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).

CATEGORY:
Astrometry

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