AD2XY
Name
AD2XY
Purpose
Compute X and Y from native coordinates and a FITS astrometry structure
Explanation
If a WCS projection (Calabretta & Greisen 2002, A&A, 395, 1077) is
present, then the procedure WCSXY2SPH is used to compute native
coordinates. If distortion is present then this is corrected.
In all cases, the inverse of the CD matrix is applied and offset
from the reference pixel to obtain X and Y.
AD2XY is generally meant to be used internal to other procedures. For
interactive purposes, use ADXY.
Calling Sequence
AD2XY, a ,d, astr, x, y
Inputs
A - R.A. or longitude in DEGREES, scalar or vector
D - Dec. or longitude in DEGREES, scalar or vector
ASTR - astrometry structure, output from EXTAST procedure containing:
.CD - 2 x 2 array containing the astrometry parameters CD1_1 CD1_2
in DEGREES/PIXEL CD2_1 CD2_2
.CDELT - 2 element vector giving increment at reference point in
DEGREES/PIXEL
.CRPIX - 2 element vector giving X and Y coordinates of reference pixel
(def = NAXIS/2) in FITS convention (first pixel is 1,1)
.CRVAL - 2 element vector giving coordinates of the reference pixel
in DEGREES
.CTYPE - 2 element vector giving projection types
.LONGPOLE - scalar longitude of north pole (default = 180)
.PV2 - Vector of additional parameter (e.g. PV2_1, PV2_2) needed in
some projections
.DISTORT - Optional substructure specifying distortion parameters
Outputs
X - row position in pixels, scalar or vector
Y - column position in pixels, scalar or vector
X,Y will be in the standard IDL convention (first pixel is 0), and
*not* the FITS convention (first pixel is 1)
Notes
AD2XY tests for presence of WCS coordinates by the presence of a dash
in the 5th character position in the value of CTYPE (e.g 'DEC--SIN').
Procedures Used
TAG_EXIST(), WCSSPH2XY
Revision History
Converted to IDL by B. Boothman, SASC Tech, 4/21/86
Use astrometry structure, W. Landsman Jan. 1994
Do computation correctly in degrees W. Landsman Dec. 1994
Only pass 2 CRVAL values to WCSSPH2XY W. Landsman June 1995
Don't subscript CTYPE W. Landsman August 1995
Understand reversed X,Y (X-Dec, Y-RA) axes, W. Landsman October 1998
Consistent conversion between CROTA and CD matrix W. Landsman October 2000
No special case for tangent projection W. Landsman June 2003
Work for non-WCS coordinate transformations W. Landsman Oct 2004
Use CRVAL reference point for non-WCS transformation W.L. March 2007
Use post V6.0 notation W.L. July 2009