StellarSolver and Parity (and ASTAP)

[Hy Murveit]

Thanks Rob,

Probably right, but easy to verify since you already have a working
solution. Also, for another check, the parity is in the wcs.fits file that
one can download from the online astrometry.net.

I am concerned that ASTAP didn't solve my positive parity image (which as I
mentioned was the exact-same-but-mirrored image it solved as negative
parity), but I assume/hope that it is a user-issue (e.g. some command line
flag or the like is off). I don't have Han's email, and don't know if he
reads this mailing list. Could you please forward these questions to him?

Thanks,
Hy

On Tue, Apr 6, 2021 at 7:30 PM Robert Lancaster <rlancaste at gmail.com> wrote:

> Sorry one correction to my last email.
>
> I just checked the internal stellarsolver parity code, there is this
> comment:
>
> // Note, negative determinant = positive parity.
>
>
> So that means if this
>
> cd11 * cd22 - cd12 * cd21
>
>
> Produces a negative number, that is a positive parity and vice versa.
>
> Sorry for the mistake in this last email.
>
> On Apr 6, 2021, at 10:27 PM, Robert Lancaster <rlancaste at gmail.com> wrote:
>
> Hi Hy,
>
> I didn’t get the parity from ASTAP because I didn’t believe it reported
> that information.  It isn’t in the INI file.  However, I just took a look
> at the astrometry.net wcs calculations and I think we may be able to
> calculate the parity from the INI file ASTAP produces.
>
> If I am reading this correctly, we can do the following:
>
> cd11 * cd22 - cd12 * cd21
>
> And if that produces a positive number, it is positive parity, but if it
> is negative, it is negative parity.
>
> Please let me know if this sounds correct.  I can always add this to
> StellarSolver if it is correct.
>
> Source:
> https://github.com/dstndstn/astrometry.net/blob/main/net/wcs.py
>
> As for the Local solver getting the parity wrong, the code I am using in
> StellarSolver to read the WCS file is the same code we were using to read
> the solution files before in KStars.  I basically just moved it.  It reads
> the “parity”  key-value pair.  I can check into this to see if something
> was missed.  In fact, if you think the above calculation would work, I
> could just do that for the local solver as well.
>
> Rob
>
>
> On Apr 6, 2021, at 9:33 PM, Hy Murveit <murveit at gmail.com> wrote:
>
> *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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