[daip] FITS header

[Eric Greisen]

Ian Pattison writes:
 > Eric,
 > 
 > Any idea why the RA and Dec. coordinates (in AIPS) are still different to
 > the coordinates in the original FITS data (as viewed with ds9)? I looked
 > at a number of pixels in both images, and the reference pixel RA and Dec.  
 > agrees in these two images, but moving out from the reference pixel the
 > coordinates diverge more as one moves further from the ref pixel, up to 
 > more than 20 asec at the map edge. 
 > 
 > I suspect it is the pixel scale and/or transformation matrix. If I change 
 > the CDELT1 and CDELT2 to -0.333 and 0.333 asec/pixel (for RA and Dec.), 
 > the coordinates in the AIPS data then 'almost' match the original 
 > coordinates (to within 1 asec or so). Not suggesting this is a fix, just 
 > wanted to try and pin down the root of this problem.
 > 
 > Any suggestions? I appreciate your help on this.

Not having your FITS file I cannot run ds9 on it - I can read its
header and aips is telling you what is in the header.  If it has been
processed between the ds9 and aips then that would be the place to
look.

Your wording is imprecise so I do not know what you are saying.  The
reference value is not affected by keywords haveing to do with
increments e.g. CDELT or CD1_1.  Changing the increments does not do a
thing to the listed coordinate (in IMHEADER - if you are using QHEAD
then you get an interpreted header - note the *s - and that would
affect the coordinates you see).  QHEAD shows the coordinate at
IMSIZE/2 while IMHEADER shows what the FITS coordinates actually are.


Eric Greisen