# Could background removal bias orbital WCS measurements?

- **Yes in principle; the direct additive effect is small in this bounded C 3/CCD 3 test.** This does not certify every exposure or rule out errors inherited by the trained ePSF.
- Scope: Sector 20 camera 3 CCD 3, six exposures:2.5 hours after each orbit's science start, one interior exposure per orbit, and 2 hours before each ending. These are representative snapshots, not complete first/last-day coverage.
- Our subtraction cannot alter the independently produced mission-header WCS. It can affect our image centroids and trained WCS. Mission processing may have its own background sensitivity, not tested here.

## What was actually fitted

- Historical centroid ECSV metadata records no additional local background estimator. The temporal fitter likewise used prepared images without a nuisance background column. The modern single-image fit uses a different image generation; its upstream harmonic background processing must not be confused with the historical lane.
- The old lane has saved background products but no frozen subtraction recipe establishing every historical parameter. Raw-minus-hp_d includes removed sources/templates; it is not a pure background estimate.
- New positional test:168 spatially selected isolated sources, Tmag 9–12.8;133 have sufficient common native-pixel support. All attempted fits converge. The 35 excluded sources are not certified.
- The same saved ePSF,11×11 pixels, uniform weights and fixed valid-pixel mask are used for zero, constant and sloping-plane background fits. Source positions and amplitudes are refitted; ePSFs are not retrained.
- Report the change in each star's background correction relative to its own orbit-interior exposure. This removes static offsets and does not test the inter-orbit jump.

## What the images show

- The large background patterns, including extra broad light at the second startup snapshot, appear in the calibrated FFI and saved background estimate. A comparable broad enhancement is not apparent in the subtracted centroid-input map.
- In the 168-star annulus diagnostic,150–151 sources are usable per exposure and 150 are common to all six. Median residual gradient magnitudes range 0.0246–0.0288 electron/s/pixel, without a strong endpoint-only elevation.
- Masked annuli still contain arcs and compact residuals. Sky background, stellar wings, neighbours and template errors remain mixed; a fitted surface is not independently known sky.

## What changes the positions

- Allowing a plane changes endpoint-minus-interior corrections by 0.236–0.349 millipixel RMS across the 133 stars. The corresponding median vector is below 0.055 millipixel in each endpoint snapshot. A millipixel is 0.001 native detector pixel.
- These quantities describe sensitivity within the new fitter, not absolute positional accuracy. No projection onto the thermal WCS modes was fitted, and the star and WCS-probe populations have different denominators; no explained fraction is claimed. Spatial maps and outliers accompany the medians.
- The new optimizer's zero-background solution differs from archived centroids at roughly 1.5–2.1 millipixel median even on full 121 pixel support. A separate 10-star audit using the original photutils path reproduces the saved positions to numerical precision; the new-solver difference is retained as a limitation, not attributed to bad saved products.
- Changing a local background term cannot undo errors already absorbed into ePSF shape/origin, source removal or template construction. That end-to-end pathway remains untested.

## Controlled injections

- Two preselected detector locations, their six saved profiles, Tmag 9/11/13, known fixed positions and source-matched annular background surfaces;2052 fits, no noise or neighbours and no ePSF retraining.
- AtTmag 13, full measured quadratic surfaces produce a median endpoint-relative bias 0.074 millipixel and maximum 0.255 millipixel when no background is fitted, across eight endpoint/source pairs. Fitting a plane leaves median 0.009 and maximum 0.036 millipixel in this shared-profile control.
- Stronger injected gradients can produce millipixel shifts. The control demonstrates the mechanism and scales for these profiles; it is not proof that all real backgrounds are this small.

## Paper consequence and next decisive test

- The sampled direct residual constant/gradient is unlikely to be the main source of the several-millipixel orbital remainder. This is a constrained negative result for one CCD and these sources/exposures.
- Keep background-induced profile/centroid bias in the physical-cause budget. Thermal correlation alone cannot rule it out.
- To close the remaining pathway, rebuild backgrounds and ePSFs from the same raw endpoint images under alternative source masks/background models, then score fixed held-source positions and residual images. Expand to other detectors and the earliest science exposures.

**Conclusion:** broad background removal appears effective in the inspected images, and direct local-background sensitivity is small for the tested cohort. Background effects inherited by the ePSF or upstream subtraction remain a live uncertainty; no blanket all-camera or full-day certification is justified.