Search Parameters

Several parameters used by imstar to decide whether a pixel is part of a star in an image can be set either on the command line or in the libwcs/lwcs.h header file. In the header file, they are
#define NSTATPIX        25      /* Stats are computed for +- this many pixels */
#define ISTATPIX        10      /* Stats are computed every this many pixels */
#define MAXWALK         20      /* Farthest distance to walk from seed */
#define BURNEDOUT       0       /* Clamp pixels brighter than this, if > 0 */
#define NITERATE        5       /* Number of iterations for sigma clipping */
#define STARSIGMA       5.0     /* Stars must be this many sigmas above mean */
#define BORDER          10      /* Ignore this much of the edge */
#define MAXRAD          20      /* Maximum radius for a star */
#define MINRAD          1       /* Minimum radius for a star */
#define MINPEAK         10      /* Minimum peak for a star */
#define MINSEP          10      /* Minimum separations for stars */

The same parameters can be reset on the command line by adding
or nspix=int
for the number of pixels used to compute mean pixel value and standard deviation for detection.
or ispix=int
for the frequency at which the mean and standard deviation are recomputed.
maxwalk=int for the maximum number of pixels the program can move from a pixel detected to be a local peak toward a brighter one.
burnedout=num for the pixel value at or above which the pixel is assumed to be saturated, and thus an unreliable peak.
or niter=int
for the number of iterations to sigma-clip each computation of a local mean.
starsig=num for the number of standard deviations above the local mean a pixel must be to signal a possible star detection.
border=int for the number of pixels in from each edge of the image to ignore in the star search.
maxrad=int for the maximum radius in pixels to be used to find the center of a star.
minrad=int for the minimum radius in pixels of a possible
minpeak=num for the minimum value a pixel must have to qualifty as a possible star. This test precedes the test for whether the pixel value meets the starsig test.
minsep=int for the minimum distance in pixels a possible star must be from an already-discovered star.

Last updated 17 June 1999 by Doug Mink

[WCS Programs] Telescope Data Center [imstar]