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BASIC CONCEPTS OF CALIBRATION FOR ATCA DATA

 

In this section I give some background on polarimetry and antenna based calibration. Ideally, this section would come after you have loaded and edited your data. However, some of the concepts discussed here are required background for some of the discussion of the task that loads your data into AIPS (ATLOD).

More extensive reading on the topics of this section can be found in the NRAO synthesis workshop books and in ``Interferometry and Synthesis in Radio Astronomy'' (Thompson, Moran and Swenson). Note however that the NRAO books tend to focus on telescopes with circularly polarized feeds (such as the VLA) when they discuss polarimetry. The ATCA has linearly polarized feeds, which has some thorny ramifications for users as discussed in the last two subsections below.

A synthesis telescope such as the ATCA records the Fourier transform of the intensity distribution of the sky. The ensemble of complex numbers recorded by a synthesis array is referred to as the observed visibilities. The definition of a single visibility is a bit murky and is really software-implementation specific. In its minimalist form, a visibility is one complex number, representing one sample of the Fourier transform of the sky measured by one baseline in one polarization in one integration in one spectral channel. In its broadest sense, and one often used in AIPS jargon, it is a collection of complex numbers representing all the channels and polarizations in one integration for one baseline.

Because the cosmic signals must traverse the ionosphere, the atmosphere, and a complicated train of electronics, the observed visibilities often bear little resemblance to the desired Fourier transform of the source. The purpose of calibration is to determine the corrections necessary to produce a good estimate of the true visibilities, which can then be Fourier transformed to the image domain.



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Next: Antenna-based gain calibration Up: Analysis of ATCA Data Previous: How to use the

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