ALL;
          default_exp.sex#INTERP_TYPE = ALL;
          default_tile.sex#INTERP_MAXXLAG = 16;
          default_tile.sex#INTERP_MAXYLAG = 16;
          default_exp.sex#INTERP_MAXXLAG = 16;
          default_exp.sex#INTERP_MAXYLAG = 16.
        default: interp_all
        options:
          interp_all:
            label: INTERP_TYPE ALL
          no_interpolation:
            label: INTERP_TYPE NONE
      spurious_detection_cleaning:
        label: Cleaning of spurious detections
        rationale: >-
          CLEAN on both passes deletes detections
          consistent with being wings of a brighter neighbour, a post-deblend
          change to the object list. Keep the same cleaning prescription in
          matching image simulations.
          Values:
          default_tile.sex#CLEAN_PARAM = 1.0;
          default_exp.sex#CLEAN_PARAM = 1.0;
          default_tile.sex#CLEAN = Y;
          default_exp.sex#CLEAN = Y.
        default: clean_1
        options:
          clean_1:
            label: CLEAN Y, CLEAN_PARAM 1.0
      blend_photometry_mask_type:
        label: Neighbour pixels in blend photometry
        rationale: >-
          MASK_TYPE CORRECT on both passes replaces neighbour pixels by their
          mirror across the object centre during photometry, changing blend
          fluxes and windowed moments. This SExtractor photometry choice is
          separate from ngmix's neighbour-pixel weighting decision.
          Values:
          default_tile.sex#MASK_TYPE = CORRECT;
          default_exp.sex#MASK_TYPE = CORRECT.
        default: correct
        options:
          correct:
            label: MASK_TYPE CORRECT
          blank:
            label: MASK_TYPE BLANK
      saturation_level:
        label: Saturation level read from each image's header
        rationale: >-
          Each SExtractor pass takes the image's saturation level from its
          SATURATE header card and sets FLAGS bit 4 on objects with saturated
          pixels. Neither .sex file sets SATUR_LEVEL, so where the card is
          absent SExtractor falls back to its built-in level (50000 ADU per
          its documentation). Delivered exposures carry the card (every CCD
          of 2114045p: 65535) and split_exp keeps each CCD's header; a sampled
          tile (CFIS.186.307) has 9558.7. The PSF-star masks require FLAGS ==
          0, so the level can remove bright stars from the PSF sample.
          Changing the card or pinning a fixed level requires checking
          the resulting bright-star selection.
          Values:
          default_exp.sex#SATUR_KEY = SATURATE;
          default_tile.sex#SATUR_KEY = SATURATE;
          MASK:star_selection.FLAGS = "== 0";
          MASK:preselect.FLAGS = "== 0";
          MASK:flag.FLAGS = "== 0";
          default_tile.sex#SATUR_LEVEL = absent;
          default_exp.sex#SATUR_LEVEL = absent.
        default: header_saturate
        options:
          header_saturate:
            label: SATURATE header card; SExtractor default level where absent
          fixed_level:
            label: Fixed SATUR_LEVEL set in the .sex files
      photometry_parameters:
        label: Kron and aperture photometry definitions
        rationale: >-
          Stock Kron, aperture and flux-fraction settings; no rationale
          recorded. On the exposure pass MAG_AUTO is the axis of the
          star-selection magnitude windows and FLUX_AUTO is PSFEx's
          photometric normalisation, so a different Kron factor moves stars
          across those cuts. The reported apertures and flux-fraction
          measurements must match the simulated catalogue they are compared
          with.
          Values:
          default_tile.sex#PHOT_AUTOPARAMS = 2.5,3.5;
          default_exp.sex#PHOT_AUTOPARAMS = 2.5,3.5;
          default_tile.sex#PHOT_APERTURES = 5;
          default_exp.sex#PHOT_APERTURES = 5;
          default_tile.sex#PHOT_FLUXFRAC = 0.5;
          default_exp.sex#PHOT_FLUXFRAC = 0.5;
          PHOTFLUX_KEY = FLUX_AUTO.
        default: kron_25_35
        options:
          kron_25_35:
            label: Kron 2.5/3.5, aperture 5 px, FLUXFRAC 0.5
      detection_source_mode:
        label: Single-image, unflagged detection on the r-band tile
        rationale: >-
          No detection image, no flag image, and the default_noimaflags.param
          column list: tiles have no instrument flag image and no detection
          coadd exists, so detection sees every tile pixel and the tile
          catalogue carries no IMAFLAGS_ISO; final_cat.param, the merge's
          exact allow-list, does not request it. Without a detection image
          sextractor_runner calls SExtractor in single-image mode, which on
          tile 186.307 reproduces the UNIONS catalogue's MAG_AUTO to 0.01 mag
          for all but 0.03% of objects. Dual-image mode with the same image
          twice is not equivalent: it changes the background, MAG_AUTO and
          FLUX_RADIUS of a fifth of the objects (#936).
          Values:
          SEXTRACTOR_RUNNER.DETECTION_IMAGE = False;
          SEXTRACTOR_RUNNER.FLAG_IMAGE = False.
        default: sx_nomask_single_image
        options:
          sx_nomask_single_image:
            label: Unflagged single-image r-band detection
            insights: [guinot22_stacked_detection]
          masked_tile:
            label: Detection on a tile masked by rasterised sky masks
            description: >-
              Not implemented; no tile pixel mask exists (see
              masking.pixel_mask_source).
          dual_image_coadd:
            label: Dual-image mode with a detection coadd
            description: Not implemented; no UNIONS detection coadd exists.
      epoch_membership_ccd_bounds:
        label: Which exposure CCDs an object belongs to (N_EPOCH)
        rationale: >-
          `all_world2pix(..., 0)` returns 0-based pixels, and the strict test
          accepts `33 < x < 2080`. In 1-based FITS coordinates this is x=35
          through x=2080 inclusive: it trims DATASEC columns 33 and 34 but
          admits its upper endpoint, column 2080. [HARDCODED] The comparisons
          are strict in make_post_process; CCD_SIZE sets only the bounds. A
          WCS inversion failure skips that CCD, lowering N_EPOCH, which caps
          how many exposures enter each galaxy's multi-epoch fit.
          Values:
          SEXTRACTOR_RUNNER.CCD_SIZE = 33,2080,1,4612;
          SEXTRACTOR_RUNNER.MAKE_POST_PROCESS = True.
        default: trimmed_bounds_33_2080
        options:
          trimmed_bounds_33_2080:
            label: "x in (33,2080), y in (1,4612), strict"
          inclusive_bounds:
            label: Same bounds, inclusive
            description: Not implemented; make_post_process uses strict comparisons.
    prior_insights:
      guinot22_sextractor_params:
        claim: >-
          The published tile detection uses DETECT_THRESH 1.5, DETECT_MINAREA
          10, the default 3x3 filter and DEBLEND_MINCONT 0.001, with every
          other SExtractor parameter at its default.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_table2
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'Table 2. SExtractor parametrisation. All other parameters are kept to their default values.'
            location: { page: 5 }
          - id: ev_guinot22_mincont
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'DEBLEND_MINCONT 0.001'
            location: { page: 5 }
          - id: ev_guinot22_minarea
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'DETECT_MINAREA 10'
            location: { page: 5 }
      guinot22_cfis_seeing:
        claim: >-
          CFIS r-band data have an average seeing of 0.65 arcsec.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_seeing
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'is taking r-band data with an average seeing of 0.65 arcsec'
            location: { page: 1 }
      guinot22_stacked_detection:
        claim: >-
          Source extraction in the published analysis is performed on the
          stacked tile images, for signal-to-noise and because artefacts are
          suppressed relative to single exposures.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_stacked_detection
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'We do the extraction on stacked images which provide a better signal-to-noise ratio, and most artifacts have a reduced amplitude with respect to single exposures'
            location: { page: 5 }

  # ═════════════════════════════════════════════════════════════════════════
  preparation:
    description: >-
      How exposures, astrometry, epoch lists and stamps are prepared before
      anything is measured.
    inputs:
      - id: exposure_files
        type: data
        source: delivered CFIS exposure triplets (image/weight/flag MEF) + tile stacks
    outputs:
      - id: epoch_stamps
        type: data
        format: sqlite
        description: Per-object multi-epoch vignets + per-CCD WCS log feeding ngmix.
        inputs: [exposure_files]
        decisions:
          [astrometric_solution_source, ccd_split_extent,
           epoch_provenance_from_tile_history, object_position_columns,
           stamp_positioning_and_padding]
    decisions:
      astrometric_solution_source:
        label: Astrometry taken verbatim from delivered per-CCD headers
        rationale: >-
          split_exp builds WCS(header) from each delivered CCD header, and
          merge_headers stores these per-tile. Every committed world-to-pixel
          step on exposures uses that solution: epoch membership,
          multi-epoch stamp placement and OFFSET, tile-side PSF interpolation
          positions, and the per-epoch ngmix Jacobian. [HARDCODED] No
          astrometric re-derivation or refinement exists. A re-fit would shift
          stamp centres and Jacobians and could change epoch membership.
        default: delivered_headers
        options:
          delivered_headers:
            label: WCS(header) verbatim, stored at split time
            insights: [guinot22_gaia_astrometry]
          astrometric_refit:
            label: Joint astrometric re-solution
            description: Not implemented.
      ccd_split_extent:
        label: All 40 MegaCam HDUs split and carried as candidate epochs
        rationale: >-
          Any HDU count other than N_HDU raises; every CCD is a candidate
          epoch wherever the WCS lands it.
