Subroutine of GETPSF to perform a one-star least-squares fit
Part of the DAOPHOT PSF photometry sequence
PKFIT, f, scale, x, y, sky, radius, ronois, phpadu, gauss, psf,
errmag, chi, sharp, Niter, /DEBUG
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
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.
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.
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
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.
No parameter checking is performed
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