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Why Not Always Observe at Maximum Resolution with a Radio Telescope Interferometer?

-- | May 18, 2014

Question: Best angular resolution is achieved with the NRAO dishes at their maximum separation. I do not understand why less than maximum separation is often used. Can you please give some examples of projects done at NRAO which actually benefited from less than best angular resolution.

I am an astronomer, although I work at visible light wavelengths with no familiarity with radio telescopes. But I should be able to understand the technical language of what you are observing far beyond the solar system.

Thank you for your service.

Sincerely yours,
Frederick Pilcher

Answer: As you correctly point out, radio telescope interferometers (or even optical interferometers) attain their highest spatial resolution when they are at their maximum separation. To understand how the separation of a telescope array elements relates to the spatial scales to which it is sensitive, consider first a single telescope. The maximum resolution attained by a single telescope is inversely proportional to the diameter of its light collecting surface (dish surface for a radio telescope, mirror for an optical telescope). You have an instrument that is sensitive to source sizes that are proportional to your spatial resolution, but nothing larger or smaller than that. If you then connect two telescopes to make an interferometer, the maximum resolution attained is inversely proportional to the separation of the two telescopes. You are now sensitive to source sizes which are proportional to your interferometer resolution, but not to source sizes which are larger or smaller than that. This is sometimes referred to as the “missing flux” problem in radio astronomy. Your radio interferometer measurement tells you about structure in your source which is equal to the resolution attained with its maximum antenna separation, but nothing about the source structure which would be measured by antenna separations smaller than your shortest antenna separation.

There are many experiments where one would like to know what the more “extended” flux in an object looks like, which warrants an interferometer configuration which uses shorter telescope configurations. These include studies of extended structure in star formation regions in our own galaxy, or studies of the emission from supernova remnants.

Jeff Mangum

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