          Values:
          SPLIT_EXP_RUNNER.N_HDU = 40.
        default: all_40_hdus
        options:
          all_40_hdus:
            label: 40 HDUs, hard-fail on any other count
            insights: [guinot22_forty_chips]
          exclude_ccd_subset:
            label: Exclude a subset of CCDs as epochs
            description: Not implemented.
      epoch_provenance_from_tile_history:
        label: Epoch sets parsed from tile FITS HISTORY cards
        rationale: >-
          The tile's primary-header HISTORY cards are its exposure list, and
          two readers parse the same cards. find_exposures takes the file name
          in column COLNUM, strips the extension and deduplicates. EXP_PREFIX
          is blank, so 2243881p keeps its p. This list decides which exposures
          are retrieved, split and merged into the tile's WCS log.
          make_post_process re-parses the HISTORY copied into the catalogue
          (the digits before an optional p) and builds the EPOCH HDUs and
          N_EPOCH that the stamps and ngmix use. It raises if a HISTORY
          exposure is missing from the WCS log. A tile whose header or HISTORY
          cannot be read fails; it never gets an empty exposure list.
          Values:
          FIND_EXPOSURES_RUNNER.COLNUM = 3.
        default: history_parse
        options:
          history_parse:
            label: HISTORY column 3, no prefix stripped, deduplicated
      object_position_columns:
        label: Windowed centroids (XWIN/YWIN) define every position
        rationale: >-
          PSF interpolation sites, tile and multi-epoch stamp centres, epoch
          membership and the catalogue position all use the tile SExtractor
          run's windowed centroid, in data and image simulations alike: in pixels
          for tile stamps and exposure-side PSF validation, in world
          coordinates for tile-side PSF interpolation and multi-epoch stamps.
          Windowed, isophotal and model centroids differ systematically for
          blends and asymmetric galaxies, and the centroid feeds the position
          seed and the centroid prior.
          Values:
          VIGNETMAKER_RUNNER_RUN_1.POSITION_PARAMS = XWIN_IMAGE,YWIN_IMAGE;
          VIGNETMAKER_RUNNER_RUN_1.COORD = PIX;
          config_tile_PiViVi_psfex.ini#PSFEX_INTERP_RUNNER.POSITION_PARAMS =
          XWIN_WORLD,YWIN_WORLD;
          VIGNETMAKER_RUNNER_RUN_2.POSITION_PARAMS = XWIN_WORLD,YWIN_WORLD;
          VIGNETMAKER_RUNNER_RUN_2.COORD = SPHE;
          config_exp_psfex.ini#PSFEX_INTERP_RUNNER.POSITION_PARAMS =
          XWIN_IMAGE,YWIN_IMAGE.
        default: xwin_windowed
        options:
          xwin_windowed:
            label: Windowed centroids everywhere
          isophotal_barycentre:
            label: Isophotal barycentre (X_IMAGE/Y_IMAGE)
            excluded: true
            excluded_reason: >-
              Noisier than the windowed centroid and pulled by asymmetric or
              deblend-clipped footprints; on the DR6 image of tile 186.307
              it sits 0.16 px (median) and 1.6 px (p99) from it for
              20 < MAG_AUTO < 24.5, and the image simulations measure the
              windowed one.
      stamp_positioning_and_padding:
        label: Nearest-pixel stamp extraction with zero padding
        rationale: >-
          [HARDCODED] stamps are cut around the pixel nearest the object's
          position, with no sub-pixel interpolation; the sub-pixel remainder
          is stored as the stamp's OFFSET, which ngmix uses as the Jacobian
          origin (shape_measurement.centroid_source), so extraction and
          centroid prior share one rounding. Multi-epoch stamps map the tile
          world coordinate through the stored per-CCD WCS. Stamps overrunning
          an image edge are kept, zero-filled outside the image; a stamp
          centre that rounds outside the image raises and fails the vignet
          run for the whole tile.
        default: round_and_zero_pad
        options:
          round_and_zero_pad:
            label: Nearest-pixel extraction, zero padding, no edge rejection
    prior_insights:
      guinot22_gaia_astrometry:
        claim: >-
          The astrometric solution the analysis relies on is the upstream
          MegaPipe/Gaia DR2 calibration, accurate to within 20 mas; no
          astrometric re-fit inside the pipeline is described.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_astrometry
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'An astrometric calibration within 20 mas was achieved using the Gaia DR2 observations'
            location: { page: 2 }
      guinot22_forty_chips:
        claim: >-
          Star selection and PSF modelling are carried out independently on
          each of the 40 MegaCam chips, with no chip excluded.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_forty_chips
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'is performed independently on each of the 40 chips that constitute the MegaCAM'
            location: { page: 3 }

  # ═════════════════════════════════════════════════════════════════════════
  star_selection_psf:
    description: >-
      Which objects constrain the PSF, the PSF model itself, and which CCD
      models are good enough to use.
    inputs:
      - id: exposure_sexcat
        type: data
        source: per-CCD exposure SExtractor catalogues (default_exp.sex run)
    outputs:
      - id: psf_model
        type: data
        format: psf
        description: >-
          Per-CCD PSF models and the PSFs interpolated at object positions,
          with HSM shape diagnostics.
        inputs: [exposure_sexcat]
        decisions:
          [star_selection_box, psf_train_validation_split,
           psfex_candidate_vetting, psf_modelling_software,
           psf_model_complexity, psf_acceptance_thresholds]
    decisions:
      star_selection_box:
        label: Stellar-locus selection, magnitude window and FWHM window scaled to the locus
        rationale: >-
          Stars are objects with FLAGS == 0 and IMAFLAGS_ISO == 0 in a
          MAG_AUTO window whose FWHM_IMAGE lies within 3 locus widths of the
          stellar-locus peak of a looser, size-limited preselection
          (FWHM_WORLD 0.3-1.5 arcsec, angular bounds from each exposure's
          WCS). mode() is the peak of a Gaussian kernel density
          estimate of the preselection FWHM, and locus_width() is that
          peak's full width at half maximum as a Gaussian sigma, with the
          kernel removed in quadrature. The box scales with the locus
          because the locus width varies from 0.06 to 0.26 px per CCD with
          seeing and focal-plane position, so a fixed box cuts broad
          corner loci (2114045p CCD 35 keeps 39 of 63 bright locus stars at
          +-0.2 px, 63 at 3 sigma) and is loose on sharp ones. k = 3 is
          where widening stops adding mostly stars: against the
          size-independent MU_MAX - MAG_AUTO star sequence (18-20.5 mag, 79
          CCDs), objects added between 2.5 and 3 sigma are 59% (0.53 arcsec
          seeing) and 71% (0.69 arcsec) stars, and between 3 and 3.5 sigma
          28% and 65%. At k = 3, 98.9% and 89.1% of locus stars are kept
          with 2.7% and 5.6% non-stars, against 95.6%, 80.9%, 1.7% and 3.0%
          for the fixed +-0.2 px box. The [HARDCODED] 0.1 px kernel
          bandwidth is close to the median locus width (0.08 px at 0.53
          arcsec, 0.10 px at 0.69 arcsec) and is not tied to each CCD's
          width: a wider kernel on a broad locus pulls
          the peak and the width into the galaxies, and a tied bandwidth
          moved box edges by up to 4.5 px when the preselection bounds
          shifted slightly on 0.69 arcsec CCDs. Measured widths make the box
          edges noisier than a fixed half-width (bootstrap 0.03-0.05 px,
          against 0.01 px), still below the locus width. Below 20 objects the centre is the median
          and the width the normalised MAD; with none both are -1, which
          selects nothing. Where galaxies capture the peak (2254826p CCD
          35), no box recovers the stars; the logged locus width flags it
          (0.32 px, against 0.06-0.26 px elsewhere).
          PSFEx's own selection is off; see psfex_candidate_vetting for what
          PSFEx may still apply.
