Using celestial world coordinate system information in the image headers, sregister computes a grid of matched pixel coordinates in a reference and input image, uses the coordinate grid to compute the coordinate mapping from the reference to the input image, uses the coordinate mapping to compute the output image grey levels by interpolating in the input image at the position of the mapped reference pixels, and copies the reference image celestial world coordinate system to the output image.
Sregister differs from the related immatch package task wregister in that: 1) it can only register images that have celestial world coordinate systems, e.g. equatorial, whereas wregister can in principal register images with any supported coordinate system and, more importantly, (2) it can register images with different fundamental coordinate systems, e.g fk4 b1950 and fk5 j2000, or which are precessed with respect to each other, e.g. fk5 j1990 and fk5 j2000, which wregister cannot do.
Sregister assumes that the reference and input image celestial coordinate systems are accurate and can be read by iraf. Prospective users should be aware that archive data do not always follow established fits standards, and that for some fits keywords only proposed standards exist. Sregister attempts to conform to the latest draft fits standards for the celestial coordinate keywords RADECSYS, EQUINOX/EPOCH, MJD-OBS/DATE-OBS but these may be subject to change in the future. See the sregister help page for the details of the celestial coordinate systems currently supported by iraf and for the currently required image header keywords.
The coordinate grids and coordinate mappings can be computed without doing the actual image registration, by using the skyxymatch and skymap tasks respectively. These tasks are useful for examining the details of the coordinate mappings and / or determining why sregister failed to register an image set properly.
Skyxmatch, skymap, and sregister are available as part of the external add-on package IMMATCH. To retrieve and install immatch, follow the installation instructions found in the readme file.
Last updated: 12December1995