In interferometry, the image formed normally by the imaging and deconvolution steps is a representation of the sky multiplied by the primary beam response of the antennas. The primary beam is typically similar to a Gaussian function, although it also has sidelobes.
MIRIAD
tasks which require knowledge of the primary beam response of a
telescope use built-in models of the responses of various telescopes
(e.g. ATCA, VLA, Hat Creek, WSRT) -- the primary beam model used is
determined by the `pbtype' or `telescop' item or variable
(pbtype, if present, takes precedence over telescop).
Currently these models assume the primary beam is circularly symmetric and time
independent. The task pbplot
produces some information and can
make a plot of the primary beam models.
If you wish to override MIRIAD
's model, or if MIRIAD
does not have a
model of the telescope of interest, you may set the primary beam
associated with an image or visibility dataset to be a particular
type or Gaussian of a
given size. This is done by setting the pbtype item using
puthd
. pbplot
(without any additional parameter) produces
a listing of the primary beam types known to MIRIAD
.
Setting a Gaussian primary beam type differs from setting
other primary beams in that you also must give an additional parameter
exclosed in brackets) giving the
FWHM of the primary beam, in arc
seconds, at the reference frequency (the `reference frequency' for a
visibility dataset is the frequency of the first channel imaged!!). For
example, to set the primary beam to be a Gaussian of the image lmc.map, use:
Although there are a few exceptions (e.g. mfspin and ellint ), MIRIAD 's analysis tasks do not correct for primary beam attenuation automatically. The task to correct an image for the primary beam is linmos . To use linmos for this function, you need only set the input and output dataset names. For example
