ASTXY2SN

Name

astxy2sn

Purpose

Astrometry conversion from image (x,y) to tangent plane ($\xi$,$\eta$)

Description

This transformation can either be based on a simple linear transformation
with rotation from the celestial sphere to linear CCD chip coordinates.
Or, it can use a full astrometric solution (including linear). The
simple linear transformation is only an approximate treatment and
will not work for very large fields. The best results will come from
the full-up treatment.

Calling Sequence

astxy2sn,x,y,info,xi,eta

Inputs

x - X coordinate in image
y - Y coordinate in image
info - Transformation information held in an anonymous structure. There
are two different groups of tags that can appear. The original
simple linear transformation needs the following tags:
pscale - Plate scale (arcsec/pixel).
rang - Rotation angle of image (radians).
xflip - -1 if image flipped in X, 1 if not.
yflip - -1 if image flipped in Y, 1 if not.
The full-up transformation requires a different set of tags:
renormfac - normalization factor
cxi - xi transformations coefficients (x,y -> xi)
ceta - eta transformations coefficients (x,y -> eta)
terms - string array with list of terms to include
prot - Rotation to get to standard coordinates (radians)
Both types need the following.
xcref - X center of image.
ycref - Y center of image.
raref - Right ascension of center of image (tangent plane).
decref - Declination of center of image (tangent plane).

Optional Input Parameters

Keyword Input Parameters

ARCSEC - Flag, if set the returned values are in arcseconds.
FULL - Flag, if set indicates the full transformation should be used.
Only the tags needed must be provided.

Outputs

xi - Tangent plane coordinates (radians)
eta - Tangent plane coordinates (radians)

Keyword Output Parameters

DX - Internal transformed x value (normalized for /full)
DY - Internal transformed y value (normalized for /full)

Common Blocks

Side Effects

Restrictions

Procedure

The following applies to the /FULL conversion.
The solution is a two-step conversion. You start with raw coordinates
that typically relate to the original position in the native coordinate
system for the device. The first transformation step is to convert
to another system related to the first by a translation and rotation.
e.g. xp = ( (x-xc)*cos(prot) + (y-yc)*sin(prot) ) / renormfac
yp = ( -(x-xc)*sin(prot) + (y-yc)*cos(prot) ) / renormfac
The astrometric transformation then maps xp,yp to ra,dec.

Modification History

2009/11/02 - Written by Marc W. Buie, Southwest Research Institute
2009/12/02, MWB, consolidate xiterms and etaterms tags into one (terms)
2010/01/13, MWB, added ARCSEC option
2016/04/07, MWB, fixed a bug in the rotation matrix for the pure linear case