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PSF star selection needs a deep-sky-object veto: M33, M101, M13 and M92 enter the PSF star sample

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Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
45/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Activo
Stack tecnológico
python
Área
data

Línea de trabajo

Read the PSF star selection configuration around masking.psf_star_mask_veto, [MASK:star_selection], and MASK_PATHS; the issue also points to sp_validation as the run-1 entry point for filtering stars before ρ/τ calculations. First trace how MASK_EXT is queried and how the bit-8 map is loaded. Done means future PSFEx fits exclude bit-8 regions and run-1 ρ/τ calculations veto those stars, with M33 covered to at least 0.7° for stars and galaxies.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Summary

Run-1 PSF star selection cuts only on instrument flags (masking.psf_star_mask_veto = instrument_flags_only), so stars, clusters and HII regions in M33, M101, M13 and M92 enter the PSF star sample. M33 alone dominates the first-bin ρ statistics and flips the sign of the fitted leakage coefficients. The galaxy catalogue is protected by the UNIONS manual mask (MASK_8_Manual), except around M33, where the manual ellipse is too small.

The current default was a deliberate measure-first choice. In #847 (43c32cde), star selection "starts from outlier rejection alone, and MASK_EXT is the configurable pickup if that proves insufficient — write the column, measure the effect, impose it only if the measurement says to." This issue is that measurement. Before #846 (eb93838a), ShapePipe's internal mask module wrote Messier/NGC discs into the exposure flags, so these regions were vetoed through IMAFLAGS_ISO. Every run-1 campaign commit descends from eb93838a. The NGC/Messier corner-overlap fix (#827) is unrelated, because no run-1 campaign ran that code.

Measurement

Run-1 products: PSF stars from the compact sample that reproduces production ρ/τ; galaxies from the shape-selected catalogue; the bit-8 footprint from the DR6 healsparse mask.

object PSF stars inside MASK_8_Manual peak PSF-star overdensity peak rms |e★ − e_PSF| vs field
M33 9,162 ×11 ×14
M13 3,241 ×15 ×1.4
M92 1,890 ×11 ×2.9
M101 437 ×1.4 ×1.6
  • Survey-wide, 51,663 of 10,380,311 PSF stars (0.5%) lie inside bit 8.
  • For M101, M13 and M92, the whole excess in star residuals lies inside bit 8. The star overdensity outside it (to ~0.3° for M13 and M92) has normal residuals; these look like genuine cluster members and usable PSF stars.
  • M33 is the exception. Its manual-mask ellipse is ≈0.55° × 0.40°, but the star residual rms stays 1.2–1.7× the field value out to ≈0.6°, and kept galaxies are 1.25–1.35× overdense at 0.5–0.65°. Both flatten beyond ≈0.7°.
  • In the cosmic-shear leakage fit, the M33 cell carries >99% of the first-bin ρ1/ρ3/ρ4 variance and drives the leakage coefficients α and η negative. Masking it moves the mean ρ4 over 20 angular bins from −7.6e-8 to −2.4e-8.

PSF stars and galaxies around the four objects, with the manual mask, the old ShapePipe Messier radius and the star-contamination circle

Radial profiles: manual-mask coverage, star and galaxy overdensity, star residual rms

Proposed fix

  1. Future runs (ShapePipe): add a DSO option to masking.psf_star_mask_veto. Point mask_query MASK_PATHS at the bit-8 (manual) map and add MASK_EXT == 0 to [MASK:star_selection]. This keeps these sources out of the PSFEx fit itself, which a downstream veto cannot do. The star-body map named for star_body_veto is no substitute: it flags 12.3% of PSF stars and is the wrong map for extended objects.
  2. Run 1, no rerun (sp_validation): before computing ρ/τ, drop PSF stars that fall inside the bit-8 map (0.5% of stars).
  3. M33 for both stars and galaxies: veto a circle of at least 0.7°, or have the manual mask's M33 footprint enlarged.

Star residuals are flat beyond the masked regions, which suggests PSF models away from these objects are not visibly affected. That is why no rerun seems necessary.

Claude on behalf of Cail

Lenguaje dominante
Python
Estrellas
18
Forks
14
Merge medio
4 d 16 h
PR fusionados (30 d)
35

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