First, the task tfinder was run on the images. It is given the epoch and the center coordinates of the image, the intended orientation of the CCD (in our case north at the bottom, and east to the right), and a good guess on the CCD (``plate'') scale (in our case 0.51 ''/pixel). It then uses the Hubble Guide Star Catalog (GSC; on CD-ROM) to find the GSC stars present in the given CCD image. The GSC stars are drawn as rings on top of the image, which is displayed in SAOimage. The user does the coarse centering, after which the task does the real centering, and outputs a catalog image.tab, in which the following columns are of particular interest: RA_DEG, DEC_DEG, X_CENTER, and Y_CENTER.
A note should be made about the two coordinate systems involved. (x,y) are the ideal plate coordinates, in radians. They are given in the columns RA_DEG and DEC_DEG, with a conversion from degrees to radians. The x-axis points towards east, and the y-axis points towards north, making it a left-handed system. (x',y') are the CCD coordinates, in pixels. They are given in the columns X_CENTER and Y_CENTER. In our case, the x'-axis points approximately towards east, and the y'-axis points approximately towards south, making it a usual right-handed system. See Figure .
The task tastrom takes the catalog
produced by tfinder and computes
a 6 coefficient co-called plate solution of the form
1PLATE SOLUTION - EMPIRICAL, 6-COEFF -------------- ------------------ X,Y = IDEAL PLATE COORDINATES (RADIANS) X = -0.001269 Y = 0.001301 + 0.2458474E-05 * XMEAS + -0.2458925E-05 * YMEAS + -0.1837236E-07 * YMEAS + -0.1840916E-07 * XMEASWe identify aij and bi as
Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997
Bo Milvang-Jensen (email@example.com)