iraf.net

NUMBER:	195
MODULE:	daophot.allstar
SYSTEM:	V2.10
DATE:	Sun Jan  3 11:04:59 MST 1993
FROM:	davis

BUG:	In crowded regions allstar could occasionally refuse to 1) fit a
	bright star or group of bright stars, or 2) fail to converge to
	reasonable x,y, and magnitude values for a group of bright
	stars by the time the number of iterations equaled maxiter,
	resulting in a group of stars with very poor subtractions and
	large chi values.

	The problem was caused by a bug in the code which steps through the
	stellar groups, subtracting off the current best fit for all the
	stars, to produce a residuals image. Due to this bug, on occasion
	stars which should have been subtracted from the residuals image
	were not being subtracted. Since the residuals image is used to
	determine the relative errors and weights, which in turn are used
	to control the bad data rejection algorithm, allstar sometimes
	refused to fit stars because the residuals were too big or could
	not converge to a reasonable value. This bug is data dependent
	but is most likely to be a problem if, 1) the stellar detection
	threshold is very low, 2) allstar has to do a lot of regrouping to
	get the groups below maxgroup in size 3) the fitting radius is
	large resulting in very large groups.

STATUS:	This bug is only a problem in 2.10 (not in the external testphot
	package) and has been fixed for the next release of IRAF. There
	is no reliable workaround for allstar, but the nstar task which fits
	fixed groups does not have this problem.