Name

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      GROUP

Purpose

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      Assign stars with non-overlapping PSF profiles into distinct groups

Explanation

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      Part of the IDL-DAOPHOT sequence

Calling Sequence

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      GROUP, X, Y, RCRIT, NGROUP

Inputs

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      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
          floating point
      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.

Outputs

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      NGROUP - integer vector, same number of elements as X and Y,
              giving a group number for each star position. Group
              numbering begins with 0.

Method

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      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.

Procedures Used

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      REM_DUP()

Revision History

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      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