NAME:
    getannul
 PURPOSE: (one line)
    Extract an annulus from a 2-D array.
 DESCRIPTION:

 CATEGORY:
    CCD data processing
 CALLING SEQUENCE:
    Getannul, image, xcen, ycen, inradius, outradius, data

 INPUTS:
    image       : CCD image array.
    xcen,ycen   : Center of annulus.
    inradius    : Radius of inner circle.
    outradius   : Radius of outer circle.

 OPTIONAL INPUT PARAMETERS:

 KEYWORD PARAMETERS:
 OUTPUTS:
    data        : Array of data from the image array.
    idx         : Optional output of 1D indices (in image) of data.

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
    Written by Doug Loucks, Lowell Observatory, April, 1993.
    Reference: getannul.c by Marc Buie.
    This version is completely different than the previous version and
    somewhat faster.
    5/12/93, DWL, Fixed a bug involving fringe pixels.  In certain cases,
                  the determination of the row indices led to a bad array
                  subscript range.
    1/5/94, DWL, Added optional output parameter idx.
    95/06/12, MWB, Fixed bug, base needed to be LONG for large sky apertures