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 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.