Three fixes and one refusal, all found by working on the wide-field
narrowband session that would not solve.
The detection floor was a fixed 12 pixels, which quietly assumed a well
sampled star. At 0.53 arcsec/px with 5.9 px seeing that is right; at 3.5
arcsec/px the stars are undersampled at 1.75 px FWHM and cover a handful
of pixels each, so the floor discarded nearly every real star and kept
blends and galaxies instead. It now scales with the measured seeing, and
that session went from 305 usable detections to 600.
Matching now happens over the frame's inscribed circle rather than a cone
reaching its corners. On a 4 degree field the old cone covered 33 square
degrees of sky against 16 of image, so half the catalogue was not in the
picture at all.
Where the header records a roll angle - and iTelescope's does - the
orientation no longer has to be recovered from scratch. Rotation, scale
and parity are applied directly and only the residual pointing error is
searched, by histogramming every detection-to-catalogue offset and taking
the peak. Asterism matching remains as the fallback for headers that say
nothing about orientation.
The refusal matters most. With catalogue depth slices added, the wide
field produced a "solution" of 25 stars at 2.50 px residual, claiming a
centre half a degree from the pointing. A genuine solve on that field
matches hundreds of stars and refines to well under a pixel. Accepting it
would have silently corrupted every position in the science catalogue, so
a solution must now reach 40 stars AND 1.5 px or it is discarded and the
session is reported as unsolved. A wrong WCS is worse than no WCS.
NGC 2030 improves from 0.31 to 0.12 arcsec on 123 stars under the
pointing-based match. NGC 2070 is honestly unsolved.
measure.py produces the numbers used to choose a registration reference
and to weight the stack - sky, noise, seeing - and caches each frame's
star list, because registration and the plate solve both need it and
detection costs far more than reading a small array back. Frames are
opened one at a time; a 4096x4096 float32 frame is 67 MB and a session
holds ninety of them.
register.py aligns everything onto a single reference and combines per
filter. One reference for ALL filters, not one per filter, which is what
makes the masters pixel-aligned so the colour composite needs no further
registration. The reference is the sharpest frame of the filter with the
most signal, because the reference sets the output grid and a poor choice
costs resolution everywhere, permanently.
The case worth the care is NGC 6744: luminance at bin1 4096x4096 and
colour at bin2 2048x2048, so frames share neither shape nor pixel scale.
Asterism matching already returns a similarity transform including scale,
so the maths was never the problem; the trap is assuming the warp output
is the same shape as its input, which would write a bin2 frame onto a
bin2 grid that silently fails to line up with a bin1 reference. Every
warp is now given the reference shape explicitly.
Verified rather than assumed: after stacking, the upsampled bin2 colour
masters align to the bin1 luminance master to within 0.04 pixels, across
about 245 matched stars per channel.
Sigma clipping is skipped below three frames, where there is nothing to
reject against and clipping would only discard signal - which matters
because one session has a single frame per filter.
Tested on three sessions covering the awkward shapes: RGB without
luminance, SHO with one frame per filter, and the mixed bin1/bin2 LRGB.
