Right Ascension, Declination from pixel coordinates

Question

I have an Excel spreadsheet that will take certain keywords from a FITS image header. Image centre RA, Dec, rotation, x centre, y centre, x scale, y scale. I enter the pixel coordinates of an object and the spreadsheet returns the RA and DEC coordinates. Close to the centre of the image the results are good, but an error increases towards the outer parts of the frame. I believe there are Python programs for doing this but my experience with Python, C++ is zero.

Does anyone know how accurate coordinates can be obtained, using arithmetic suitable for excel and Visual Basic?

Do you take into account the projection used in the fits file? Especially for large fields of views this can have a big impact. — samcarter_is_at_topanswers.xyz, Sep 08 '19 at 08:57
That was a new one to me! I had a look and my system uses
CTYPE1 = 'RA---TAN' / X-axis coordinate type
CTYPE2 = 'DEC--TAN' / Y-axis coordinate type — George Carey, Sep 08 '19 at 10:02
https://arxiv.org/abs/astro-ph/0207413 gives an overview on the different projections used by fits — samcarter_is_at_topanswers.xyz, Sep 08 '19 at 10:33
If these are images taken through a camera or telescope you'll probably need to correct for lens distortion also. — Greg Miller, Sep 08 '19 at 14:32
This may be pushing the bounds of what makes sense to do in a spreadsheet. There are lots of tutorials for astronomy Python out there including Python4Astronomers which has one specifically on WCS transformations which is what trying to do (map x,y pixels to RA,Dec or viceversa) — astrosnapper, Sep 08 '19 at 19:15
Thanks. I was trying to avoid Python as I have no experience with it. But I have downloaded it and will have a look. — George Carey, Sep 09 '19 at 08:41
It's complicated: https://photo.stackexchange.com/questions/6111/pixel-to-ra-dec-mapping-in-digitized-astrophotography — , Sep 09 '19 at 18:00

Right Ascension, Declination from pixel coordinates

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