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?