New Image Wcs / Astrometry Tasks


Three new image world coordinate system (wcs) / astrometry tasks, ccmap, cctran, and ccsetwcs have been added to the image matching package immatch IMMATCH. The main function of these tasks is to compute a plate solution for an image, covert it to an image wcs, and store the wcs in the image header where it may be accessed by other wcs driven tasks. The tasks can also be used to compute simple geometric parameters for images, e.g. plate scale, and to do simple astrometry.

Ccmap computes the plate solution for an image given a text file of matched pixel and celestial coordinates (such as may be generated by the finder package tfinder task for example) and the celestial coordinates of the tangent point (these default to the mean of the input celestial coordinates if not supplied by the user), and optionally stores the solution in the image header. The computed plate solution is written to an output database file for later user by the ccsetwcs and cctran tasks. The mapping task can be run interactively (the default) or non-interactively.

Ccsetwcs creates an image wcs from the computed plate solution or from a list of parameters supplied by the user including the position of the tangent point in pixel and celestial coordinates, the x and y plate scales in arcseconds per pixel, and the image orientation in degrees.

Cctran task uses the computed plate solution to transform from pixel to celestial coordinates and vice versa.

These tasks are not intended to be fully featured astrometry tasks. However the default ccmap parameter settings produce comparable results with the finder package tastrom task's 6-coeff fit to within machine precision. Ccmap and cctran are capable of creating and evaluating higher order polynomial fits to the coordinate data but the form of these functions is somewhat arbitrary. In any case the wcs written to the image header includes only the linear 6-coeff terms of the plate solution, as there is yet no standard way to represent these higher order terms in fits format. Users also need to be aware that the tasks conform to the fits convention of considering the integer coordinates to be the center of a pixel (e.g. x,y coordinates of 1.0,1.0 refer to the center of pixel 1,1), if they import coordinate lists from other sources.

Ccmap, cctran, and ccsetwcs 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.


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Last updated: 17May1996