How Does a Radio Interferometer Work?

Question: I just toured the VLA today with a friend (both of us are engineers) and both of us were wondering how radio telescopes work. We’re curious at the 25-words-or-less level, as opposed to a deep and full understanding.

Cameras capture visible light, a certain area of sky, on film or a CCD chip. In other words, a lot of pixels are captured at once.

Radio telescopes, I would think, would capture only one pixel at only one frequency (per antenna). To get a second pixel, wouldn’t you have to move the antenna slightly? How do you keep the array aligned so that they’re both looking at the same star/galaxy? What kind of resolution can you get from one dish and then also all of them? Thanks very much!  — Paul

Answer: In fact, radio telescopes operate in two different ways.  The simplest way for a radio telescope to make astronomical measurements is for it to operate in a stand-alone mode.  In this mode the radio telescope collects signals from space just like a single optical telescope.  At the focal point of the radio telescope one can place either a detector with a single pixel or one with multiple pixels, much like the way one places CCD detectors at the focal points of optical telescopes.  The second way to make astronomical measurements with a radio telescope, but this time to make very high resolution measurements, is to connect two or more of them electronically while having them point in the same direction of the sky.  This is how the VLA works, by using a technique called interferometry.  Since a radio telescope interferometer can combine measurements from each of the pairs of antennas in an array simultaneously, it can make a very high resolution measurement of a specific point in the focal plane of the radio telescope.  By combining many pairs of antennas one can create an image of a specific point in the sky.  For a very nice description and graphic showing how a radio telescope interferometer works, see Brian Koberlein’s “How Does Interferometry Work” page.  The graphic which shows how a radio interferometer image is built-up is a particularly nice description of how radio interferometric images are made.

The spatial resolution that one can attain with a radio interferometer is a function of how widely separated the antennas in the array are placed.  Since we can place antennas at separations as large as the diameter of the Earth (and farther for radio telescopes in orbit), one can attain spatial resolutions as high as a few micro-arcseconds.  This compares to the few milli-arcsecond resolution images that one can get with optical and infrared telescopes.  The spatial resolution that a single radio telescope can attain is a function of its diameter and the frequency at which the measurements are made.  A typical single antenna radio telescope can attain  a spatial resolution of a few arcseconds.

Jeff Mangum