Level 2 Help for PSF

INP is the name of the image file from which the PSF will be extracted,
using the AREA parameter.

OUT is the name of the output file into which the properly formatted PSF
will be written.

SIZE is the standard Vicar2 parameter containing:
 
  (Starting Line, Starting Sample, Number of Lines, Number of Samples)
 
Note that in this program it refers only to the output file, and therefore
the first two elements are ignored.  This parameter is included only for
unformity in Vicar2 application program interface.

See HELP SIZE.

See HELP SIZE.

NL specifies the size of the image in the line direction, i.e., the
number of lines in the image.

NS specifies the size of the image in the sample direction, i.e.,
the number of pixels per line in the image.

AREA specifies the area in the input file (INP) from which the PSF will be
extracted, after subtraction of the background DN value.  It has the
same structure as the SIZE parameter, i.e.:
 
  (Starting Line, Starting Sample, Number of Lines, Number of Samples).
 
Note that the last two elements should be less than or equal to the
corresponding elements in SIZE (or NL and NS) for meaningful results.

If NOSHIFT is specified, then the final PSF will remain strictly inside
this area after determination of the centroid.  Otherwise, the final
PSF will be of the same size as specified by AREA, but centered on the
centroid of the function.

This parameter controls whether the PSF will be allowed to shift outside
the area specified by the AREA parameter after determination of its
centroid.

If SHIFT is specified, then the final PSF will be of the size specified 
by the AREA parameter, but centered on the centroid of the function. (If
part falls outside the image, the size is reduced accordingly.)  This is
the normal choice when the user does know the exact location of the PSF
in the input.

If NOSHIFT is specified, then the area of the PSF remains that specified
by AREA, but the quartering is done about the centroid, so that the four
quarters may be unequal.  This choice is useful if the location of the
PSF in the input is accurately known and it is important that data
outside this area be excluded.