AIPS HELP file for GUARD in 31DEC22
As of Wed Nov 29 12:27:46 2023
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
0 => use 0.3 * sqrt(taper weight 0.3 from edge)
IMAGR.......Wide-field and/or wide-frequency Cleaning / imaging
SCIMG.......Full-featured imaging plus self-calibration loop with
SCMAP.......Imaging plus self-calibration loop with editing
UVMAP.......Makes images from calibrated UV data.
LINIMAGE....Build image cube from multi-IF data set. GUARD is
passed to IMAGR.
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.