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Saturated pixels reach ngmix unflagged: survey flag bit 8 marks only the first pixel of each saturated run

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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
computer-vision

调研方向

Start in split_exp, tracing how per-CCD flag files are produced and how SExtractor and the vignets consume them. Review default_exp.sex for the SATUR_LEVEL setting, then use the saturation evidence in /n17data/cdaley/scratch/saturation/ to check the proposed clipping, growth, and bleed-tail masks. Done means saturated pixels and their proposed extensions are flagged consistently without masking more good pixels than expected.

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描述

Saturated pixels reach ngmix unflagged and at full weight. In the CFIS survey flag map, bit 8 marks only the first (lowest-column) pixel of each horizontal run of saturated pixels. A 1-px bleed trail therefore looks fully flagged, while wider trail sections and star cores are mostly unflagged. Nothing downstream catches the rest:

  • The v2 pipeline has no mask rule, and prepare_ngmix_weights never tests image values.
  • SExtractor's saturation flag is inert, because SATURATE = 65535 in the headers while the int16 images clip at 32760.

trail
Exposure 2086786p, CCD 23. Blue: survey flag bit 8. Red: clipped, unflagged, weight > 0. Right: the trail widens to two columns, and only the left one is flagged. Each red pixel there is about 2,400σ.

Scale (4 exposures × 40 CCDs): 80–88% of clipped pixels are unflagged, 66k–99k per exposure. In horizontal runs of clipped pixels at least 2 px wide, 76% have only the first pixel flagged. The pattern is the same on both amplifiers.

Impact on shapes, so far small:

  • Among galaxy-like epoch stamps on tile 202.301, after the neighbour mask, 0.05% contain such a pixel, and 0.02% within 8 px of the centre. Most of those are coadd detections lying on a trail.
  • PSF stars (MAG_AUTO > 18) sit below the clipping onset at mag 17.0–17.4 (inferred, not checked star by star).
  • Shear impact is not measured.

Proposed fix, in split_exp, ORed into each per-CCD flag file so that SExtractor and the vignets both see it:

  1. flag image ≥ 0.9 × 32760, which catches all of them at 0.02% of good pixels;
  2. grow the mask by 1 px (3×3) for the charge spilled into the next column, about +1.5 to 3σ on the side away from the readout amplifier;
  3. extend it ±20 rows along the column for the bleed tails, which are still +2σ at 20 px. Total cost about 0.06–0.07% of good pixels.

Dilating bit 8 instead catches at most 50%, at 0.9% of the area. Separately, set SATUR_LEVEL ≈ 29000 in default_exp.sex so that exposure FLAGS bit 4 works.

Open: why the upstream flag-map production marks only the first pixel of each run. Someone who knows the CFIS flag pipeline would know.

Evidence on candide: /n17data/cdaley/scratch/saturation/ (scripts, the 4 exposures, figures, including fig_stats.png).

— Claude Opus 5.5, on behalf of Cail

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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。

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