Obtain the X and Y coordinates of a line of constant right ascension
Return a set of X pixel values given an image with astrometry,
(1) a set of Y pixel values, and a scalar right ascension (or
(2) a set of right ascension values, and a scalar Y value.
In usage (1), CONS_RA can be used to determine the (X,Y) values
of a line of constant right ascension. In usage (2), CONS_RA can
used to determine the X positions of specified RA values, along a
line of constant Y.
X = CONS_RA( RA, Y, ASTR, [ DEC] )
RA - Right Ascension value in DEGREES (0 < RA < 360.). If Y is a
vector, then RA must be a scalar
Y - Specified Y pixel value(s) for line of constant right ascension
If RA is a vector, then Y must be a scalar
ASTR - Astrometry structure as extracted from a FITS header by the
X - Computed set of X pixel values. The number of elements of X
is the maximum of the number of elements of RA and Y.
DEC - Computed set of declinations (in DEGREES) for X,Y, coordinates
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 Calabretta & Greisen (2002, A&A, 395, 1077).
These documents are available from
Implemented only for the TANgent, SIN and CARtesian projections
Written, Wayne Landsman STX Co. April, 1988
Algorithm adapted from AIPS memo No. 27 by Eric Greisen
New astrometry structure
Converted to IDL V5.0 W. Landsman September 1997
Added SIN projection W. Landsman January 2000
Fix possible sign error introduced Jan. 2000 W. Landsman May 2000
Work for the CARee' projection W. Landsman May 2003
For TAN projection ensure angles between -90 and 90 W. Landsman Jan 2008