wcs2dr -- Convert WCS header information into drizzle parameters for image registration
wcs2dr image refim
This task uses the world coordinate information in the headers of a list of images and reference image to deduce the parameters which must be given to the "drizzle" task to register the input images with the reference.
Only the linear part of the WCS is used. This is assumed to consist of eight values - CRVAL1/2, CRPIX1/2 and the four elements of the CD matrix (CD1/2_1/2). It is further assumed that the two images are in TAN projection (although nor necessarily with the same reference point).
The transform is deduced by starting with a unit square at the center of the input image. The position of each corner of this square is then rigorously transformed, via equatorial coodinates, into a pixel position on the reference grid. A check is made to see whether the result is significantly non-square (ie, where there are differences in the X and Y scale or skew is present) and if so the user is informed. The shift of the center of the pixel, along with the measured rotation and scale change are then displayed in the form which the drizzle task needs. The user is also informed the values for the drizzle parameters shft_un, shft_fr and align which must be used.
If "files" is a list of images which are to be aligned with the reference image "ref" then suitable drizzle parameters can be deduced using: cl> wcs2dr @files ref style="para" # drizzle.data="test1" drizzle.shft_fr="output" drizzle.shft_un="output" drizzle.align="center" drizzle.xsh= 552.733 drizzle.ysh= 482.749 drizzle.scale= 1.009 drizzle.rot= -2.871 # drizzle.data="test2" drizzle.shft_fr="output" drizzle.shft_un="output" drizzle.align="center" drizzle.xsh= 532.142 drizzle.ysh= 498.043 drizzle.scale= 1.009 drizzle.rot= -2.871 # drizzle.data="test3" drizzle.shft_fr="output" drizzle.shft_un="output" drizzle.align="center" drizzle.xsh= 519.552 drizzle.ysh= 378.566 drizzle.scale= 1.009 drizzle.rot= -2.871 or alternatively: cl> wcs2dr test ref style="line" # Image xsh ysh scale rot # Note: shft_fr="output" shft_un="output" align="center" test1 552.733 482.749 1.009 -2.871 test2 532.142 498.043 1.009 -2.871 test3 519.552 378.566 1.009 -2.871 cl> where in this case "files" contains: test1 test2 test3
It is, as one would expect, very quick!
1. Only the linear WCS is considered. 2. Only the TAN projection can be handled.
drizzle, traxy, tranback