          Values:
          MASK:star_selection.FLAGS = "== 0";
          MASK:preselect.FLAGS = "== 0";
          MASK:star_selection.IMAFLAGS_ISO = "== 0";
          MASK:preselect.IMAFLAGS_ISO = "== 0";
          MASK:star_selection.MAG_AUTO = ["> 18.", "< 22."];
          MASK:star_selection.FWHM_IMAGE = ["<= mode(FWHM_IMAGE{preselect})
          + 3 * locus_width(FWHM_IMAGE{preselect})", ">=
          mode(FWHM_IMAGE{preselect}) - 3 * locus_width(FWHM_IMAGE{preselect})"];
          MASK:preselect.MAG_AUTO = ["> 0", "< 21"];
          MASK:preselect.FWHM_WORLD = ["> 0.3 / 3600", "< 1.5 / 3600"];
          PLOT:fwhm_field.SCATTER = "FWHM_WORLD{star_selection}*3600";
          LOCUS_BANDWIDTH = 0.1;
          LOCUS_WIDTH_FLOOR = 0.02;
          LOCUS_MIN_SIZE = 20;
          SAMPLE_AUTOSELECT = N.
        default: locus_scaled_box
        options:
          locus_scaled_box:
            label: Box +-3 locus widths about the locus peak, mag 18-22
          mode_centred_box:
            label: Fixed box +-0.2 px (0.04 arcsec) about the FWHM mode, mag 18-22
            insights: [guinot22_star_box]
          size_mag_locus_fit:
            label: Fitted size-magnitude stellar locus
            description: Not implemented.
          psfex_autoselect:
            label: PSFEx SAMPLE_AUTOSELECT vetting on top
            insights: [guinot22_psfex_preselection_off]
            excluded: true
            excluded_reason: >-
              Disabled because the pipeline does its own star selection
              (Guinot+22); PSFEx's other SAMPLE_* cuts still apply
              (psfex_candidate_vetting).
      psf_train_validation_split:
        label: Seeded 80/20 star split, model fit vs held-out validation
        rationale: >-
          The 80% sample fits the PSFEx model and feeds the tile multi-epoch
          interpolation; the 20% sample is the independent residual
          diagnostic (psfex_interp VALIDATION mode). The split trades
          training stars per CCD, which interacts with the acceptance gate,
          against an independent residual test. The permutation is seeded
          from the digits of the unit's file number, so a CCD gets the same
          split on every run.
          Values:
          RAND_SPLIT:star_split.RATIO = 20;
          PSFEX_RUNNER.FILE_PATTERN = star_split_ratio_80;
          PSFEX_INTERP_RUNNER.FILE_PATTERN =
          star_split_ratio_80,star_split_ratio_20,psfex_cat;
          PSFEX_INTERP_RUNNER.ME_DOT_PSF_PATTERN = star_split_ratio_80.
        default: split_80_20_seeded
        options:
          split_80_20_seeded:
            label: 80% train / 20% validation, seeded from the file number
            insights: [guinot22_star_split]
          split_80_20_unseeded:
            label: Same split from an unseeded random draw
            excluded: true
            excluded_reason: >-
              Makes the PSF star sample, and every shape downstream of it,
              irreproducible run-to-run.
          no_holdout:
            label: All stars in the model, no held-out diagnostic
            excluded: true
            excluded_reason: Loses the independent residual and rho-statistic input.
      psfex_candidate_vetting:
        label: PSFEx built-in candidate cuts, unpinned
        rationale: >-
          default.psfex sets SAMPLE_AUTOSELECT N (compiled default Y) and
          omits every other SAMPLE_* key, so [HARDCODED] PSFEx's compiled
          defaults govern them and can change with its version. psfex -dd
          (PSFEx 3.21.1, develop-runtime image) gives SAMPLE_FWHMRANGE
          2.0,10.0, SAMPLE_VARIABILITY 0.2, SAMPLE_MINSN 20, SAMPLE_MAXELLIP
          0.3, SAMPLE_FLAGMASK 0x00fe, SAMPLE_WFLAGMASK 0x0000 and
          SAMPLE_IMAFLAGMASK 0x0. Per the PSFEx source (not re-read here),
          MINSN, MAXELLIP, FLAGMASK, FWHMRANGE and VARIABILITY still cut
          candidates with autoselect off; SETools cuts neither S/N nor
          ellipticity, so MINSN and MAXELLIP can reject stars it kept.
          BADPIXEL_FILTER N and PSF_RECENTER N, also compiled defaults, leave
          flagged star vignets unfiltered and candidates unrecentred.
          Compiled-in values admit no `= value` assertion, so the anchors
          assert only that each SAMPLE_* key is absent; open issue #919 would
          pin them in default.psfex.
          Values:
          SAMPLE_AUTOSELECT = N;
          BADPIXEL_FILTER = N;
          PSF_RECENTER = N;
          SAMPLE_FWHMRANGE = absent;
          SAMPLE_VARIABILITY = absent;
          SAMPLE_MINSN = absent;
          SAMPLE_MAXELLIP = absent;
          SAMPLE_FLAGMASK = absent;
          SAMPLE_WFLAGMASK = absent;
          SAMPLE_IMAFLAGMASK = absent.
        default: builtin_defaults
        options:
          builtin_defaults:
            label: PSFEx 3.21.1 built-in SAMPLE_* values, no bad-pixel filter
          pinned_explicit:
            label: Write the SAMPLE_* values explicitly into default.psfex
      psf_modelling_software:
        label: PSF model, PSFEx per CCD or MCCD over the focal plane
        rationale: >-
          psf_model in workflow/config.yaml's input_types: table selects the
          exposure and tile config pair and is exported as SP_PSF to the tile
          interpolation. The committed value is psfex, which fits each split
          CCD independently (image sims default to fake, the true PSF with no
          exposure fit). MCCD (Liaudat+2021) fits one hybrid local+global
          model per exposure over the focal plane. Its ShapePipe chain exists
          and matches PSFEx's up to the model: SExtractor reads the split
          image, weight and instrument flag, then mask_query, then setools.
          However, the Snakefile refuses psf_model=mccd at parse time, because
          exp_persist and star_cat_merge read only PSFEx products. Choosing
          MCCD therefore needs MCCD-aware persistence and star-catalogue
          merging first.
          Values:
          INSTANCE.N_COMP_LOC = 8;
          INSTANCE.D_COMP_GLOB = 8;
          INSTANCE.FP_GEOMETRY = CFIS;
          INSTANCE.RMSE_THRESH = 1.25;
          INPUTS.MIN_N_STARS = 20;
          FIT.LOC_MODEL = hybrid;
          config_exp_mccd.ini#EXECUTION.MODULE =
          sextractor_runner,mask_query_runner,setools_runner,mccd_preprocessing_runner,mccd_fit_val_runner,merge_starcat_runner,mccd_plots_runner;
          SEXTRACTOR_RUNNER.INPUT_DIR =
          $SP_RUN/output/run_sp_exp_Sp/split_exp_runner/output;
          SEXTRACTOR_RUNNER.FILE_PATTERN = image,weight,flag;
          SEXTRACTOR_RUNNER.FLAG_IMAGE = True.
        default: psfex
        options:
          psfex:
            label: PSFEx, independent per-CCD models
            insights: [guinot22_psfex_software, farrens22_two_psf_methods]
          mccd_focal_plane:
            label: MCCD hybrid local+global focal-plane model
            description: >-
              Refused by workflow/Snakefile (refuse_unpersistable_psf) until
              persistence and star-catalogue merging read MCCD products.
            insights: [farrens22_two_psf_methods]
      psf_model_complexity:
        label: PSFEx pixel basis with degree-2 spatial variation per CCD
        rationale: >-
          A pixel basis with polynomial variation in XWIN/YWIN, fit per CCD
          at native sampling. PSF_ACCURACY is the fractional accuracy PSFEx
          assumes for PSF pixel values; per the PSFEx documentation it enters
          the fit weights, and so how closely the model follows bright stars.
          Model flexibility trades overfitting against PSF leakage, the
          dominant additive systematic in cosmic shear. A more flexible model
          must be supported by the surviving training stars and checked on
          held-out residuals under the acceptance gate; successful optimisation
          alone does not establish a usable PSF.
          Stock values; no rationale recorded.
