imcntr -- locate the center of a stellar image
imcntr input x_init y_init
- The list of images which contain the star to be centered.
- The approximate column coordinate as a starting point for the centering.
- The approximate line (row) coordinate as a starting point for the centering.
- cboxsize = 5
- The size of the extraction box to be used during the centering process.
Given the approximate coordinates of the center of an object, (x_init, y_init), IMCNTR will compute a more accurate center using the algorithms described in the Kitt Peak publication "Stellar Magnitudes from Digital Images" under the Mountain Photometry Code section. Briefly, this algorithm computes the sum of all the rows and the sum of all the columns in the extraction box. These are called "marginal distributions". The center in x (column value) is then the center of gravity of the row marginal, and the center in y is the center of gravity of the column marginal. If the resultant x or y center value deviates from the original input approximate starting points by more than 1 pixel, the process is repeated once more around the new center. Only one iteration is attempted to avoid runaway if a bright star is nearby.
Because the centers are computed independently for x and y, the result may be considered inferior to a true two-dimensional centering algorithm. Nevertheless, in practice the results appear to be very usable.
The value for the box size should be an odd value. If chosen too large, nearby objects will affect the result. If too small, the center will be poorly defined.
1. The following example locates the center of a star near (123, 234)
in 3 images.
cl> imcntr m92red,m92blu,m92grn 123 234
The routine will probably fail if the desired object is within 2 or 3 pixels of the image boundary.