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PKFIT

PKFIT

Name


PKFIT

Purpose


Subroutine of GETPSF to perform a one-star least-squares fit

Explanation


Part of the DAOPHOT PSF photometry sequence

Calling Sequence


PKFIT, f, scale, x, y, sky, radius, ronois, phpadu, gauss, psf,
errmag, chi, sharp, Niter, /DEBUG

Inputs


F - NX by NY array containing actual picture data.
X, Y - the initial estimates of the centroid of the star relative
to the corner (0,0) of the subarray. Upon return, the
final computed values of X and Y will be passed back to the
calling routine.
SKY - the local sky brightness value, as obtained from APER
RADIUS- the fitting radius-- only pixels within RADIUS of the
instantaneous estimate of the star's centroid will be
included in the fit, scalar
RONOIS - readout noise per pixel, scalar
PHPADU - photons per analog digital unit, scalar
GAUSS - vector containing the values of the five parameters defining
the analytic Gaussian which approximates the core of the PSF.
PSF - an NPSF by NPSF look-up table containing corrections from
the Gaussian approximation of the PSF to the true PSF.

Input-output


SCALE - the initial estimate of the brightness of the star,
expressed as a fraction of the brightness of the PSF.
Upon return, the final computed value of SCALE will be
passed back to the calling routine.

Outputs


ERRMAG - the estimated standard error of the value of SCALE
returned by this routine.
CHI - the estimated goodness-of-fit statistic: the ratio
of the observed pixel-to-pixel mean absolute deviation from
the profile fit, to the value expected on the basis of the
noise as determined from Poisson statistics and the
readout noise.
SHARP - a goodness-of-fit statistic describing how much broader
the actual profile of the object appears than the
profile of the PSF.
NITER - the number of iterations the solution required to achieve
convergence. If NITER = 25, the solution did not converge.
If for some reason a singular matrix occurs during the least-
squares solution, this will be flagged by setting NITER = -1.

Restrictions


No parameter checking is performed

Revison History


Adapted from the official DAO version of 1985 January 25
Version 2.0 W. Landsman STX November 1988
Converted to IDL V5.0 W. Landsman September 1997



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