idlastro / FITS Astrometry and Calibration: CONS_DEC

[Source code]

NAME
CONS_DEC
PURPOSE
Obtain the X and Y coordinates of a line of constant declination
EXPLANATION
Returns a set of Y pixels values, given an image with astrometry, and 
     either
(1)  A set of X pixel values, and a scalar declination value, or
(2)  A set of declination values, and a scalar X value
Form (1) can be used to find the (X,Y) values of a line of constant
declination.  Form (2) can be used to find the Y positions of a set
declinations, along a line of constant X.
CALLING SEQUENCE
Y = CONS_DEC( DEC, X, ASTR, [ ALPHA ])
INPUTS
DEC - Declination value(s) in DEGREES (-!PI/2 < DEC < !PI/2).  
        If X is a vector, then DEC must be a scalar.
X -   Specified X pixel value(s) for line of constant declination 
        If DEC is a vector, then X must be a scalar.
ASTR - Astrometry structure, as extracted from a FITS header by the
        procedure EXTAST
OUTPUT
Y   - Computed set of Y pixel values.  The number of Y values is the
        same as either DEC or X, whichever is greater.
OPTIONAL OUTPUT
ALPHA - the right ascensions (DEGREES) associated with the (X,Y) points
RESTRICTIONS
Implemented only for the TANgent, SIN and CAR projections
NOTES
The algorithm (and notation) is based on AIPS Memo 27 by Eric Greisen,
with modifications for a coordinate description (CD) matrix as 
described in Paper II of Greisen & Calabretta (2002, A&A, 395, 1077).
These documents are available from 
http://www.cv.nrao.edu/fits/documents/wcs/wcs.html
REVISION HISTORY
Written, Wayne Landsman  STX Co.                          April 1988
Use new astrometry structure,     W. Landsman    HSTX     Jan. 1994
Use CD matrix, add SIN projection   W. Landsman  HSTX     April, 1996
Converted to IDL V5.0   W. Landsman   September 1997
Fix case where DEC is scalar, X is vector   W. Landsman RITSS Feb. 2000
Fix possible sign error introduced Jan. 2000   W. Landsman  May 2000
Work for the CARee' projection W. Landsman   May 2003