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. CATEGORY: Astrometry 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