Primary Beams and Primary Beam Correction

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. The model of the ATCA primary beam is described in Mark Wieringa & Mike Kesteven's memo (AT Memo 39.3/024).

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 enclosed 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, use:

PUTHD Set pbfwhm of
value=gaus(1200) Set the primary beam FWHM to 20 arcmin (1200 arcsec).

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

LINMOS Image to primary beam correct.
out=lmc.pbcorr Output, corrected, image.
options Leave unset.

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