          Values:
          BASIS_TYPE = PIXEL;
          PSF_ACCURACY = 0.01;
          BASIS_NUMBER = 20;
          PSFVAR_DEGREES = 2;
          PSF_SAMPLING = 1;
          PSFVAR_KEYS = XWIN_IMAGE,YWIN_IMAGE;
          PSFVAR_GROUPS = 1,1;
          MEF_TYPE = INDEPENDENT.
        default: pixel_basis_deg2_per_ccd
        options:
          pixel_basis_deg2_per_ccd:
            label: PIXEL basis, degree 2, per CCD
            insights: [guinot22_psf_no_oversampling]
          deg3:
            label: Degree-3 spatial variation
            description: Needs more stars per CCD than the acceptance gate guarantees.
      psf_acceptance_thresholds:
        label: Per-CCD PSF-model quality gate
        rationale: >-
          A CCD whose model has ACCEPTED < STAR_THRESH or CHI2 > CHI2_THRESH
          is not interpolated on either the validation or multi-epoch pass.
          ACCEPTED counts stars that survive PSFEx's own sample cuts; PSFEx
          fits the 80% training split, so STAR_THRESH applies the published
          22-star floor to that accepted count. In the science path the CCD's
          epoch is dropped for every object on it, and an object left with no
          epoch has no shape.
          The pipeline has no minimum-epoch floor: NGMIX_N_EPOCH records what
          survived and epoch-count cuts happen downstream. Guinot+22 discards
          the CCD from PSF estimation rather than gating at interpolation.
          Values:
          config_exp_psfex.ini#PSFEX_INTERP_RUNNER.STAR_THRESH = 22;
          config_exp_psfex.ini#PSFEX_INTERP_RUNNER.CHI2_THRESH = 2;
          config_tile_PiViVi_psfex.ini#PSFEX_INTERP_RUNNER.STAR_THRESH = 22;
          config_tile_PiViVi_psfex.ini#PSFEX_INTERP_RUNNER.CHI2_THRESH = 2.
        default: stars22_chi2_2
        options:
          stars22_chi2_2:
            label: ">= 22 stars and chi2 <= 2 on both passes"
            insights: [des_psf_blacklist_local, guinot22_star_floor_22_local]
          stars20_science_path:
            label: ">= 20 stars on the science path"
            excluded: true
            excluded_reason: >-
              A 20-star science-path threshold would pass models with 20 or 21
              PSFEx-accepted training stars, below the 22-star floor retained
              on the validation path.
          des_25:
            label: DES Y3 threshold (25 stars)
            excluded: true
            excluded_reason: >-
              CFIS CCDs are smaller than DECam's; the floor is survey-specific.
    prior_insights:
      des_psf_blacklist_local:
        claim: >-
          DES Y3 blacklists any CCD with fewer than 25 stars surviving
          outlier rejection in the PSF fit.
        created_at: "2026-07-16T00:00:00Z"
        evidence:
          - id: ev_jarvis_y3_local
            doi: "10.48550/arXiv.2011.03409"
            quote:
              exact: "fewer than 25 stars survived the outlier rejection"
            location: { page: 10 }
      guinot22_star_floor_22_local:
        claim: >-
          The published ShapePipe/UNIONS analysis discards a CCD from the PSF
          estimation when fewer than 22 stars are selected on it.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_star_floor_local
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'The dashed line represents the cut at 22 stars/CCD below which the CCD is discarded for the PSF estimation.'
            location: { page: 4 }
      guinot22_star_box:
        claim: >-
          The published star selection keeps objects whose FWHM lies within
          0.04 arcsec of the mode of a size preselection, restricted to the
          magnitude range 18 < r < 22.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_star_box
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'From this pre-selection we keep objects for which the FWHM is within 0.04 arcsec of the mode. In addition to these size cuts, we only use star candidates in the magnitude range 18 < r < 22.'
            location: { page: 4 }
      guinot22_star_split:
        claim: >-
          The published analysis randomly splits the star sample in two, 80%
          building the PSF model and 20% held out for the validation tests.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_star_split
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'To be able to perform these tests properly, our star sample has been randomly divided in two:'
            location: { page: 7 }
      guinot22_psfex_preselection_off:
        claim: >-
          PSFEx's internal pre-selection is disabled because the pipeline
          does its own star selection; the PSF is fit on the entire star
          sample.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_preselection_off
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'Since we carry out our own star selection (see Sect. 4.1), we disable the internal PSFEx pre-selection, and the PSF is thus obtained using the entire star sample.'
            location: { page: 4 }
      guinot22_psfex_software:
        claim: >-
          The published UNIONS shear catalogue uses PSFEx for PSF modelling,
          with MCCD named as upcoming rather than current work.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_psfex_software
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'We make use of the PSFEx software package'
            location: { page: 4 }
      farrens22_two_psf_methods:
        claim: >-
          ShapePipe ships two PSF modelling methods, PSFEx and MCCD, either or
          both of which may be run and used for the galaxy shape measurement.
        created_at: "2022-06-01T00:00:00Z"
        evidence:
          - id: ev_farrens22_two_psf
            doi: "10.48550/arXiv.2206.14689"
            quote:
              exact: 'ShapePipe allows either or both methods to be run and subsequently used for the galaxy shape measurement.'
            location: { page: 3 }
      guinot22_psf_no_oversampling:
        claim: >-
          The PSFEx parametrisation is tabulated in the paper (pixel basis,
          degree-2 spatial variation), with the deliberate choice not to
          over-sample the PSF models.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_psf_complexity
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'The PSFEx parameters we used are presented in Table 1. We have chosen not to over-sample the PSF models.'
            location: { page: 5 }

  # ═════════════════════════════════════════════════════════════════════════
  shape_measurement:
    description: >-
      Joint multi-epoch ngmix Gaussian fits with metacalibration. Most
      choices are fixed in code, so silent defaults concentrate here.
    inputs:
      - id: vignets
        type: data
        source: multi-epoch galaxy, weight, flag, background and background-RMS vignets + interpolated PSFs
    outputs:
      - id: ngmix_cat
        type: data
        format: fits
        description: Per-tile metacal shear catalogue chunks (ngmix_runner family).
        inputs: [vignets]
        decisions:
          [ngmix_seed_mode, galaxy_model, fit_initialisation, fit_priors,
           metacal_scheme, centroid_source, epoch_flux_rescaling,
           psf_epoch_averaging, galaxy_pixel_weights, psf_likelihood_noise,
           megacam_ccd_flip, defect_fill, blend_handling,
           epoch_masked_fraction_cut, central_defect_veto]
    decisions:
      ngmix_seed_mode:
        label: Per-object RNG seeded from sky position
        rationale: >-
          Each object's RNG (noise realisations, guesses, priors) is seeded
          from its position: [HARDCODED] 3-arcsec sky boxes offset by the
          first epoch's CCD number, folded and Cantor-paired mod 2^32. Results
          therefore do not depend on how a tile is chunked, and metacal's
          fixnoise counter-noise cancels across image-simulation branches that
          share an object's box.
        default: position_seed
        options:
          position_seed:
            label: Per-object seed from (ra, dec, ccd), 3-arcsec boxes
          tile_seed:
            label: One tile-wide RandomState consumed in object order
            excluded: true
            excluded_reason: >-
              Makes every noise stream depend on chunk boundaries and object
              order.
      galaxy_model:
        label: Single-Gaussian galaxy and PSF models
        rationale: >-
          [HARDCODED] ngmix Fitter(model='gauss') for galaxy and PSF. The
          standard defence, that model bias largely cancels in the metacal
          response, is not stated in the code.
        default: gauss
        options:
          gauss:
            label: Single Gaussian
            insights: [guinot22_gaussian_model]
          exp_or_bdf:
            label: exp / bdf galaxy models
            description: Not exposed; make_runners fixes the model.
      fit_initialisation:
        label: Fit guesses and retries
        rationale: >-
          [HARDCODED] the galaxy guesser (TPSFFluxAndPriorGuesser) starts
          from T = 0.25 and a PSF-flux fit; the PSF guesser (TFluxGuesser)
          from T = 0.25 and the catalogue flux. The galaxy runner tries 5
          times, the PSF runner twice. With a non-convex likelihood, guess and
          retries decide which objects converge. A failed fit is flagged; an
          exception during an object's fit drops it with no ngmix row, leaving
          it to the catalogue sentinels (catalogue_assembly.failure_sentinels).
        default: prior_guess_t025_ntry5_2
        options:
          prior_guess_t025_ntry5_2:
            label: T guess 0.25; PSF-flux (galaxy) / catalogue flux (PSF); ntry 5 / 2
          hsm_initialisation:
            label: Guesses from HSM adaptive moments (Guinot+22)
            insights: [guinot22_hsm_initialisation]
            description: Not implemented.
      fit_priors:
        label: ngmix joint prior
        rationale: >-
          [HARDCODED] ellipticity GPriorBA with sigma 0.4; flat T and F
          priors with negative support (the bounds decide which noisy fits
          survive and which rail); a centroid prior one pixel wide, where
          each object's pixel is sqrt|det J| of its epochs' WCS Jacobians,
          averaged over those epochs. The PSF fitter is built with the same
          joint prior. Prior width drives noise
          bias; no rationale recorded. Guinot+22 used a wider flux prior and a
          flat prior on r50 rather than T.
