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.help imcntr Dec85 proto
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NAME
imcntr -- locate the center of a stellar image
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USAGE
imcntr input x_init y_init
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PARAMETERS
.ls input
The list of images which contain the star to be centered.
.le
.ls x_init
The approximate column coordinate as a starting point for the centering.
.le
.ls y_init
The approximate line (row) coordinate as a starting point for the centering.
.le
.ls cboxsize = 5
The size of the extraction box to be used during the centering process.
.le
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DESCRIPTION
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.
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EXAMPLES
1. The following example locates the center of a star near (123, 234)
in 3 images.
.sp 1
.nj
.nf
cl> imcntr m92red,m92blu,m92grn 123 234
.fi
.ju
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BUGS
The routine will probably fail if the desired object is within 2 or 3 pixels
of the image boundary.
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SEE ALSO
pradprof
.endhelp
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