PSF_GAUSSIAN

       Create a 1-d, 2-d, or 3-d Gaussian with specified FWHM, center

       Return a point spread function having Gaussian profiles,
       as either a 1D vector, a 2D image, or 3D volumetric-data.

       psf = psf_Gaussian( NPIXEL=, FWHM= , CENTROID =
                     [ /DOUBLE, /NORMALIZE, ST_DEV=,  NDIMEN= ] )
 or:
       psf = psf_Gaussian( parameters, NPIXEL = ,NDIMEN = )

       NPIXEL = number pixels for each dimension, specify as an array,
               or just one number to make all sizes equal.

       CENTROID = floating scalar or vector giving position of  PSF center.
               default is exact center of requested vector/image/volume.
               The number of elements in CENTROID should equal the number of
               dimensions.    **The definition of Centroid was changed in
               March 2002, and now an integer defines the center of a pixel.**
       /DOUBLE  = If set, then the output array is computed in double precision
               the default is to return a floating point array.
       FWHM = the desired Full-Width Half-Max (pixels) in each dimension,
               specify as an array, or single number to make all the same.
       NDIMEN = integer dimension of result: either 1 (vector), 2 (image), or
                3 (volume), default = 2 (an image result).
       /NORMALIZE causes resulting PSF to be normalized so Total( psf ) = 1.
       ST_DEV = optional way to specify width by standard deviation param.
                Ignored if FWHM is specified.
       XY_CORREL = scalar between 0 and 1 specifying correlation coefficient
               Use this keyword, for example, to specify an elliptical
               Gaussian oriented at an angle to the X,Y axis.   Only valid
               for 2-dimensional case.
 INPUTS (optional):
       parameters = an NDIMEN by 3 array giving for each dimension:
                       [ maxval, center, st_dev ],  overrides other keywords.

       (1) Create a 31 x 31 array containing a normalized centered Gaussian
       with an X FWHM = 4.3 and a Y FWHM = 3.6
       IDL> array = PSF_GAUSSIAN( Npixel=31, FWHM=[4.3,3.6], /NORMAL )
       (2) Create a 50 pixel 1-d Gaussian vector with a maximum of 12,
          centered at  pixel 23 with a sigma of 19.2
       IDL> psf = psf_gaussian([12,23,19.2],npixel=50)

       function Gaussian()

       To improve speed, floating underflow exceptions are suppressed (using
       the MASK=32  keyword of CHECK_MATH() rather than being flagged.

       Written, Frank Varosi NASA/GSFC 1991.
       Converted to IDL V5.0   W. Landsman   September 1997
       Suppress underflow messages, add DOUBLE keyword. **Modified centroid
       definition so integer position is pixel center** W. Landsman March 2002
       Allow use of the ST_DEV (not STDEV) keyword W. Landsman Nov. 2002

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