          Values:
          get_prior.T_range = -1,1e3;
          get_prior.F_range = -100,1e9;
          NGMIX_RUNNER.PIXEL_SCALE = absent.
        default: gpriorba04_flat
        options:
          gpriorba04_flat:
            label: GPriorBA 0.4 + flat T/F with negative support
            insights: [guinot22_ngmix_priors]
          nonneg_informative:
            label: Non-negative or informative T/F priors
            excluded: true
            excluded_reason: >-
              Truncating negative support biases the noshear ensemble mean;
              metacal wants a symmetric noise response.
      metacal_scheme:
        label: Metacalibration protocol
        rationale: >-
          [HARDCODED] the five metacal types and step, fixnoise with the noise
          image, and ignore_failed_psf (an epoch whose PSF fit fails is
          dropped). The reconvolution kernel, METACAL_PSF, defaults to
          fitgauss and is unset in the committed config; it moves the response
          directly. No sheared-PSF types run, so the catalogue has no PSF
          response term.
          Values:
          METACAL_TYPES = noshear,1p,1m,2p,2m;
          do_ngmix_metacal.metacal_pars[types] = noshear,1p,1m,2p,2m;
          do_ngmix_metacal.metacal_pars[step] = 0.01;
          do_ngmix_metacal.metacal_pars[fixnoise] = True;
          do_ngmix_metacal.metacal_pars[use_noise_image] = True;
          NGMIX_RUNNER.METACAL_PSF = absent.
        default: five_types_step001_fitgauss
        options:
          five_types_step001_fitgauss:
            label: noshear+1p/1m/2p/2m, step 0.01, fitgauss, fixnoise
            insights: [guinot22_metacal_five_images]
          with_psf_response:
            label: Add sheared-PSF types for R_psf
            description: Not implemented.
      centroid_source:
        label: Jacobian origin at the coadd centroid
        rationale: >-
          The Jacobian origin, where the centroid prior centres, is the
          sub-pixel offset the stamp extractor stored when cutting the stamp
          ("wcs", the default at every level; the committed config sets no
          CENTROID_SOURCE). Extraction and prior share one projection and one
          rounding, so they cannot disagree near a rounding tie.
          "hsm" re-centres on adaptive moments measured from the stamp.
          Values:
          NGMIX_RUNNER.CENTROID_SOURCE = absent.
        default: wcs
        options:
          wcs:
            label: Coadd-centroid offset from the stamp extractor
          hsm:
            label: HSM adaptive-moment centroid
            excluded: true
            excluded_reason: >-
              Noisy, notably for stars, and follows the light rather than the
              astrometry that placed the stamp.
      epoch_flux_rescaling:
        label: Epochs put on a common flux scale by header FSCALE
        rationale: >-
          [HARDCODED] each epoch's image is multiplied by its header FSCALE and
          its weight divided by FSCALE squared (background RMS scaled with the
          image) before the joint fit, so the epochs share the tile's
          zero-point.
        default: fscale
        options:
          fscale:
            label: Rescale by header FSCALE
      psf_epoch_averaging:
        label: Catalogue PSF quantities averaged over epochs by galaxy weight
        rationale: >-
          [HARDCODED] the catalogue PSF shape and size (original image PSF
          and metacal reconvolution kernel) are epoch averages weighted by each
          epoch's summed galaxy inverse variance; epochs whose PSF fit failed
          are left out. These columns feed downstream PSF-leakage estimates.
        default: galaxy_weight_sum
        options:
          galaxy_weight_sum:
            label: Weighted by summed galaxy inverse variance
      galaxy_pixel_weights:
        label: Per-pixel inverse variance from background-RMS vignets
        rationale: >-
          With BKG_RMS_VIGNET_PATH set, each pixel's weight is 1/rms^2 from the
          SExtractor background-RMS map (all or nothing; a missing file
          raises), and the noise realisations use the same per-pixel RMS; the
          fallback is a scalar 1/sigma_mad^2. Each epoch is
          background-subtracted with the SExtractor background vignet
          (BKG_SUB, on unless set False).
          Values:
          NGMIX_RUNNER.BKG_RMS_VIGNET_PATH =
          $NGMIX_VIGNET_DIR/vignetmaker_runner_run_2/output/background_rms_vignet{file_number_string}.sqlite;
          NGMIX_RUNNER.BKG_SUB = absent.
        default: rms_vignet_weights
        options:
          rms_vignet_weights:
            label: Per-pixel background-RMS weights
          scalar_sigma_mad:
            label: Scalar sigma_mad per epoch
            excluded: true
            excluded_reason: Mis-reports errors wherever the RMS map varies.
      psf_likelihood_noise:
        label: Flat PSF-observation weight
        rationale: >-
          [HARDCODED] each PSF observation carries a flat weight
          1/PSF_NOISE^2. Without it, ngmix's unit weight lets the GPriorBA(0.4)
          prior swamp the original-PSF prefit and pull the *_psf_orig
          ellipticity to zero; the metacal shears are unchanged
          (test_psf_weight_leaves_metacal_shear_invariant). The recovered PSF
          shape and size are flat for PSF_NOISE from 1e-4 to 1e-6 on the
          digital twin.
          Values:
          PSF_NOISE = 1e-5.
        default: psf_noise_1em5
        options:
          psf_noise_1em5:
            label: PSF_NOISE 1e-5
      megacam_ccd_flip:
        label: 180-degree rotation of tile vignets for CCDs below 18 and 36-37
        rationale: >-
          [HARDCODED] "MegaPipe has CCDs that are upside down": the tile vignet
          and segmentation stamp are rotated to register with the epoch stamp.
          A wrong flip mis-registers the tile coverage flag against the epoch,
          changing flagged pixels and the masked-fraction cut. The docstring
          warns it is wrong for THELI CCDs.
        default: megapipe_flip
        options:
          megapipe_flip:
            label: Flip CCDs < 18 and 36/37 (MegaPipe orientation)
      defect_fill:
        label: Image content of defect pixels before metacal
        rationale: >-
          A defect pixel (nonzero instrument flag, zero exposure weight or
          invalid background RMS) gets weight 0 in prepare_ngmix_weights. Its
          image value still matters: ngmix's metacal deconvolves, shears and
          reconvolves an InterpolatedImage of the whole image and copies the
          weights through, so a zero-weight pixel's content spreads into the
          weighted pixels within about a PSF width. DES's ngmixer fills for
          that reason: "it may be important for codes that take moments or use
          FFTs". In the committed default (`BLEND_HANDLING = noisefill`),
          masked pixels are replaced with independent noise at the per-pixel
          background RMS when supplied, or the stamp's robust noise scale
          otherwise. With `BLEND_HANDLING = uberseg`, the image is left
          untouched and weights are zeroed on defects and neighbour-side
          pixels. [LINT] the prepare_ngmix_weights docstring says noisefill
          keeps the weight of filled pixels (the code zeroes it), and the
          ngmix_runner comment says noisefill fills neighbour pixels (it fills
          flagged pixels and leaves neighbours untouched). The committed fill
          uses the unsymmetrized defect set: DES symmetrized its masks, but
          four-fold symmetrization quadruples m and still leaves an additive c1
          (symmetrized_4fold_noise). The cost of not symmetrizing, a hole in
          the galaxy light, is bounded by central_defect_veto. Noise stays the
          default until a survey A/B against interpolation.
