NAME

fgauss -- Convolve a FITS image with an elliptical Gaussian function

USAGE

fgauss infile outfile sigma

DESCRIPTION

FGAUSS convolves the FITS image in \fIinfile\fR with the Gaussian kernel specified by \fIsigma\fR, \fIratio\fR, \fItheta\fR and \fInsigma\fR and places the convolved images in \fIoutfile\fR FITS file. Out of bounds pixels are computed using the algorithm specified by \fIboundary\fR. The FITS image to be smoothed can be in either the primary array or in an image extension. The resulting smoothed image will be placed in either the output files primary array or in an image extension depending on three things; the location of the image in the input file, the value of the copyprime flag and the value of the copyall flag.

If the input image is in the primary array then the resulting image will always be placed in the primary array. In this case, if the copyprime flag is \fIyes\fR then all the keywords from the input primary header will be copied to the output primary header, otherwise only the required keywords will be placed in the output primary header. Setting the copyall flag will cause all the FITS extensions in the input FITS file to be copied over to the output FITS file.

If the input image is in an image extension the the resulting image will be placed in one of three places depending on the values of the copyprime and copyall flags. If the copyprime flag is \fIno\fR the the resulting smoothed image will always appear in the primary array. If the copyall flag is also set then all extension will be copied to the output FITS file and the input primary array will be stored in the image extension that the image was originally stored in the input FITS file. If the copyprime flag is \fIyes\fR then the primary array of the input FITS file will always be copied over to the primary array of the output FITS file. In this case, if the copyall flag is \fIno\fR then the smoothed image will be stored in the first extension of the output FITS file, otherwise the smoothed image will appear in the same extension in both the input and output FITS file and all other extensions will be copied to the output FITS file.

The Gaussian kernel has an elliptical cross-section and Gaussian profile and is defined mathematically as follows.

     1. Circularly Symmetric Gaussian Function
     
         ratio = 1   theta = 0.0   N = normalization factor
     
         G = N * exp (-0.5 * (r / sigma) ** 2)
     
     2. Elliptical Gaussian Function (Theta = 0, 90 or 180)
     
         sigmax = sigma   sigmay = ratio * sigmax   N = normalization factor
     
         A = cos (theta) ** 2 / sigmax ** 2 - sin (theta) ** 2 / sigmay ** 2
     
         B = 0.0
     
         C = sin (theta) ** 2 / sigmax ** 2 + cos (theta) ** 2 / sigmay ** 2
     
         z = A * x ** 2 + B * x * y + C * y ** 2 
     
         G = N * exp (-0.5 * z)
     
     3. Elliptical Gaussian Function (Arbitrary Theta)
     
         sigmax = sigma   sigmay = ratio * sigmax   N=normalization factor
     
         A = cos (theta) ** 2 / sigmax ** 2 - sin (theta) ** 2 / sigmay ** 2
     
         B = -2 * (1 / sigmax ** 2 - 1 / sigmay ** 2) * sin (theta) * cos (theta)
     
         C = sin (theta) ** 2 / sigmax ** 2 + cos (theta) ** 2 / sigmay ** 2
     
         z = A * x ** 2 + B * x * y + C * y ** 2 
     
         G = N * exp (-0.5 * z)

PARAMETERS

infile [file name]

Input FITS file and extension number to be convolved with the elliptical Gaussian function.

outfile [file name]

Output FITS file where the convolved image will be written.

sigma [double]

The sigma of the Gaussian function in pixels along the direction \fItheta\fR of the major axis of the Gaussian function.

(ratio = 1.0) [double]

The ratio of the sigma in the minor axis direction to the sigma in the major axis direction of the Gaussian function. If \fIratio\fR is 1 the Gaussian function is circular.

(theta = 0.0) [double]

The position of the major axis of the elliptical Gaussian function. \fITheta\fR is measured counter-clockwise from the x axis and must be between 0 and 180 degrees.

(nsigma = 4.0) [double]

The distance along the major axis of the Gaussian function at which the kernel is truncated in \fIsigma\fR pixels.

(boundary = "nearest") [double]

The algorithm used to compute the values of the out of bounds pixels. The options are:

nearest

Use the value of the nearest boundary pixel.

constant

Use a constant value.

reflect

Generate a value by reflecting around the boundary.

wrap

Generate a value by wrapping around to the opposite side of the image.

(constant = 0.0) [double]

The constant for constant-valued boundary extension.

(datatype = "-") [character]

The data type for the resulting smoothed image. A "-" or " " results in storing the smoothed image with the same data type as the original input image. The letter B or b represents byte, the letter I or i represents short integer, the letter J or j represents integer, the letter E or e represents single precision reals and the letter D or d represents double precision reals.

(nullval = 0.0) [double]

Value to substitute for any undefined pixels within the image.

(copyprime=yes) [boolean]

If true, all of the primary header keywords, the primary array (if any) and all of the extension keywords of the first input file are copied to the output file. If false, only minimal headers are created.

(copyall=no) [boolean]

If true, all other extensions, including the primary array are copied to the output file.

(clobber = no) [boolean]

If true, an existing output file of the same name will be overwritten.

EXAMPLES

1. Convolve an image with a circular Gaussian function of sigma 2.0, and size 4.0 sigma using nearest neighbor boundary extension.

         cl> fgauss image.fits gimage.fits 2.0

2. Convolve an image with an elliptical Gaussian function whose sigma in the major and minor axis direction is 2.0 and 1.5 respectively, and whose position angle is 45 degrees, using wrap around boundary extension.

         cl> fgauss img.fits gimg.fits 2.0 ratio=0.75 theta=45.0 bound=wrap

3. Convolve an image in the 3rd extension with a circular Gaussian function of sigma 2.0, and size 4.0 sigma using nearest neighbor boundary extension and store the resulting image in the primary array.

         cl> fgauss imgx.fits[3] gimg.fits 2.0 copyprime=no

BUGS

SEE ALSO

fboxcar

CATEGORY