[fitsbits] Plate Center on POSS2 Plates

[William Pence]

Those keywords presumably give the coordinate of the center of the 
wide-field photographic plate, which could be degrees away from the center 
of the small image that you retrieved.  The approximate coordinates of the 
center of your image appear to be given by the OBJCTRA and OBJCTDEC 
keywords.  The exact coordinates of any pixel in the image can be calculated 
from a complicated polynomial expression whose coefficients are given by the 
PPnn, AMDXn, and AMDYn header keywords, but a much simplier way to get the 
coordinates is to display the image using a program such as 'ds9' that does 
the calculation for you.

Bill Pence

K. L. Tah wrote:
> Hi,
> I've just retrieved a fits file from
> http://archive.stsci.edu/cgi-bin/dss_form
> and would like to know the coordinates which the center pixel of this
> image corresponds to.
> I notice that there are comments "Plate center RA" and "plate
> center DEC" next to keywords PLTRAH,PLTRAM,PLTRAS etc. but for the most
> part, these seem not to correspond to the center of the actual fits image
> since they are ~ few minutes different from
> the epoch 2000 coordinates and the star of interest lies fairly close to
> the center (~ tens of pixels from center). Can someone explain this to me?
> Thanks,
> KL
-- 
____________________________________________________________________
Dr. William Pence                          William.D.Pence at nasa.gov
NASA/GSFC Code 662         HEASARC         +1-301-286-4599 (voice)
Greenbelt MD 20771                         +1-301-286-1684 (fax)