          Values:
          VIGNETMAKER_RUNNER_RUN_1.MASKING = False;
          VIGNETMAKER_RUNNER_RUN_2.MASKING = False.
        default: noise
        options:
          noise:
            label: Independent noise on the unsymmetrized defect set
            description: >-
              Masked pixels get independent noise using the per-pixel
              background RMS when supplied, or the stamp's robust noise scale
              otherwise; their inverse-variance weights stay zero. Metacal's
              fixnoise noise image covers every pixel.
            insights: [mask_metacal_acts_on_whole_stamp, mask_bad_column_symmetrize]
          interpolate:
            label: Interpolate short bounded runs; noise-fill the rest
            description: >-
              Not implemented. PR #916 measures c1 = -1.3e-3 for a column
              8 px from a 0.5 arcsec galaxy and +2e-6 when the weight is also
              zeroed on its quarter-turn orbit. For a 3-px bleed 6 px from that
              galaxy, PR #916 measures m11 = +0.89% when the fill is
              symmetrized, against +0.19% otherwise.
            insights: [mask_interpolate_with_noise, mask_sharp_edges_ring]
          symmetrized_4fold_noise:
            label: Four-fold-symmetrized defect set (M | rot90 | rot180 | rot270), then noise fill
            description: Not implemented.
            excluded: true
            excluded_reason: >-
              PR #915 measures m11 = -2.7% with four-fold symmetrization
              against -0.64% without it for a 3-px bleed 10 px from a galaxy
              with 0.5 arcsec half-light radius and a 0.7 arcsec PSF. Through
              a PSF with ellipticity (0.05, 0.02), it measures c1 = 7.3e-4
              with symmetrization against 0.7e-4 without it.
            insights: [mask_bad_column_symmetrize, mask_des_defect_practice, mask_fixed_orientation]
          raw:
            label: "No fill: raw defect values (BLEND_HANDLING = uberseg)"
            description: >-
              With BLEND_HANDLING = uberseg, defect pixels keep weight 0 but
              their image values stay untouched in the image that metacal
              deconvolves, shears and reconvolves.
            excluded: true
            excluded_reason: >-
              Metacal acts on every pixel regardless of weight, so raw defects
              leak into the weighted pixels. No published metacal pipeline does
              this: DES dropped, model-filled or interpolated defects.
            insights: [mask_metacal_acts_on_whole_stamp, mask_des_defect_practice]
      blend_handling:
        label: Neighbour treatment before metacal
        rationale: >-
          Covers only pixels shared with a neighbour; defect_fill is coupled to
          it through BLEND_HANDLING. noisefill (the default; the committed
          config sets no key) leaves neighbours fully weighted and untouched.
          uberseg zeroes the weight of pixels nearer a neighbour's coadd
          segmentation footprint than the target's (DILATE_NEIGHBOUR, default 1)
          and leaves the image untouched, as official uberseg does:
          esheldon/meds get_uberseg returns a weight map (a
          nearest-segment-pixel Voronoi split). DES Y1's fiducial metacal and
          the last Y3 config ran on uberseg-weighted stamps with raw neighbour
          light (an earlier Y3 config subtracted MOF neighbours first); Y6 masks
          no neighbours before the shear step, using uberseg only as the fit
          weight after metacal. No DES or Rubin metacal/metadetect pipeline
          noise-fills the neighbour side. Raw neighbour light is consistent with
          metacal: it is real sky, and the artificial shear shears it with the
          target, as the real shear does; the reconvolution spreads it slightly
          further across the Voronoi boundary than the PSF already had. Known
          residuals: about +2% m for uberseg-only relative to MOF subtraction in DES Y1
          simulations, and a blending bias dominated by shear-dependent
          detection, which no pixel treatment fixes and which DES Y3 calibrated
          with simulations (mask_des_y1_uberseg_only,
          mask_blend_bias_detection). Noise-filling the neighbour side would
          instead cut the target's own light along an unsheared boundary, a
          sharp edge that rings in the FFTs. The recommended comparison arm is
          uberseg (weight-only), with defect_fill held equal across arms.
        default: none
        options:
          none:
            label: No neighbour treatment (BLEND_HANDLING = noisefill)
            description: >-
              Neighbour pixels keep their full weight and image values, so the
              fit sees all neighbour light, which biases shapes toward
              neighbours (Jarvis et al. 2016); this masks less than even their
              plain segmentation map. A candidate cause of the FLAGS=2 B-modes
              investigated in #814.
            insights: [mask_uberseg_neighbour_bias]
          uberseg:
            label: UberSeg, weight-only
            description: >-
              As in DES Y1/Y3. Needs the coadd segmentation stamp
              (SEG_VIGNET_PATH). DILATE_NEIGHBOUR absorbs the coadd-vs-epoch
              overlay offset: ShapePipe reuses one coadd seg stamp for every
              epoch where MEDS reprojects it.
            insights: [mask_uberseg_weight_only, mask_uberseg_neighbour_bias, mask_des_y1_uberseg_only, mask_blend_bias_detection]
          uberseg_fill:
            label: UberSeg plus noise fill of neighbour-side pixels
            description: >-
              Not implemented. Also replace neighbour-side pixels with noise,
              removing neighbour light from metacal at the cost of a sharp,
              unsheared edge through the target's wings (FFT ringing). No
              published pipeline does this; useful only as a diagnostic arm,
              where a smooth (apodized) taper would do less damage.
            insights: [mask_sharp_edges_ring, mask_blend_bias_detection]
          mof_subtract:
            label: Subtract neighbour models, then UberSeg (DES Y1 alternative)
            description: >-
              Not implemented. Subtract multi-object-fit models of the
              neighbours before metacal and keep uberseg weights. This removed
              the ~2% uberseg-only bias in DES Y1 simulations, although Sheldon
              et al. 2020 found similar blend biases with and without MOF.
            insights: [mask_des_y1_uberseg_only, mask_blend_bias_detection]
      central_defect_veto:
        label: Per-epoch veto on a defect near the stamp centre
        rationale: >-
          Not on develop; implemented on feat/defect-fill-veto (7777181b).
          There an epoch is dropped when a defect pixel lies strictly closer
          to the stamp centre than its fill's radius, beside the
          masked-fraction cut in the epoch loop. The veto reads only the
          defect mask, so it selects on nothing shear-responsive; for the same
          reason the radii are fixed rather than scaled by galaxy size: 10 px
          for noise-filled pixels (EPOCH_CENTRAL_DEFECT_RADIUS) and 7 px for
          interpolated ones (EPOCH_INTERPOLATED_DEFECT_RADIUS,
          feat/defect-interpolation). Calibrated on 51-px stamps at known RMS
          and high S/N, through a round and a (0.05, 0.02) elliptical 0.7
          arcsec PSF, on galaxies of half-light radius 0.3 and 0.5 arcsec, and
          for interpolation also 0.7 and 0.9 arcsec. Known limits: at 10 px,
          wide defects through the elliptical PSF sit at the 1% bound (m11 =
          -0.98%; -0.24% at 11 px), and noise fill needs 14 px for 0.7 and 0.9
          arcsec galaxies. Measured on feat/defect-fill-veto and
          feat/defect-interpolation.
        default: disabled
        options:
          disabled:
            label: No central veto (committed code)
          fixed_radii:
            label: Fixed radii, 10 px for noise-filled and 7 px for interpolated defects
            description: >-
              Implemented on feat/defect-fill-veto and feat/defect-interpolation,
              not on develop.
          size_scaled_radius:
            label: Veto radius scaled by galaxy size
            excluded: true
            excluded_reason: >-
              Galaxy size responds to shear, so a size-scaled veto selects on
              a shear-responsive quantity.
      epoch_masked_fraction_cut:
        label: Per-epoch masked-fraction cut
        rationale: >-
          [HARDCODED] on develop, an epoch whose stamp has more than 1/3 of its
          pixels flagged (any nonzero flag bit, including the tile-coverage bit
          2**10 set where the tile vignet is off-image) is dropped from the
          multi-epoch fit; an object with no surviving epoch has no shape.
          Zero-weight and invalid-RMS pixels are not counted. On
          feat/defect-fill-veto the cut counts the raw, unsymmetrized defect
          set (flagged, zero-weight and invalid-RMS pixels) against
          EPOCH_MASKED_FRACTION_CUT, default 1/3. Before the cut, an epoch is
          dropped silently if its galaxy stamp is all zeros or its
          background-subtracted sigma_mad is not positive. DES was stricter:
          Y1 rejected any epoch with a masked or zero-weight pixel, or with
          its central 4-pixel region masked; Y3 cut at 10% raw zero-weight
          pixels; Y6 drops images more than 10% missing and keeps objects at
          mfrac < 0.1 (mask_multi_epoch_drop). UNIONS has fewer epochs than
          DES, so the cost in effective number density has to be measured,
          not assumed. A defect near the centre is handled separately
          (central_defect_veto).
        default: one_third
        options:
          one_third:
            label: 1/3 of the stamp in the defect set
            description: >-
              Drop an epoch only if more than 1/3 of the stamp pixels are
              defects (on develop, flagged pixels).
          ten_percent:
            label: 10% (DES Y3 / Y6)
            description: >-
              Not a default; EPOCH_MASKED_FRACTION_CUT = 0.1 on
              feat/defect-fill-veto. Matches DES Y3 max_zero_weight_frac and
              Y6 max_masked_fraction.
            insights: [mask_multi_epoch_drop]
          any_masked:
            label: Any masked pixel (DES Y1)
            description: >-
              Not implemented. Drop an epoch with any masked stamp pixel, so no
              fill is ever needed. DES could afford this with about 10 epochs
              per band; UNIONS likely cannot.
            insights: [mask_multi_epoch_drop, mask_des_defect_practice]
    prior_insights:
      guinot22_ngmix_priors:
        claim: >-
          The published ngmix fit uses a Gaussian centroid prior of width one
          pixel scale, a flat prior on the half-light radius r50 in [-10, 1e6]
          arcsec and a flat flux prior in [-1e4, 1e9].
