DAOERF

       Calulates the intensity, and derivatives, of a 2-d Gaussian PSF

       Corrects for the finite size of a pixel by integrating the Gaussian
       over the size of the pixel.    Used in the IDL-DAOPHOT sequence.

       DAOERF, XIN, YIN, A, F, [ PDER ]

       XIN - input scalar, vector or array, giving X coordinate values
       YIN - input scalar, vector or array, giving Y coordinate values, must
               have same number of elements as XIN.
       A - 5 element parameter array describing the Gaussian
               A(0) - peak intensity
               A(1) - X position of peak intensity (centroid)
               A(2) - Y position of peak intensity (centroid)
               A(3) - X sigma of the gaussian (=FWHM/2.345)
               A(4) - Y sigma of gaussian

       F - array containing value of the function at each (XIN,YIN)
           The number of output elements in F and PDER is identical with
               the number of elements in X and Y

       PDER - 2 dimensional array of size (NPTS,5) giving the analytic
               derivative at each value of F with respect to each parameter A.

       Written: W. Landsman                October, 1987
       Converted to IDL V5.0   W. Landsman   September 1997

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