Imaging Flanking Fields

Overview

The ability to image flanking fields will be introduced in CASA v. 3.0.0. For the purposes of this guide, flanking fields are the locations of neighboring radio sources that can contribute significant, confusing signal to your science target. The confusing signal is introduced through sidelobes, which are local maxima of the point response function, or beam, of the array.

There are two angular scales that are important to the discussion: the synthesized beam and the primary beam. The synthesized beam is the effective angular resolution of the array and is determined by the longest baselines, or telescope separations. The primary beam is the angular resolution of a single telescope in the array, and is determined by its physical diameter. The response to radio sources outside the primary beam is very poor, and, unless they are peculiarly bright, such radio sources will contribute negligible confusing signal. The scale of both synthesized beam and the primary beam can be approximated by the Rayleigh criterion.

To achieve the highest quality image of the science target, any neighboring radio sources should be imaged and deconvolved to reduce the sidelobe contribution. One approach is to image a very large field around your science target; however, long baseline configurations require small pixels to sample the synthesized beam, and the number of pixels required to map the primary beam becomes inconveniently large. To illustrate, the following table lists the image sizes required to map the VLA primary beam at observing frequency 1.4 GHz and for the four primary array configurations. The primary beam at this frequency is ~32'.

! Configuration	A	B	C	D
! Clean Beam(")	1.4	4.6	15	50
! Pixels	4200	1300	380	120

--Jgallimo 15:06, 12 November 2009 (UTC)