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_cen_prior
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'Gaussian distribution centred on 0 with σ = pixel scale'
            location: { page: 7 }
          - id: ev_guinot22_r50_prior
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'r50: flat distribution in'
            location: { page: 7 }
          - id: ev_guinot22_flux_prior
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'F: flat distribution in'
            location: { page: 7 }
      guinot22_hsm_initialisation:
        claim: >-
          The published fit is initialised from adaptive moments measured on
          each sheared version of the object.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_hsm_init
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'We thus run first an adaptive moments algorithm to initialise the least-square operation.'
            location: { page: 7 }
      guinot22_gaussian_model:
        claim: >-
          The published shape measurement models galaxies with a single
          Gaussian profile, arguing the resulting model bias is small and
          largely absorbed by metacalibration.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_gaussian
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'Despite being very simple, the model bias (Kacprzak et al.'
              suffix: ' 2014) is small.'
            location: { page: 7 }
      guinot22_metacal_five_images:
        claim: >-
          Metacalibration in the published analysis creates four sheared
          images for calibration plus one for measurement, with a shear step
          of 0.01 and a 90-degree-rotated noise image to cancel noise
          correlations.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_metacal
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'This method creates four images used for the calibration, and one for the measurement.'
            location: { page: 6 }
      mask_uberseg_weight_only:
        label: UberSeg is a weight-map operation (Jarvis 2016)
        claim: >-
          UberSeg (DES SV) zeroes the WEIGHT of pixels in another object's
          coadd segmentation footprint or closer to another object than to the
          target; MEDS weights are also zeroed wherever a mask flag is set.
          The image is not modified: in the forward-model fits it was designed
          for, a zero-weight pixel drops out of the likelihood exactly.
          esheldon/meds get_uberseg, which returns a weight map, matches.
        created_at: "2026-09-26T00:00:00Z"
        derived: true
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.1507.05603
            version: 3
            quote:
              exact: We then set pixels in the weight map to zero if they were either associated with other objects in the segmentation map or were closer to any other object than to the object of interest.
          - id: ev2
            doi: 10.48550/arXiv.1507.05603
            version: 3
            quote:
              exact: are both of the model-fitting variety
      mask_uberseg_neighbour_bias:
        label: Neighbour light biases shapes toward neighbours (Jarvis 2016)
        claim: >-
          With a plain segmentation-map mask, light from a bright neighbour
          just outside its footprint biased the shape toward the neighbour;
          UberSeg made that bias undetectable in end-to-end simulations.
          ShapePipe's default (no neighbour treatment) masks less than even
          the plain segmentation map.
        created_at: "2026-09-26T00:00:00Z"
        derived: false
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.1507.05603
            version: 3
            quote:
              exact: when using ordinary segmentation maps we found a significant bias of the galaxy shape in the direction toward neighbors.
          - id: ev2
            doi: 10.48550/arXiv.1507.05603
            version: 3
            quote:
              exact: we no longer detected any systematic bias in the shape estimates due to unmasked flux from neighboring objects
      mask_metacal_acts_on_whole_stamp:
        label: Metacal transforms every stamp pixel, weights unseen
        claim: >-
          Metacal builds an interpolated image of the whole postage stamp,
          then deconvolves, shears and reconvolves it; its Fourier transforms
          cannot accommodate missing data, so zero-weight pixels' image values
          feed the sheared images. The method papers leave masking
          unaddressed. ngmix does exactly this: an InterpolatedImage of
          obs.image, weights copied through.
        created_at: "2026-09-26T00:00:00Z"
        derived: true
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.1702.02600
            version: 1
            quote:
              exact: For each galaxy and PSF postage stamp, we first create an
          - id: ev2
            doi: 10.48550/arXiv.1702.02600
            version: 1
            quote:
              exact: We have made no attempt to deal with the effects of masked pixels or blending
          - id: ev3
            doi: 10.48550/arXiv.1702.02601
            version: 2
            quote:
              exact: convolutions cannot accommodate missing data
      mask_bad_column_symmetrize:
        label: Filled bad columns give additive e1; symmetrize the mask
        claim: >-
          Sheldon & Huff 2017 found that filling bad columns (with the
          best-fit model, not even noise) gave a large additive e1 bias and a
          few-per-mille multiplicative bias, both removed by adding a
          compensating column rotated 90 degrees about the stamp centre.
          Sheldon et al. 2020 recommend the same compensating mask; DES Y6,
          like previous DES pipelines, OR-s each bad-pixel mask with its
          90-degree rotation to cancel additive biases.
        created_at: "2026-09-26T00:00:00Z"
        derived: true
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.1702.02601
            version: 2
            quote:
              exact: as well as a multiplicative bias of a few parts in a thousand
          - id: ev2
            doi: 10.48550/arXiv.1702.02601
            version: 2
            quote:
              exact: at 90 degree rotation about the center of the image, restoring symmetry to the image
          - id: ev3
            doi: 10.48550/arXiv.1911.02505
            version: 2
            quote:
              exact: An additional compensating mask, rotated at right angles to the real mask, can be used to restore symmetry to the image
          - id: ev4
            doi: 10.48550/arXiv.2501.05665
            version: 2
            quote:
              exact: we rotate the bad pixel mask by 90 degrees and apply it via a logical
          - id: ev5
            doi: 10.48550/arXiv.2501.05665
            version: 2
            quote:
              exact: This process helps to cancel additive biases in the final shape measurement.
      mask_des_defect_practice:
        label: DES never passed raw defects through metacal
        claim: >-
          Every DES metacal generation removed defects from the image before
          metacal: Y1 dropped any epoch with a masked pixel, Y3 filled
          symmetrized defects with the best-fit central model (ngmix-y1-config
          / ngmix-y3-config), and Y6
          interpolated symmetrized defects following "previous DES shear
          measurement pipelines". None left raw defect values in a zero-weight
          pixel, as ShapePipe's uberseg path does.
        created_at: "2026-09-26T00:00:00Z"
        derived: true
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.2501.05665
            version: 2
            quote:
              exact: Following previous DES shear measurement pipelines
          - id: ev2
            doi: 10.48550/arXiv.2501.05665
            version: 2
            quote:
              exact: Then, we apply a two-dimensional Clough-Tocher interpolation
      mask_interpolate_with_noise:
        label: Interpolate defects, and the noise image identically
        claim: >-
          The metadetect papers interpolate masked regions (bad columns,
          cosmic rays, saturation) before the shear step, taking care not to
          create a spurious shear, and pass the noise image used for metacal's
          correlated-noise correction through the same interpolation.
        created_at: "2026-09-26T00:00:00Z"
        derived: true
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.1911.02505
            version: 2
            quote:
              exact: The FFT does not permit missing data, so the masked regions must be interpolated in some way.
          - id: ev2
            doi: 10.48550/arXiv.1911.02505
            version: 2
            quote:
              exact: so the noise field used for correcting correlated noise effects must also be propagated through the same coadding and interpolation
          - id: ev3
            doi: 10.48550/arXiv.2303.03947
            version: 2
            quote:
              exact: Before warping, we interpolated the simulated artifacts in each image such as cosmic rays, bad columns, and saturated pixels.
