## 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