How do we use an array of antennas to make a radio picture of the sky?
- The bigger a telescope, the finer detail we can see on the sky.
- Instead of one very big telescope we can use an array of many small antennas.
- How do we use this to make a picture of the sky?
This could be a bit ambitious, but…
an exercise could be to place antennas interactively with a mouse on the “ground” (use script to read grid coordinates), simulate an observation and see what sort of picture comes out. A bit like the “Virtual Radio Interferometer” (VRI) but perhaps a simpler interface if feasible. Of course the details are too complex to go into, but can look at some pretty patterns… could use e.g. scripts in MIRIAD to generate images if possible. Will look into this more.
Could use model data. Picture of the “real” sky brightness distribution. [Picture of how the radiation pattern from the source appears on the ground (FT of sky)?]
Each antenna receives a signal. Signals from each pair of antennas get multiplied. Show lines connecting pairs of antennas to illustrate “interferometers”. [Picture of how the radiation is sampled by the pairs of elements (u,v plane)?]
We then (using software) do some maths with these measurements to get an image. We call this the “dirty image” - how it looks depends where the antennas were placed. Because we know where the antennas were placed, we can “clean” the image (again using software to help) to get a result that looks (hopefully) very close to the real sky. The distances between the antenna pairs determine how much detail we can see.
The more antennas we have, the more sensivity we have - analogous to putting more buckets out to catch more rain! That allows us to see fainter objects in the sky, just like looking through binoculars or an optical telescope allows you to see more, fainter stars. We can also look longer to see fainter sources, like setting a camera to a long exposure for night-time photos. [I think trying to include something on Earth rotation synthesis is too much…]