- NAME
MAKE_ASTR
- PURPOSE
Build an astrometry structure from input parameter values
- EXPLANATION
This structure can be subsequently placed in a FITS header with
PUTAST
- CALLING SEQUENCE
MAKE_ASTR, astr, CRPIX =, CRVAL =, [CD = , DELT =, CTYPE =, $
LATPOLE = , LONGPOLE =, PV2 =, NAXIS =, AXES =, PV1 =, $
RADECSYS =, EQUINOX =, DATEOBS =, MJDOBS =]
- OUTPUT PARAMETER
ASTR - Anonymous structure containing astrometry info. See the
documentation for EXTAST for descriptions of the individual
tags
- REQUIRED INPUT KEYWORDS
CRPIX - 2 element vector giving X and Y coordinates of reference pixel
(def = NAXIS/2). VALUES MUST BE IN FITS CONVENTION (first pixel
is [1,1]) AND NOT IDL CONVENTION (first pixel is [0,0]).
CRVAL - 2 element double precision vector giving R.A. and DEC of
reference pixel in DEGREES
- OPTIONAL INPUT KEYWORDS
CD - 2 x 2 array containing the astrometry parameters CD1_1 CD1_2
in DEGREES/PIXEL CD2_1 CD2_2
DELT - 2 element vector giving physical increment at reference pixel
in DEGREES/PIXEL default = [-1.0D, 1.0D]/3600. (1 arcsec/pixel)
CTYPE - 2 element string vector giving projection types, default
['RA---TAN','DEC--TAN']
LATPOLE - Scalar latitude of the north pole, default = +90
LONGPOLE - scalar longitude of north pole
PV2 - Vector of projection parameters associated with latitude axis.
Not required for some projections (e.g. TAN) and optional for
others (e.g. SIN).
Usually a 2 element vector, but may contain up to 21 elements
for the Zenithal Polynomial (ZPN) projection. Corresponds to
the keywords PV2_1, PV2_2... Defaults to 0.0
dded for version 2 astrometry structure:
XES - 2 element integer vector giving the FITS-convention axis
numbers associated with astrometry, in ascending order.
Default [1,2].
AXIS - 2 element integer vector giving number of pixels on each axis
V1 - Vector of projection parameters associated with longitude axis
Elements 4 & 5 (if present) are equivalent to LONGPOLE & LATPOLE
and take precedence if both are specified, i.e. LONGPOLE & LATPOLE
in the structure are forced to agree with PV1.
ADECSYS - String giving RA/Dec system e.g. 'FK4', 'ICRS' etc.
QUINOX - Double giving the epoch of the mean equator and equinox
ATEOBS - Text string giving (start) date/time of observations
JDOBS - Modified julian date of start of observations.
(specify one or other of DATEOBS or MJDOBS)
- NOTES
(1) An anonymous structure is created to avoid structure definition
conflicts. This is needed because some projection systems
require additional dimensions (i.e. spherical cube
projections require a specification of the cube face).
(2) The name of the keyword for the CDELT parameter is DELT because
the IDL keyword CDELT would conflict with the CD keyword
(3) The astrometry structure definition was slightly modified in
July 2003; all angles are now double precision, and the
LATPOLE tag was added. In April 2007 the CRPIX tag was also
changed to double precision.
- REVISION HISTORY
Written by W. Landsman Mar. 1994
Added LATPOLE, all angles double precision W. Landsman July 2003
Use PV2 keyword rather than PROJP1, PROJP2 W. Landsman May 2004
Make .CRPIX tag double precision, change CDELT default to 1"/pixel
W. Landsman April 2007
Default plate scale is now 1"/pixel (not 1 deg/pix) WL Oct. 2010
Oct 2010 change should only apply when CD matrix not given
M. Cushing/W.L. Aug 2011
added v2 parameters; more filling out of defaults; default
LATPOLE changed to 90 (FITS standard) J. P. Leahy Jul 2013