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## 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

## 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
If the input A and D are arrays with 2 or more dimensions,
they will be converted to a 1-D vectors.
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
.PV1 - Vector of projection parameters associated with longitude axis
.AXES - 2 element integer vector giving the FITS-convention axis
numbers associated with astrometry, in ascending order.
Default [1,2].
.REVERSE - byte, true if first astrometry axis is Dec/latitude
.COORDSYS - 1 or 2 character code giving coordinate system, including
'C' = RA/Dec, 'G' = Galactic, 'E' = Ecliptic, 'X' = unknown.
.RADECSYS - String giving RA/Dec system e.g. 'FK4', 'ICRS' etc.
.EQUINOX - Double giving the epoch of the mean equator and equinox
.DATEOBS - Text string giving (start) date/time of observations
.MJDOBS - Modified julian date of start of observations.
.X0Y0 - Implied offset in intermediate world coordinates if user has
specified a non-standard fiducial point via PV1 and also
has set PV1_0a =/ 0 to indicate that the offset should be
applied in order to place CRVAL at the IWC origin.
Should be *added* to the IWC derived from application of
CRPIX, CDELT, CD to the pixel coordinates.
.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').

## Common Blocks

BROYDEN_COMMON - Used when solving for a reverse distortion tranformation
(either SIP or TGV) by iterating on the forward transformation.

## Procedures Used

CGErrorMsg (from Coyote Library)
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
Allows use of Version 2 astrometry structure & optimised for
large input arrays. Wrap test for cylindrical coords. J. P. Leahy July 2013
Wrap test failed for 2d input arrays
T. Ellsworth-Bowers/W.Landsman July 2013
Tweaked to restore shape of arrays on exit JPL Aug 2013.
..and make them scalars if input is scalar JPL Aug 2013
Iterate when forward SIP coefficients are supplied but not the reverse
coefficients. Don't compute poles if not a cylindrical system
W. Landsman Dec 2013
Evaluate TPV distortion (SCAMP) if present W. Landsman Jan 2014
Support IRAF TNX projection M. Sullivan U. of Southhamptom Mar 2014
No longer check that CDELT[0] differs from 1 W. Landsman Apr 2015

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