NAME:
	PSF_GAUSSIAN

 PURPOSE:
	Create a 1-d, 2-d, or 3-d Gaussian with specified FWHM, center
 EXPLANATION:
	Return a point spread function having Gaussian profiles,
	as either a 1D vector, a 2D image, or 3D volumetric-data.

 CALLING SEQUENCE:
	psf = psf_Gaussian( NPIXEL=, FWHM= , [/NORMALIZE, /ST_DEV,  )
 or:
	psf = psf_Gaussian( parameters, NPIXEL =  )

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

 OPTIONAL KEYWORDS:

	NDIMEN = dimension of result: 1 (vector), 2 (image), or 3 (volume),
		default = 2 (an image result).

	FWHM = the desired Full-Width Half-Max (pixels) in each dimension,
		specify as an array, or single number to make all the same.

	CENTROID = pixels numbers of PSF maximum ( 0.5 is center of a pixel ),
		default is exact center of requested vector/image/volume.

	STDEV = optional way to specify width by standard deviation param.

	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

	/NORMALIZE causes resulting PSF to be normalized so Total( psf ) = 1.

 INPUTS (optional):

	parameters = an NDIMEN by 3 array giving for each dimension:
			[ maxval, center, stdev ],  overrides other keywords.

 CALLED BY:
	FILTER_IMAGE, hsi_polar_clean
 EXAMPLE:
	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

 EXTERNAL CALLS:
	function Gaussian

 HISTORY:
	Written, Frank Varosi NASA/GSFC 1991.
	Converted to IDL V5.0   W. Landsman   September 1997