          - id: ev4
            doi: 10.48550/arXiv.2303.03947
            version: 2
            quote:
              exact: we must also run the noise image through the same procedures
      mask_sharp_edges_ring:
        label: Sharp mask edges ring through metacal; apodize large masks
        claim: >-
          Sharp mask edges ring in the metacal FFTs, so bright-star regions
          are zeroed with an apodized (smoothly tapered) edge, and the noise
          image gets the same masking. The same reasoning argues against a
          hard noise fill cut along a neighbour's Voronoi boundary.
        created_at: "2026-09-26T00:00:00Z"
        derived: true
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.2303.03947
            version: 2
            quote:
              exact: Masks with sharp features can cause ringing in the FFTs used by the
          - id: ev2
            doi: 10.48550/arXiv.2501.05665
            version: 2
            quote:
              exact: we further mask bright stars with apodization to avoid FFT artifacts during the deconvolution, shearing, and reconvolution processes
          - id: ev3
            doi: 10.48550/arXiv.2501.05665
            version: 2
            quote:
              exact: we also applied the same masking and apodization to the
      mask_multi_epoch_drop:
        label: Drop heavily masked epochs (10% in DES)
        claim: >-
          With many dithered epochs, problematic data can simply be dropped
          (Sheldon & Huff 2017). DES Y6 drops input images more than 10%
          missing and cuts objects at masked fraction mfrac < 0.1, which its
          image simulations show avoids shear calibration bias. ShapePipe's
          cut is 1/3.
        created_at: "2026-09-26T00:00:00Z"
        derived: true
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.1702.02601
            version: 2
            quote:
              exact: data deemed problematic can simply be left out of the fit
          - id: ev2
            doi: 10.48550/arXiv.2501.05665
            version: 2
            quote:
              exact: Images with a missing pixel fraction higher than 10
          - id: ev3
            doi: 10.48550/arXiv.2501.05665
            version: 2
            quote:
              exact: is sufficient not to introduce shear calibration biases
      mask_des_y1_uberseg_only:
        label: 'DES Y1 metacal: uberseg-only fiducial, ~2% m residual'
        claim: >-
          DES Y1 metacal's fiducial catalogue handled neighbours with uberseg
          only (raw neighbour light in the image). In dense deblending
          simulations it carried m = 2.18 +/- 0.16 per cent at S/N > 10,
          removed by subtracting MOF models of the neighbours; in data the
          relative uberseg-vs-MOF m was 0.023 +/- 0.009.
        created_at: "2026-09-26T00:00:00Z"
        derived: false
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.1708.01533
            version: 2
            quote:
              exact: masks pixels close to neighbours rather than assigning a fraction of the light in each pixel to them
          - id: ev2
            doi: 10.48550/arXiv.1708.01533
            version: 2
            quote:
              exact: We detect no bias when subtracting the light from neighbours.
      mask_blend_bias_detection:
        label: Stamp-metacal blend bias is mostly shear-dependent detection
        claim: >-
          Sheldon et al. 2020 find that per-stamp metacal's few-percent
          blending bias comes from shear-dependent detection, not blended
          light: it is similar with and without neighbour subtraction, and
          metacal shears the space between objects coherently. DES Y3 kept Y1's
          approach (Gatti et al. list no change to neighbour or mask handling)
          and calibrated the resulting 2-3% with image simulations.
        created_at: "2026-09-26T00:00:00Z"
        derived: true
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.1911.02505
            version: 2
            quote:
              exact: We show that this bias is not due to blending itself, but rather to shear-dependent object detection.
          - id: ev2
            doi: 10.48550/arXiv.1911.02505
            version: 2
            quote:
              exact: we see similar biases when no deblending is performed
          - id: ev3
            doi: 10.48550/arXiv.1911.02505
            version: 2
            quote:
              exact: the space between objects is sheared coherently
          - id: ev4
            doi: 10.48550/arXiv.2011.03408
            version: 3
            quote:
              exact: shape catalogue differs from DES Y1 in the following ways
          - id: ev5
            doi: 10.48550/arXiv.2011.03408
            version: 3
            quote:
              exact: which affects the shear estimates when objects are blended
      mask_fixed_orientation:
        label: Fixed-orientation cameras make mask anisotropy coherent
        claim: >-
          Masks have a preferred direction (columns, bleeds, spikes). On a
          camera with a fixed sky orientation (DECam, and CFHT/MegaCam on its
          equatorial mount), mask-induced shape errors add up coherently;
          with Rubin's camera rotation, unmasked trails averaged away. DES Y3 has
          an unexplained mean e1 of 3.5e-4 that its simulations, which include
          the real bad-pixel masks, do not reproduce; whether masks cause it is
          not established.
        created_at: "2026-09-26T00:00:00Z"
        derived: true
        evidence:
          - id: ev1
            doi: 10.48550/arXiv.1708.01533
            version: 2
            quote:
              exact: The DES focal plane does not rotate, so these effects always correspond to the same orientation in sky coordinates.
          - id: ev2
            doi: 10.48550/arXiv.2303.03947
            version: 2
            quote:
              exact: We find that these unmasked trails do not cause a shear bias, which we attribute to the camera rotations that randomize the direction of the trail on the sky.
          - id: ev3
            doi: 10.48550/arXiv.2011.03408
            version: 3
            quote:
              exact: Our shear catalogue is characterized by a non-null mean shear in one of the two components, whose origin is unknown.

  # ═════════════════════════════════════════════════════════════════════════
  catalogue_assembly:
    description: >-
      The per-tile science catalogue: shape chunks merged and joined to the
      detection catalogue and PSF quantities.
    inputs:
      - id: ngmix_chunks
        type: data
        source: per-tile ngmix catalogue chunks + tile sexcat + PSF diagnostics
    outputs:
      - id: tile_final_cat
        type: data
        format: fits
        description: The assembled per-tile science catalogue (final_cat family).
        inputs: [ngmix_chunks]
        decisions:
          [star_galaxy_classification, tile_overlap_handling,
           failure_sentinels]
    decisions:
      star_galaxy_classification:
        label: Star/galaxy separation deferred out of the pipeline
        rationale: >-
          [HARDCODED] ShapePipe does no star/galaxy classification: make_cat
          keeps every detection, and the catalogue carries no classification
          column. Separation happens downstream, in sp_validation's size-based
          selection. The spread-model classifier Guinot+22 used
          (guinot22_spread_model_cut) is removed from the pipeline.
        default: deferred_downstream
        options:
          deferred_downstream:
            label: No in-pipeline classification; catalogue ships all objects
          spread_model_inline:
            label: spread_model classification in make_cat
            insights: [guinot22_spread_model_cut]
            excluded: true
            excluded_reason: >-
              It did not work reliably when last run, and it separates stars
              from galaxies worse than the downstream classification.
      tile_overlap_handling:
        label: Tile-overlap duplicates neither removed nor flagged
        rationale: >-
          Adjacent tiles overlap, and the UNIONS per-tile catalogue spans the
          whole tile image, so an object in an overlap is measured once in
          each tile. [HARDCODED] make_cat keeps every row and adds TILE_ID and
          TILE_UNIQUE_ID (tile_id * 10**6 + NUMBER). The second is unique per
          tile row, so an overlap object gets a different ID in each tile. No
          config key deduplicates or flags overlap rows. sp_validation removes
          the duplicates by keeping each object only inside its own tile's
          nominal box (classification_galaxy_overlap_ra_dec, from TILE_ID and
          XWIN_WORLD/YWIN_WORLD).
        default: no_dedup_in_pipeline
        options:
          no_dedup_in_pipeline:
            label: >-
              Ship overlap duplicates; sp_validation cuts each tile to its
              nominal box
          overlap_flagging:
            label: Flag overlap objects in the catalogue
            description: Not implemented.
          nearest_tile_centre:
            label: Keep each object only in its nearest tile
            excluded: true
            excluded_reason: >-
              Not needed in the pipeline: the cut needs only each object's
              TILE_ID and position, and sp_validation applies it downstream.
              Leaving it out keeps every tile-catalogue row in the shape
              catalogue.
      failure_sentinels:
        label: Objects without shape measurements kept, with sentinel values
        rationale: >-
          [HARDCODED] detections with no ngmix row (no surviving epoch, or an
          exception during the fit) stay in the catalogue with sentinels:
          sizes, fluxes and magnitudes 0, flux and magnitude errors -1,
          ellipticities and their errors -10, size errors 1e30,
          NGMIX_N_EPOCH 0. Their flag columns carry ngmix's own
          LM_FUNC_NOTFINITE bit (2**12; FLAGS_<SHEAR> and NGMIX_MCAL_FLAGS)
          and NGMIX_MCAL_TYPES_FAIL is 5, as for a metacal type that reports
          success without a finite shear, so the MCAL_FLAGS == 0 and
          MCAL_TYPES_FAIL == 0 cut rejects them; NGMIX_N_EPOCH == 0 tells
          them apart from fitted objects.
        default: sentinel_values
        options:
          sentinel_values:
            label: Keep with sentinels (flags LM_FUNC_NOTFINITE, e -10, T_ERR 1e30)
          drop_unmatched:
            label: Drop objects without shapes
            excluded: true
            excluded_reason: >-
              Loses the photometry-only population and hides attrition.
    prior_insights:
      guinot22_spread_model_cut:
        claim: >-
          The published analysis selects galaxies inside the pipeline with the
          spread model, at s + 2 sigma_s > 0.0003 together with s > 0 and
          20 < MAG_AUTO < 26.
        created_at: "2022-04-01T00:00:00Z"
        evidence:
          - id: ev_guinot22_spread_model
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 'We then use the spread model to make the selection presented below:'
            location: { page: 5 }
          - id: ev_guinot22_spread_model_cut
            doi: "10.48550/arXiv.2204.04798"
            quote:
              exact: 's + 2σs > 0.0003'
            location: { page: 5 }