Name

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GETPSF

Purpose

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To generate a point-spread function (PSF) from observed stars.

Explanation

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The PSF is represented as a 2-dimensional Gaussian
(integrated over each pixel) and a lookup table of residuals.
The lookup table and Gaussian parameters are output in a FITS
image file. The PSF FITS file created by GETPSF can be
read with the procedure RDPSF. Adapted from the 1986 STSDAS
version of DAOPHOT

Calling Sequence

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GETPSF, image, xc, yc, apmag, sky, [ronois, phpadu, gauss, psf,
idpsf, psfrad, fitrad, psfname, /DEBUG ]

Inputs

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IMAGE - input image array
XC - input vector of x coordinates (from FIND), these should be
IDL (first pixel is (0,0)) convention.
YC - input vector of y coordinates (from FIND)
APMAG - vector of magnitudes (from APER), used for initial estimate
of gaussian intensity. If APMAG is multidimensional, (more
than 1 aperture was used in APER) then the first aperture
is used.
SKY - vector of sky values (from APER)

Optional Inputs

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The user will be prompted for the following parameters if not supplied.
RONOIS - readout noise per pixel, (in electrons, or equivalent photons)
PHPADU - photons per analog digital unit, used to scale the data
numbers in IMAGE into photon units
IDPSF - subscripts of the list of stars created by
APER which will be used to define the PSF. Stars whose
centroid does not fall within PSFRAD of the edge of the frame,
or for which a Gaussian fit requires more than 25 iterations,
will be ignored when creating the final PSF.
PSFRAD - the scalar radius, in pixels, of the circular area within
which the PSF will be defined. This should be slightly larger
than the radius of the brightest star that one will be
interested in.
FITRAD - the scalar radius, in pixels of the circular area used in the
least-square star fits. Stetson suggest that FITRAD should
approximately equal to the FWHM, slightly less for crowded
fields. (FITRAD must be smaller than PSFRAD.)
PSFNAME- Name of the FITS file that will contain the table of residuals,
and the best-fit Gaussian parameters. This file is
subsequently required for use by NSTAR.

Optional Outputs

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GAUSS - 5 element vector giving parameters of gaussian fit to the
first PSF star
GAUSS(0) - height of the gaussian (above sky)
GAUSS(1) - the offset (in pixels) of the best fitting gaussian
and the original X centroid
GAUSS(2) - similiar offset from the Y centroid
GAUSS(3) - Gaussian sigma in X
GAUSS(4) - Gaussian sigma in Y
PSF - 2-d array of PSF residuals after a Gaussian fit.

Procedure

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GETPSF fits a Gaussian profile to the core of the first PSF star
and generates a look-up table of the residuals of the
actual image data from the Gaussian fit. If desired, it will then
fit this PSF to another star (using PKFIT) to determine its precise
centroid, scale the same Gaussian to the new star's core, and add the
differences between the actual data and the scaled Gaussian to the
table of residuals. (In other words, the Gaussian fit is performed
      only on the first star.)

Optional Keyword Input

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DEBUG - if this keyword is set and non-zero, then the result of each
fitting iteration will be displayed.

Procedures Called

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DAOERF, MAKE_2D, MKHDR, RINTER(), PKFIT, STRNUMBER(), STRN(), WRITEFITS

Revison History

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Adapted from the 1986 version of DAOPHOT in STSDAS
IDL Version 2 W Landsman November 1988
Use DEBUG keyword instead of !DEBUG W. Landsman May 1996
Converted to IDL V5.0 W. Landsman September 1997