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Find positive brightness perturbations (i.e stars) in an image


Also returns centroids and shape parameters (roundness & sharpness).
Adapted from 1991 version of DAOPHOT, but does not allow for bad pixels
      and uses a slightly different centroid algorithm.
      Modified in March 2008 to use marginal Gaussian fits to find centroids

Calling Sequence

FIND, image, [ x, y, flux, sharp, round, hmin, fwhm, roundlim, sharplim


image - 2 dimensional image array (integer or real) for which one
wishes to identify the stars present

Optional Inputs

FIND will prompt for these parameters if not supplied
hmin - Threshold intensity for a point source - should generally
be 3 or 4 sigma above background RMS
fwhm - FWHM (in pixels) to be used in the convolve filter
sharplim - 2 element vector giving low and high cutoff for the
sharpness statistic (Default: [0.2,1.0] ). Change this
default only if the stars have significantly larger or
or smaller concentration than a Gaussian
roundlim - 2 element vector giving low and high cutoff for the
roundness statistic (Default: [-1.0,1.0] ). Change this
default only if the stars are significantly elongated.

Optional Input Keywords

      /MONITOR - Normally, FIND will display the results for each star
                only if no output variables are supplied. Set /MONITOR
                to always see the result of each individual star.
/SILENT - set /SILENT keyword to suppress all output display
PRINT - if set and non-zero then FIND will also write its results to
a file find.prt. Also one can specify a different output file
name by setting PRINT = 'filename'.

Optional Outputs

x - vector containing x position of all stars identified by FIND
y- vector containing y position of all stars identified by FIND
flux - vector containing flux of identified stars as determined
by a Gaussian fit. Fluxes are NOT converted to magnitudes.
sharp - vector containing sharpness statistic for identified stars
round - vector containing roundness statistic for identified stars


(1) The sharpness statistic compares the central pixel to the mean of
      the surrounding pixels. If this difference is greater than the
      originally estimated height of the Gaussian or less than 0.2 the height of the
Gaussian (for the default values of SHARPLIM) then the star will be
      (2) More recent versions of FIND in DAOPHOT allow the possibility of
      ignoring bad pixels. Unfortunately, to implement this in IDL
      would preclude the vectorization made possible with the CONVOL function
      and would run extremely slowly.
      (3) Modified in March 2008 to use marginal Gaussian distributions to
      compute centroid. (Formerly, find.pro determined centroids by locating
      where derivatives went to zero -- see cntrd.pro for this algorithm.
      This was the method used in very old (~1984) versions of DAOPHOT. )
      As discussed in more detail in the comments to the code, the centroid
      computation here is the same as in IRAF DAOFIND but differs slightly
      from the current DAOPHOT.

Procedure Calls


Revision History

Written W. Landsman, STX February, 1987
ROUND now an internal function in V3.1 W. Landsman July 1993
Change variable name DERIV to DERIVAT W. Landsman Feb. 1996
Use /PRINT keyword instead of TEXTOUT W. Landsman May 1996
Changed loop indices to type LONG W. Landsman Aug. 1997
      Replace DATATYPE() with size(/TNAME) W. Landsman Nov. 2001
      Fix problem when PRINT= filename W. Landsman October 2002
      Fix problems with >32767 stars D. Schlegel/W. Landsman Sep. 2004
      Fix error message when no stars found S. Carey/W. Landsman Sep 2007
      Rewrite centroid computation to use marginal Gaussians W. Landsman
                Mar 2008
      Added Monitor keyword, /SILENT now suppresses all output
                  W. Landsman Nov 2008
      Work when threshold is negative (difference images) W. Landsman May 2010

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