I promised earlier this week to talk a little bit more about how radio telescope arrays work, and I try to keep my promises. If anything in this explanation isn’t clear, feel free to ask questions.

One thing astronomers like to have is images that can resolve fine detail in structure. In optical wavelengths, the resolution of images is limited by the atmosphere, which blurs optical light. This is why Hubble is such an amazing telescope and why there is a lot of work on adaptive optics to correct for atmospheric blurring for ground telescopes. At radio wavelengths, the atmosphere doesn’t blur the signals we are trying to detect. This means we are operating in “diffraction-limited” mode. The physics of optics sets an absolute limit to the achievable resolution of a system, which is called the diffraction limit. The diffraction limit depends on the size of your system and the wavelength you want to give at. (The diffraction-limited resolution, in radians, is given by $\theta = \frac{1.22 \, \lambda}{D}$ where D is the size of your telescope and $\lambda$ is the wavelength of observations.)

Now, you might think that since radio telescopes don’t have to worry about atmospheric blurring, they would offer good resolution. However, the wavelength difference between radio and optical wavelengths is substantial. Typical resolution limits achieved for optical telescopes are on the order of one arcsecond. (There are sixty arcseconds in one arcminute and sixty arcminutes in one degree. The moon and sun are both about half a degree across.) Arecibo, the largest single dish radio telescope, has a resolution of about 3.5 arcminutes at a wavelength of 21 cm (which is where we observe). This is almost 200 times worse than the resolution one would have with a small optical telescope. In order to achieve comparable resolution at 21 cm, you would need a radio telescope with a diameter of over 40 km. Clearly, building such a telescope is not feasible.

Radio astronomers wanted a way to achieve good resolutions, though. Since it’s not possible to build a single telescope large enough to achieve high resolution, they came up with the idea of using multiple smaller antennas to synthesize a larger dish. This is the motivation behind the VLA and other radio telescope arrays. Some number of telescopes work together, observing the same source. The signals from the separate telescopes are then combined and processed so that you can produce an image with a resolution set by the largest baseline (separation between any two dishes) rather than the size of the radio dishes.  This means that you don’t need one giant dish; rather, you can have lots of smaller (and cheaper) telescopes spread over a large area.

Now, the details of how you combine the data and process it are quite complicated – that’s part of the reason why I’ve been in Socorro for the past few days. In fact, a Nobel Prize was awarded to Martin Ryle and Tony Hewish for figuring out the details of this process and how to reconstruct an image.  Therefore, I’m not going to go into the details of how this process works.  Rather, I’m going to be happy that it does work.