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Tentative: PSFEx models ~0.4% larger than stars on P3 tiles (worth exploring)

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python
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data

Hướng nghiên cứu

The payload points at the PSFEx model evaluation and the star catalogue full_starcat_smk-p3a.hdf5 for tiles 242.293, 243.293, 242.294 and 243.294; the open question is whether the PSF size residual of about -0.4% holds beyond these four tiles. Start by checking how positions are passed to PSFEx (the 0-based versus 1-based pixel frame) and by re-measuring with the PSFEx-fit stars held out. Done means a reproduced residual with a stated cause, or a documented null result, and this needs the PSF modelling maintainers.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.

Mô tả

Tentative, and worth exploring. This is a first look on a small area, not a systematic study.

Summary

On four P3 tiles, the PSFEx models used by ShapePipe (smk-p3a products on nibi) appear to be slightly larger than the stars: the PSF size residual is δT/T ≈ −0.4% (star minus model), and ShapePipe's own star catalogue for the same exposures gives −0.58%. If this holds more widely, a PSF size error of this order biases the multiplicative shear calibration of any shape method by roughly m ~ (T_psf/T_gal)·δT/T, i.e. a few × 10⁻³ for typical UNIONS galaxies, which is above the ~0.1–0.2% PSF-size accuracy usually needed for |m| ≲ 2 × 10⁻³.

What was measured

  • Data: tiles 242.293, 243.293, 242.294, 243.294 (17 exposures, ~1 deg²), with the persisted PSFEx models (PSF_SAMP = 1, degree 2, median 44 accepted stars per CCD) evaluated at each star's CCD position and normalised to unit sum.
  • Stars: 1,615 Gaia DR3 sources (18 ≤ G ≤ 20.5, 5/6-parameter solutions, RUWE < 1.4), at Gaia positions with proper motion applied; core sample with no masked pixel within 8 px and raw peak below 50,000 ADU.
  • Method: HSM adaptive moments of each star stamp vs the model shifted (in Fourier space) to the star's sub-pixel centre, both in the CCD pixel frame.
G star epochs δT/T (robust mean, |δT/T| < 0.1) median
18–19 1,419 −0.34 ± 0.04% −0.32%
19–20 2,036 −0.43 ± 0.09% −0.48%
20–20.5 1,294 −0.43 ± 0.11% −0.45%
  • full_starcat_smk-p3a.hdf5 (the same 17 exposures, 7,835 stars, HSM star vs PSF) independently gives −0.58 ± 0.03%.
  • The plain (non-robust) mean over all stars is +0.13 ± 0.08%, pulled by ~1.5% outliers (binaries, neighbours), so the sign depends on outlier handling; the robust statistics and the ShapePipe star catalogue agree on a negative residual.
  • Per-exposure means span −0.50% to +0.95% (rms 0.33%), and the per-star rms is ~3% at G 18–19, above the measurement noise, which suggests spatial residuals that a degree-2 polynomial on ~44 stars per CCD may not follow.
  • An independent check points the same way: with a moments-based shape estimator, the shear response measured on the Gaia stars is slightly negative (R ≈ −0.003, about −1% of the galaxy response), as expected when the PSF model is larger than the true PSF. The stars' mean ellipticity is zero to 2 × 10⁻⁴, so this does not look like a WCS offset.

Candidate causes (none tested)

  • PSFEx's own resampling or interpolation of the star vignets;
  • the star selection used for the PSFEx fit (possibly related to #316 and #919);
  • a smaller observation, unverified: PSFEx defines its position polynomial in 1-based pixel coordinates, and ShapePipe appears to pass 0-based positions (a 1-px offset in where the model is evaluated). This is probably negligible for a degree-2 polynomial, but it may be worth checking.

Caveats

1 deg² only, a single star-selection choice, and HSM moments as the size measure. A check over more tiles, and with the stars actually used in the PSFEx fit held out, would show whether this is real and survey-wide.

The measurement code and the full write-up are in a separate project (genshear) and can be shared on request.

—
Posted by Claude (AI assistant, Anthropic) on behalf of Calum Murray (@calumhrmurray), who asked for this to be raised.

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