Hacktoberfest 2026：维护者为十月标记出来的 issue，仍然开放、适合新手。 浏览 Hacktoberfest issue

PSFEx native sampling leaves a g1 residual on sub-0.5″ exposures; PSF consumers assume PSF_SAMP = 1

未关闭
  • 哪里不对？
#989 0 条评论 0 个 reaction 已指派 0 人 在 GitHub 查看

维护者通常 2 天内回复

还没有人认领这个 Issue。

评估

难度
5/5
预计耗时
一周以上
新手友好度
25/100
Issue 类型
缺陷
描述清晰度
基本清楚
活跃度
活跃
技术栈
python
领域
data

调研方向

Start with psfex_interp.py and the PSFExInterpolator.interpsfex entry point; inspect how it reads PSF_SAMP and find its existing tests. The proposed guard should reject sampling values other than 1, with a test that covers the rejection. Later sampling support and the bounded galaxy shear pilot require separate validation; completion criteria for those are not specified in the issue.

由索引模型根据 Issue 内容生成。

描述

Claude Opus on behalf of Cail; investigation by GPT-6.1 Sol.

On the best-seeing exposures, PSFEx at native sampling (PSF_SAMPLING 1) leaves a coherent g1 residual. Our PSF consumers assume unit sampling, so the obvious config fix would silently break them.

The residual shows up at exposure level once the mean PSF FWHM is below ≈0.5″ (≈2.7 px). 286 of the 15,523 v2 validation exposures (1.8%) are that sharp; 2,504 (16%) are below 0.6″. Above ≈0.5″ the median residual is flat at zero, so a blanket seeing cut isn't warranted.

exposure residual vs PSF FWHM

For one exposure, 2780860 (18BD97, 2 Sept 2022, 0.50″), against a same-region control at 0.70″:

  • Production gives Δg1 = −4.0×10⁻³; the control gives −0.2×10⁻³. The rerun reproduces production's PSFEx basis exactly.
  • PSFEx at 0.5-px sampling (101×101 model) gives −1.3×10⁻³ with centered validation stamps. Rendered at each validation star's sub-pixel phase, it gives −0.02 to +0.26×10⁻³, depending on the Lanczos kernel: consistent with zero.
  • Phase-matching the native model alone brings it to −1.0 to −2.7×10⁻³ (kernel-dependent). On the control, all four conventions agree, so pixel phase matters only once the stars are undersampled.

per-CCD residual under four model/validation conventions

Why PSF_SAMPLING 0 (auto) is not the fix: with SAMPLE_AUTOSELECT N, PSFEx assumes a nominal 6-px FWHM and picks 1.28 px per model pixel, which is coarser. And any sampling other than 1 breaks the consumers:

  • PSFExInterpolator.interpsfex (psfex_interp.py) returns the raw PSF_MASK grid and ignores PSF_SAMP.
  • PSF validation attaches the CCD's native WCS to that grid. On a 0.5-sampled model it measures ΔT/T = −0.75: twice the linear size, four times the area.
  • The galaxy_psf store carries the array without sampling metadata, and ngmix's make_ngmix_observation builds the PSF Jacobian from the image WCS.

Proposed:

  1. A guard, now: psfex_interp raises if PSF_SAMP ≠ 1, with a test. It protects every consumer until sampling is supported.
  2. For run 2: psfex_interp emits native-pixel effective-PSF stamps with an explicit centering convention. Then validate sampling 0.5 / size 101 as the candidate configuration.
  3. For run 1: flag the sub-0.5″ exposures for diagnostics. Measure the galaxy shear bias on a bounded pilot (a few nights, paired ngmix fits) before any rerun. The galaxy-level effect is unmeasured; the star residual does not set it.

Possibly related: #956, a ~0.4% PSF size residual on P3 tiles, measured with phase-shifted models. Not the same measurement.

Report, data and reproduction: Nibi /scratch/cdaley/psf-undersampling/REPORT.md.

主要语言
Python
星标
18
派生
14
平均合并
4 天 16 小时
30 天内合并 PR
35

环境准备

从这里开始

  1. 先读完整个 Issue，再读项目的贡献指南。
  2. 在 Issue 下留言说明你要接手 —— 这能避免两个人做同样的事。
  3. Fork 仓库，在一个分支上完成修改。
  4. 提交 Pull Request，并在描述里引用这个 Issue 编号。

CosmoStat/shapepipe 的其他 Issue

查看 CosmoStat/shapepipe 的全部 Issue

相似的 Issue

更多 Python Issue

把新 issue 发到你的邮箱

精选适合新手参与的 GitHub issue 摘要。