# Can we explain all orbital WCS change?

- **Not yet.** Known velocity aberration and a temperature-conditioned model account for much of the measured field evolution, but coherent startup residuals and common pointing remain incompletely predicted.
- The first/last days here mean available science observations, not the unobserved perigee/downlink interval.
- Distortion prediction supplies contemporaneously measured attitude. That removal is not a physical prediction of pointing.
- All-camera measurements here are FFI-header WCS fields; direct stellar validation remains a separate gate. A spatial feature in a header map may include a fitting artifact.

## Temperature and apparent expansion

- Sensor2 adds predictive information beyond orbit-specific linear time trends in all four cameras.
- The archived conditional sky-map scale slopes range from −19.04 to −22.87 parts per million per degree Celsius. This is a mapping coefficient, not a measured material expansion coefficient.
- A sky angle per pixel scale and a pixels per sky angle scale have opposite fractional signs. Do not call the negative coefficient physical contraction without specifying that convention.
- Lens spacings, effective focal length, refractive properties, detector pitch and alignment can all affect the mapping. A single thermometer is a proxy for a thermal state.
- The revised scatter plot removes the identical time/orbit terms from both axes. Full-sample descriptive slopes and reserved-block predictions are labelled separately.

## What extra camera temperatures tell us

- New tests compare the same endpoint observations, using time-only, mean sensors2–5, sensor contrasts, rates and a fixed12h history term. Whole test days and0.25day buffers are excluded; ridge selection uses training-only inner folds.
- Orbit47 startup: adding the tested temperature contrasts does not beat mean sensors2–5. The12h history predictor is worse.
- Orbit48 startup on cameras2–4: extra contrasts do help modestly, especially mean sensors6/7 relative to sensors2–5; rates help further in this exploratory comparison. Remaining spatial structure is not eliminated.
- The different outcome across the two startups rules out a claim that the tested simple thermal predictor is universally sufficient. It does not rule out a more complete thermal model.
- These endpoints have been examined repeatedly. Comparisons are exploratory, not a new untouched final validation or a basis for selecting the best row and reporting it as unbiased performance.
- Component and contrast feature families span the same linear space; ridge regularization can make their predictions differ. That difference is not new physical information.

## Sensor identity

| Channel | Sector20 FITS description | Instrument Handbook TableA.3 |
|---|---|---|
|2|Lens Barrel|L1|
|3|Lens Barrel|L2|
|4|Lens Barrel|L3/4|
|5|Lens Barrel|L4/3|
|6|FPE Housing|Generic PT1000 sensor6|
|7|FPE Housing|Base Plate|
|9|FPA Housing|CCD Case|
|10|FPA Mosaic|Mosaic|

- Preserve exact channel numbers. The flight-file and design-document descriptions do not uniquely identify the mechanically responsible element.
- Sensor differences are temperature contrasts, not gradients per metre: no calibrated sensor separation has been used.
- The handbook describes multiple thermal paths, including the lens hood heat sink and isolation between focal-plane electronics and assembly. That motivates component-resolved tests without proving a particular residual's cause.
- Primary source: [TESS Instrument Handbook, §4.4 and AppendixA](https://archive.stsci.edu/files/live/sites/mast/files/home/missions-and-data/active-missions/tess/_documents/TESS_Instrument_Handbook_v0.1.pdf).

## Shape of the withheld remainder

- The archived endpoint errors contain coherent per-CCD offsets and non-affine structure. Per-CCD isotropic scale accounts for only about1.5–3.6% of their squared residual amplitude.
- A spatial constant within each CCD is not an independent spacecraft-pointing measurement. Common attitude has already been removed.
- Orbit48 camera3 retains residuals across CCD interiors. Some other CCDs show strong outer-probe concentrations; both physical distortion and WCS/centroid fitting artifacts remain plausible.
- Window-mean maps show persistent patterns; exposure-averaged squared errors retain their time-variable part. The page plots both, without mixing their denominators.

- A separate whole-camera radial-template projection (free camera amplitudes at every exposure) captures only1.09–6.49% of the remaining squared error across these endpoints. This descriptive fit addresses the CCD-centering ambiguity and limits simple isotropic expansion as the missing pattern; it does not rule out other thermal deformations.

## What could cause the remainder?

| Candidate | Signature to test | Present limit |
|---|---|---|
|Unequilibrated optics or unmeasured thermal gradients|History-dependent scale/distortion at matched sensor temperatures|Tested contrasts/history do not close both startups|
|Camera or detector alignment changes|Relative CCD translations, rotation/shear and smooth nonradial fields|Spatial decomposition identifies shape, not the moving component|
|Guide response or attitude convention|Common rotation with a distinct relative-camera pattern|Measured attitude is supplied; controller/commanded-attitude reconstruction remains open|
|PSF/centroid or WCS-fitting bias|Magnitude/colour/pixel-phase dependence or large edge-localized residuals|Header maps alone cannot distinguish physical motion from estimator errors|

## Tests that would make the paper's explanation stronger

- Re-measure reserved startup/interior/ending images with the same profile-centre convention and held-source support; compare spatial modes with header maps.
- Test a fixed model in another thermal/guiding regime; avoid repeatedly tuning against these two startups.
- Resolve sensor locations/calibration and timing; add explicitly identified driver/interface and on-chip channels only after checking their data quality.
- Test matched-temperature but different-history cases and predict a distinct spatial mode before attributing a component.
- Report coherent residuals against an independently measured noise floor. An unexplained but bounded residual is a valid result; it must not be labelled fully explained.

**Conclusion:** temperature is a useful predictor, and the residual's spatial shape narrows the next tests. Complete physical attribution and an independently verified full-orbit WCS accuracy budget are not yet established.