This routine will query the FITS header of an image extracted from the Digitized Sky Survey (DSS) CD-ROM set and convert pixel coordinates into equatorial coordinates (Right Ascension and Declination) in equinox J2000.0
The coefficients of the astrometric plate solutions are stored in a number of FITS header keywords. This program will read the coefficients and solve for the equatorial coordinates for any (C,R) location in the image. To work, the image must be unprocessed. That is, the image cannot be rotated, binned, shifted, flipped, or have any of the essential header keywords removed (especially the CNPIX1 and CNPIX2 cards).
The PHOT keyword gets the star (x,y) pixel coordinates from the currently active photometry file, either entered using a program like MARKSTAR, or using the GET PHOT=fspec command. This is the most convenient way to find coordinates for a large number of stars on a DSS image. In general, using MARKSTAR RADIUS=1 gives good centroids, as the stellar images on the DSS images are slightly under-sampled.
Alternatively, (RA,DEC) coordinates may be determined for any location in an image (not just for a stellar centroid) using either the P=, V=, or TTY keywords. P=(col,row) returns the RA and DEC for a given column and row location (for either fractional or integer pixel locations). V=n (n=0-9) gives the (RA,DEC) for the TV cursor location loaded with one of the number keys. TTY will ask the user to enter the (x,y) coordinate(s) to be converted into (RA,DEC).
The STANDARD keyword tells DSSCOORD to also printout the computed standard coordinates, (xi,eta), in arcseconds of the pixel location(s) of interest. These are useful for using the stellar centroids for finding approximate astrometric plate solutions for CCD or IR Array Images using stars common to your image and the DSS to define an approximate secondary reference frame.
Coordinates derived from the DSS images are usually good to an arcsecond or so, which is to say sufficient for pointing and guide star selection, and for approximate astrometric calibration of CCD images to determine the pixel scale and field orientation (rotation relative to the cardinal axes). At this writing (December 1994), details of the DSS data set are still awaiting publication. Future versions of this help file will elaborate as necessary.