Assign stars with non-overlapping PSF profiles into distinct groups
Part of the IDL-DAOPHOT sequence
GROUP, X, Y, RCRIT, NGROUP
X - vector, giving X coordinates of a set of stars.
Y - vector, giving Y coordinates of a set of stars.
If X and Y are input as integers, then they will be converted to
RCRIT - scalar, giving minimum distance between stars of two
distinct groups. Stars less than this distance from
each other are always in the same group. Stetson suggests
setting the critical distance equal to the PSF radius +
the Fitting radius.
NGROUP - integer vector, same number of elements as X and Y,
giving a group number for each star position. Group
numbering begins with 0.
Each position is initially given a unique group number. The distance
of each star is computed against every other star. Those distances
less than RCRIT are assigned the minimum group number of the set. A
check is then made to see if any groups have merged together.
Written W. Landsman STX April, 1988
Major revision to properly merge groups together W. Landsman Sep 1991
Work for more than 32767 points W. Landsman March 1997
Converted to IDL V5.0 W. Landsman September 1997
Avoid overflow if X and Y are integers W. Landsman Feb. 1999