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CNTRD

CNTRD

## Purpose

Compute the centroid of a star using a derivative search

## Explanation

CNTRD uses an early DAOPHOT "FIND" centroid algorithm by locating the
position where the X and Y derivatives go to zero. This is usually a
more "robust" determination than a "center of mass" or fitting a 2d
Gaussian if the wings in one direction are affected by the presence
of a neighboring star.

## Calling Sequence

CNTRD, img, x, y, xcen, ycen, [ fwhm , /KEEPCENTER, /SILENT, /DEBUG
EXTENDBOX = ]

## Inputs

IMG - Two dimensional image array
X,Y - Scalar or vector integers giving approximate integer stellar
center

## Optional Input

FWHM - floating scalar; Centroid is computed using a box of half
width equal to 1.5 sigma = 0.637* FWHM. CNTRD will prompt
for FWHM if not supplied

## Outputs

XCEN - the computed X centroid position, same number of points as X
YCEN - computed Y centroid position, same number of points as Y,
floating point
Values for XCEN and YCEN will not be computed if the computed
centroid falls outside of the box, or if the computed derivatives
are non-decreasing. If the centroid cannot be computed, then a
message is displayed and XCEN and YCEN are set to -1.

## Optional Output Keywords

/SILENT - Normally CNTRD prints an error message if it is unable
to compute the centroid. Set /SILENT to suppress this.
/DEBUG - If this keyword is set, then CNTRD will display the subarray
it is using to compute the centroid.
EXTENDBOX = {non-negative positive integer}. CNTRD searches a box with
a half width equal to 1.5 sigma = 0.637* FWHM to find the
maximum pixel. To search a larger area, set EXTENDBOX to
the number of pixels to enlarge the half-width of the box.
Default is 0; prior to June 2004, the default was EXTENDBOX= 3
/KeepCenter = By default, CNTRD finds the maximum pixel in a box
centered on the input X,Y coordinates, and then extracts a new
to skip then step of finding the maximum pixel, and instead use
a box centered on the input X,Y coordinates.

## Procedure

Maximum pixel within distance from input pixel X, Y determined
from FHWM is found and used as the center of a square, within
which the centroid is computed as the value (XCEN,YCEN) at which
the derivatives of the partial sums of the input image over (y,x)
with respect to (x,y) = 0. In order to minimize contamination from
neighboring stars stars, a weighting factor W is defined as unity in
center, 0.5 at end, and linear in between

## Restrictions

(1) Does not recognize (bad) pixels. Use the procedure GCNTRD.PRO
in this situation.
(2) DAOPHOT now uses a newer algorithm (implemented in GCNTRD.PRO) in
which centroids are determined by fitting 1-d Gaussians to the
marginal distributions in the X and Y directions.
(3) The default behavior of CNTRD changed in June 2004 (from EXTENDBOX=3
to EXTENDBOX = 0).
(4) Stone (1989, AJ, 97, 1227) concludes that the derivative search
algorithm in CNTRD is not as effective (though faster) as a
Gaussian fit (used in GCNTRD.PRO).

## Modification History

Written 2/25/86, by J. K. Hill, S.A.S.C., following
algorithm used by P. Stetson in DAOPHOT.
Allowed input vectors G. Hennessy April, 1992
Fixed to prevent wrong answer if floating pt. X & Y supplied
W. Landsman March, 1993
Convert byte, integer subimages to float W. Landsman May 1995
Converted to IDL V5.0 W. Landsman September 1997
Better checking of edge of frame David Hogg October 2000
Avoid integer wraparound for unsigned arrays W.Landsman January 2001
Handle case where more than 1 pixel has maximum value W.L. July 2002
Added /KEEPCENTER, EXTENDBOX (with default = 0) keywords WL June 2004
Some errrors were returning X,Y = NaN rather than -1,-1 WL Aug 2010

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