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.
