AIPS HELP file for GUARD in 31DEC24

As of Thu Apr 25 11:13:14 2024

HELP SECTION

GUARD
Type:  Adverb (Real array, 2 elements)
Use:   Specifies the width of the outer portion of the UV plane
       which is to receive no data during the gridding process.
       Subscript 1 refers to the U axis and 2 to the V axis and
       the units are in fractions of the maximum U and V on the
       plane being gridded.
Null Value:  -1 => use only a few cells to avoid mathematical
                   error
              0 => use 0.3 * sqrt(taper weight 0.3 from edge)
Tasks:
  IMAGR.......Wide-field and/or wide-frequency Cleaning / imaging
  SCIMG.......Full-featured imaging plus self-calibration loop with
              editing
  SCMAP.......Imaging plus self-calibration loop with editing
  UVMAP.......Makes images from calibrated UV data.
Procedures:
  LINIMAGE....Build image cube from multi-IF data set.  GUARD is
              passed to IMAGR.

Discussion:
   AIPS imaging allows the user to make a UV-plane grid with data
   samples up to and off the edge of the plane.  This is a dangerous
   thing to do, however, since it introduces high-frequencies into the
   image plane that cannot be deconvolved properly.  In those cases in
   which the user cannot include all the data in the gridded plane
   (because a larger CELLSIZE or smaller IMSIZE is desired), the user
   may discard data automatically simply by allowing it to fall off
   the uv plane.  The resulting images will not deconvolve well.  The
   user may limit the data more severely, using either a circular
   limit from adverb UVRANGE or a rectangular limit from GUARD.  The
   default value for GUARD is NOT enough to prevent high-frequency
   effects in the deconvolution.  Values more like 0.4 are needed for
   that in many cases.

EXPLAIN SECTION