StellarSolver and Parity (and ASTAP)

[Hy Murveit]

*Background*:
Jasem and I have been working on an issue related to solving and polar
alignment. It turns out that the KStars WCS code has been ignoring what
astrometry.net calls "parity", and that causes methods like
FITSData::wcsToPixel or pixelToWCS to give wrong answers for off-center
pixels when the image has positive parity (which tends to happen e.g. for
RASA telescopes (reflectors at prime focus). Most setups have negative
parity, which is why we haven't seen tons of complaints before.

*Solution:*
There's likely a very simple fix--negating the FITS header keyword CDELT1
when the parity is positive, but we need to know the parity of the solved
image. StellarSolver's Solution struct contains a (QString) parity field.

However...

I just checked with a fits image of mine (which is negative parity) and
tried the 4 possible StellarSolver solving options (both in KStars and
StellarSolverTester) and all methods solved the image, and the parity field
was filled as follows:



   - Internal Solver returned  (negative) parity
   - Local ASTAP *did not return any parity*
   - Local Astrometry returned (negative) parity
   - Online Astrometry returned (negative) parity.

Then, I flipped that image (using FastRotation-->Horizontal Mirror in
PixInsight, saving to a .fits). The image should now solve with the same
coordinates but have positive parity. Running the same test I found:

   - Internal Solver returned  (positive) parity
   - Local ASTAP *failed to solve the image*.
   - Local Astrometry returned (*negative*) parity
   - Online Astrometry returned (positive) parity.

The bold lines above indicate issues. Basically:

   - Local ASTAP seems to fail on positive parity images (on an image it
   quickly solved with the other parity), and does not return parity for the
   negative parity images it does solve (all via StellarSolver--I didn't try
   ASTAP natively). Perhaps StellarSolver is setting some wrong command-line
   flag?
   - Local Astrometry did not return the right parity for positive parity
   images. Does it always return negative (all via StellarSolver--I didn't try
   it natively).

Rob, Han, any suggestions/comments? Is there a configuration I'm missing?

Thanks,
Hy
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