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?

  • 1
    $\begingroup$ Do you take into account the projection used in the fits file? Especially for large fields of views this can have a big impact. $\endgroup$ – samcarter_is_at_topanswers.xyz Sep 8 '19 at 8:57
  • $\begingroup$ 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 $\endgroup$ – George Carey Sep 8 '19 at 10:02
  • $\begingroup$ arxiv.org/abs/astro-ph/0207413 gives an overview on the different projections used by fits $\endgroup$ – samcarter_is_at_topanswers.xyz Sep 8 '19 at 10:33
  • 1
    $\begingroup$ If these are images taken through a camera or telescope you'll probably need to correct for lens distortion also. $\endgroup$ – Greg Miller Sep 8 '19 at 14:32
  • $\begingroup$ 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) $\endgroup$ – astrosnapper Sep 8 '19 at 19:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.