ASTSN2XY

Name

astsn2xy

Purpose

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

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).

Calling Sequence

astsn2xy,xi,eta,info,x,y

Inputs

xi - Tangent plane coordinates (radians)
eta - Tangent plane coordinates (radians)
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 - xi terms flag array
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

FULL - Flag, if set indicates the full transformation should be used.
Only the tags needed must be provided.
ARCSEC - Flag, if set indicates that the input xi,eta values are in
arcseconds. Otherwise they are treated as radians.

Outputs

x - X coordinate in image
y - Y coordinate in image

Keyword Output Parameters

Common Blocks

Side Effects

Restrictions

Procedure

Modification History

2009/11/02 - Written by Marc W. Buie, Southwest Research Institute
2009/11/12, MWB, added ARCSEC keyword
2011/03/21, MWB, drastic improvement to starting guess. Works on anything
now. Didn't work before if x,y was rotated by 45 degrees.
2012/05/24, MWB, changed linear case to use optical axis, not reference.
2015/08/12, MWB, changed back to use xcref,ycref. Optical axis tags
are now removed from all tools. Use ref instead.