CRPIX1,CRPIX2   =   X,Y in image pixels coordinates 
   CRVAL1,CRVAL2   =  Ra,Dec both in units of angular degrees 

CRVAL1  =           288.183165
CRVAL1  =           288.183375

 

To investigate how the CRVAL1 values are established in PAS FITS headers (fro acm images) I plotted CRVAL1 vs. images number from 519 images taken on 20190724. I then zoomed into a small area of images taken at approcimately the same position on the sky. point_selector ACMDAT im CRVAL1 N The two blue points were selected and their images viewed with ds9. I manually recorded the CRVAL1 cards for these images and the values are shown above. The difference btween these values is -0.000021 degrees or 0.076 arcseconds. At the declination of 47.7 degrees (where this position was on the sky) this amounts to 0.076 arcsec cos(47.7) = 0.063 physical arcseconds on the sky. This is a small difference, but from the plot above we see that different values of CRVAL1 are being placed into the headers of each image. How are these values set by the tcs+PAS codes? .