IDL astronomical library and FITS world coordinates system

The IDL astronomical library provides several routines to manage the coordinates. Extended explanations can be found in the online manual pages for each one of them. Here we mention a few which are called within ISOCONT.

• EXTAST.pro: It is used to extract the world coordinate system from the different FITS headers in an internal IDL structure called ast. It will look first for CD001001 and CDELT, if not found, for CD1_2 , and if CD1_2 is not found for CROTA2. CROTA1 has the same treatment as CROTA2 (bug !).
• PUTAST.pro: It is sed to put in an existing header the internal IDL structure ast. It is the inverse of EXTAST. If there is only a CROTA2, the CROTA2 will be updated. However, if there is both CROTA2 and a matrix in the FITS header, the matrix will be updated but not the CROTA2.
• AD2XY.pro: It gives the pixel coordinates x and y for a point of the sky with known RA and Dec using the astrometry of an image stored in an IDL structure ast.
• XY2AD.pro: It gives the sky coordinates RA and Dec for a given point of the image with known pixel coordinates x,y and for a given image astrometry. It is the inverse of AD2XY.
• HASTROM.pro: It makes an orthogonal transformation of an image and updates the FITS header accordingly. It calls: EXTAST, PUTAST.
• WCSSPH2XY.pro and WCSXY2SPH.pro are called by ad2xy and xy2ad when the projection is not tangential.

These routines had several bugs which we tried to correct. If one has a FITS header with the GSSS astrometry structure a transfromation of it to a more ``standard'' header using the PALOMAR_TO_ISO routine, is strongly encouraged, since ISOCONT will be able to display it properly otherwise. The PALOMAR_TO_ISO routine, creates a new transformation matrix from the GSSS header and recomputes the whole image for this transformation matrix.