The first apparently significant all-sky candidate was wrong. Reading the margin and grouping repeated photographic images leads to a strongly supported field near Selected Area 20.
Forty-six clean pairs recur at a native displacement of approximately (25.63, 0.64) pixels; shuffled controls reach at most eleven. The written intervals appear to be 20 and 2 minutes. The detailed evidence below supports a long-plus-short double exposure. Weaker displacement peaks are not established third exposures. Group means and individual-image positions must be kept distinct.
Several stars appear as shifted pairs or groups. A source list containing every image mixes exposure positions and can mislead pattern matching.
The mirrored margin appears to read “R4073”, “Kap 20”, “1920 Jan 21” and “Seed 30”. The Selected Area 20 interpretation supplied a search hint, not an independent historical instrument identification.
The revised layer uses local contrast in both polarities, expanded inspected writing regions and contextual recovery of short fragments and round digits. Known photographic targets and sampled spectral apertures are protected. It is not OCR and does not reconstruct hidden sky or invent pen strokes.
The selection covers 5.11% of this scan (1,079,163 pixels). Faint marks, scratches and contacts between ink and photographic images remain ambiguous. The cyan overlay shows the wider exclusion footprint used by the photographic remainder; the stroke mask is supplied separately.
The writing PNG is a white, contrast-rendered stencil of bright and dark observed marks. The observed-pixel PNG preserves their stretched scan appearance. Both use original scan coordinates. The photographic remainder displays excluded pixels as black holes. It is neither a clean-sky reconstruction nor a new photometric product. Keep the mask alongside any measurements. Native original-intensity TIFF/FITS products, where generated, remain in the analysis workspace and are not duplicated in this handoff.
This revision recovers short letter fragments, round digits and inspected dark marks, with broader writing regions. The stroke selection grows from 376,719 to 1,079,163 pixels (2.86×). The photographic remainder uses a separate 3-pixel exclusion margin, covering 13.56% of the scan.
The wider selection deliberately favors removing writing. Nearby stars, scratches and ambiguous fragments can also be excluded. Explicit photographic protection regions retain known target images and sampled spectra; this does not establish perfect separation elsewhere.
An initial grouped all-sky candidate near RA 124.613°, Dec +45.90° had only six matches and failed an independent image/reference correlation check; it was rejected. A Selected Area 20 position from Landolt (2013), searched within 3 degrees, led to 48 solver matches and odds 2.75 × 10²⁰. This is a label-assisted solution. The recovered center is 00h 45m 17.88s, +45° 59′ 13.02″.
The DSS2 image is resampled into the recovered plate orientation and footprint. Its photographic epochs, bands, resolution and contrast differ. Brightness and morphology need not match. It is context from the recovered WCS, not an additional independent fit check.
Among 1,200 compact candidates away from the revision-2 writing exclusion, 46 pairs share a displacement of (25.63, 0.64) original pixels. This is about 48 arcsec at the recovered scale. Median displacement scatter is 0.71 native pixels. A matching bright image and fainter companion recur across the plate.
At that fixed displacement, 60 controls with shuffled Y coordinates produce a median of 5 pairs and a maximum of 11. This is a descriptive control after peak selection, not a global trial-corrected significance test. Lesser displacement peaks in the saved audit should not be counted as established additional exposures.
The readable margin appears to record sidereal intervals 2h54m–3h14m and 3h15m–3h17m: approximately 20 and 2 minutes. The two intervals and consistently unequal photographic copies strongly favor a long-plus-short double exposure. A scan alone does not independently establish the shutter sequence, guiding behavior or scanning history.
The earlier solver grouping used a less restrictive candidate set. Its accepted-group count is not a count of independent shutter exposures. The saved WCS still registers arithmetic exposure-group centers; individual exposure components retain their offsets.
Forty-six clean pairs recur at a native displacement of approximately (25.63, 0.64) pixels; shuffled controls reach at most eleven. The written intervals appear to be 20 and 2 minutes. The detailed evidence below supports a long-plus-short double exposure. Weaker displacement peaks are not established third exposures. Group means and individual-image positions must be kept distinct.
A common-catalog check holds out ten entire pairs and fits thirty, using the same forty catalog identities for mean, brighter and fainter coordinates. TAN median residuals are 0.748, 0.752 and 0.902 native pixels; quadratic SIP gives 0.654, 0.707 and 0.631. No coordinate choice eliminates all residual structure. This smaller subset is not directly comparable to the earlier sixty-association cubic result; no replacement WCS is installed from it.
Across these scans, repeated photographic images need not imply the same mechanism. Shutter timing, guiding offsets, objective optics and different pointings can leave related patterns. Establish consistent component membership before fitting a smooth warp. No result here isolates glass deformation from telescope optics, emulsion, scanner geometry and centroid bias.
Sixty reference associations provide 46 training stars and 14 held out from fitting. TAN yields a median/p90 of 0.637/1.463 original pixels. Cubic SIP yields 0.512/1.796 pixels: the typical residual improves, while the upper tail worsens. The selected cubic model is consequently a tradeoff, not a uniform improvement.
The saved TAN-SIP WCS maps the mean positions of exposure groups. It does not register each individual exposure image separately. No pixel-resampled TIFF has been generated here. Apply the model with particular caution at the sparse edges and on individual components of a group.
| Model | Held-out median | Held-out p90 | Retained |
|---|---|---|---|
| TAN | 0.637 | 1.463 | — |
| TAN-SIP 2 | 0.712 | 1.800 | — |
| TAN-SIP 3 | 0.512 | 1.796 | Yes |
The reference footprint describes where the model is constrained. A smooth polynomial outside that footprint can look plausible while remaining unmeasured.
Reference catalogs were not propagated to the plate epoch. Proper motion, centering errors, repeated images, optical projection, emulsion, the glass and the scanner may all contribute to residuals. One scan cannot separate these physical causes.
Batch held-out stars were excluded from distortion fitting, but may have participated in the initial field identification. They are not a wholly independent astrometric experiment.
Select a numbered pin or a note. Locations are editorial review pointers, not measured confidence contours; plate-wide limitations also apply.
The historical interpretation of “Kap 20” needs a logbook check. Its role as a search prior is disclosed, so the solve is not presented as blind.
A mean group coordinate is not the location of every member. Missing or uneven components can bias centroids.
Cubic SIP worsens the held-out 90th percentile. The coverage plot locates support, but does not establish a reliable local glass map.
Recover individual exposure translations, fit each exposure consistently, and compare a simpler model after epoch correction. Check whether poor tail residuals correspond to overlapping or incomplete groups.
The field is well supported after use of a disclosed historical hint. The distortion result is useful but modest, with an explicit median-versus-tail compromise.
All scientific numbers are snapshots of the saved analysis products. The original 16-bit scans remain the measurement source. JPEG derivatives, contrast stretching and masked previews are inspection tools; do not derive photographic density or absolute response from their appearance.
