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SOLVE_ASTRO

SOLVE_ASTRO

Name


    SOLVE_ASTRO

Purpose


    Solve for an TANgent-plane astrometric plate solution with optional
    distortion terms

Explanation


    SOLVE_ASTRO takes an input matched xpixel/ypixel/ra/dec list, and returns
    an IDL astrometry structure containing the astrometric
    solution. Optional sigma clipping on the fit. Similar in
    principle to IRAF's ccmap.

Calling Sequence


    SOLVE_ASTRO, ra_degrees, dec_degrees, xpixel, ypixel,
    [XIORDER=, ETAORDER=, XTERMS= DISTORT=, CRVAL= NITER=, REJECT=,
    XIRMS=, ETARMS= SUCCESS=,VERBOSE= NORTERMS= NREJ=, N_TPVTERMS=,
    XIRESID=, ETARESID=, WFIT= ]

Input Argument


    ra_degrees - the right ascensions in degrees of the matched objects
    dec_degrees - the declinations in degrees
    xpixel - the x pixel values
    ypixel - the y pixel values

Returns


    astr - an IDL astrometric structure containing the plate solution

Optional Input Keywords


    distort - type of distortion to fit. Options: 'none': just
                  fit the linear solution, 'tpv' fit the TPV
                  convention, 'tnx': fit the iraf tnx
                  convention. default: 'none'. See:
                  http://iraf.noao.edu/projects/ccdmosaic/tnx.html
                  for TNX
                  http://iraf.noao.edu/projects/ccdmosaic/tpv.html
                  for TPV
    crval - the ra/dec of the reference pixel in
                  degrees. Default is mean of the input coordinates
    nrej - Number of rejection iterations
    reject - the sigma of the rejection (input to RESISTANT_MEAN)
    etaorder - The order of the TNX coefficients in eta
    xiorder - The order of the TNX coefficients in xi
    xterms - TNX cross-terms type (0: none, 1: full, 2: half)
    n_tpvterms - Number of TPV coefficients to include. Always
                  includes rterms even if /NORTERMS is passed. Useful
                  values include: 7 (quadratic), 12 (cubic), 17
                  (quartic).
    norterms - Do not fit the "r" terms if using a TPV distortion
    naxis1 - xpixel size of image being fit. If passed this is
                  inserted into the final astrometry structure, but
                  are not required.
    naxis2 - ypixel size of image being fit. If passed this is
                  inserted into the final astrometry structure, but
                  are not required.
    verbose - Give verbose output

Optional Output Keyword


    xirms - the xi fit rms in arcseconds
    etarms - the eta fit rms in arcseconds
    xiresid - Residuals of each point in xi entering the final
                  (fit indices of input points given by wfit).
    etaresid - Residuals of each point in eta entering the final
                  (fit indices of input points given by wfit).
    wfit - index of input datapoints that were used in the
                  final fit
    success - Did the code run successfully?

Notes


  The implmentation is slightly limited. It can only fit the
  polynomial terms in TNX and not the legendre or chebyshev. The x/y
  pixels must be previously matched with the ra/decs. The use of
  MPFIT2DFUN is probably not needed and could be re-written to run
  faster with LAPACK libraries. For tnx projections, orders in eta and xi cannot currently
  be specified separately.
  There is code duplication between tpv_eval, tnx_eval and this routine
  Input/Output is always assumed to be FK5/equinox 2000. Could be changed!

Procedures Used


      STRN() - in astrolib
      RESISTANT_MEAN - in astrolib
      MPFIT2DFUN(), MPFIT - in MPFIT library, can be downloaded from either
              from http://cow.physics.wisc.edu/~craigm/idl/fitting.html
              or http://idlastro.gsfc.nasa.gov/ftp/pro/markwardt/
      LA_LEAST_SQUARES() - IDL built-in

Revision History


      Written, M. Sullivan, March 2014
      Modified to better handle fields that straddle RA=0
      Modified to populate final astrometry structure with naxis1/2
        if these are passed



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