The gastro program determines the astrometric transformation for an image based on the cmp/smp file. This file contains both the table of object data and the complete image FITS header. Using the header astrometric information as a starting point, gastro loads the reference astrometry objects appropriate for the region and determines the matching objects between the image and the reference. These objects are used, with iteration to improve the sample, to determine the astrometric transformation with up to 3rd order polynomials.

    gastro filtname.cmp
    options:
      -h
      -help
      -v
      -p (photcode)
      -plot
      -maglims
      -nstars
      -fx
      -fy
      -coords (ra) (dec)
      -dump
      -mdmp
      -cdmp
      -loneos

    gastro configuration variables:
    OFFSET_RADIUS     - max allowed offset in gcenter
    MIN_MATCHES       - min number of matched stars for linear fit
    MINIMUM_RADIUS    - target matching radius in gfit iterations
    MAX_ERROR         - maximum allowed error in solution
    MAX_NONLINEAR     - maximum angle between axes
    MAX_PRECISE       - unused?
    DEFAULT_RADIUS    - unused?

    ROUGH_ASTROMETRY  - source of initial astrometry terms

    CCD_PC1_1         - initial guess pc1_1
    CCD_PC2_2         - initial guess pc2_2
    CCD_PC1_2         - initial guess pc1_2
    CCD_PC2_1         - initial guess pc2_1
    ASEC_PIX          - initial guess arcsec / pix

    POLAR_ALIGNMENT   - provide polar alignment of telescope mount?
    POLAR_AXIS_RA     - RA of telescope polar axis
    POLAR_AXIS_DEC    - DEC of telescope polar axis
    RA_OFFSET         - RA offset
    DEC_OFFSET        - DEC offset (degenerate?)

    NFIELD            - select stars from region NFIELD x detector size
    NPOLYTERMS	  - number of orders to allow
    MMIN              - minimum magnitude of allowed star
    ROT_ZERO          - zero of tested rotation range
    dROT              - rotation step size
    NROT              - number of rotation tests = 2NROT + 1
    GSCFILE           - name of HST GSC table file
    GSCDIR            - name of HST GSC data directory
    LONEOS_REGIONS    - table of location coordinates
    USNO_CDROM        - name of USNO data directory
    ASTRO_REFCAT      - reference catalog format to read
    CATDIR            - name of ptolemy database
    PHOTCODE_FILE     - table of photcodes