Ask an Astronomer

Over the last year or so, several people have asked me questions at workshop meetings. Some of them I've known the answer to, but most I've had to go away and find out about. Chances are, if one person has come up with a question, others would also be interested in the answer.

So, with this in mind, I thought that a new section in the newsletter might be a good idea. If you have a question (preferably an astronomy-related one!), however simple or difficult, just drop me an email and I'll see what I can find out.

Send your questions to me at m dot argo at physics dot org or write them down and pass them to me at meetings.

How does wind affect the telescope? Alan Banks (February 2005)

"I know the big dish can be parked if the weather is going to be particularly windy, but I assume like any telescope vibration can effect the steadiness of an image. Do the best results come when the wind is near zero, and at what wind speed do results become seriously impaired?"

When it is very windy you may notice the Lovell telescope is "parked" pointing directly up at the zenith. This is really a structural issue as the surface of the telescope presents quite a large sail area. If the wind catches it a lot of strain is put on the girders and the bearings which hold it up. This is a major limiting factor on how big radio telescopes can be built. Some telescopes are constructed inside giant "radomes" to minimise the effects of wind on the dish. Again this is not practical for very large telescopes, although it was suggested that one be constructed around the Lovell and painted yellow to represent the Sun in the Spaced Out project!

For the telescopes controlled from Jodrell Bank, the wind alarms in the control room start to sound at speeds of around 30 mph. At this point observations are restricted to elevations above about 40 degrees. Once the wind speed increases to about 40 mph the telescopes are generally parked.

In terms of image stability, you have to consider the "beam" of the telescope. If you picture the telescope as a transmitter rather than a receiver, the beam is the cone of radio waves that would be sent out into space. The two situations are in fact mathematically identical. If your telescope is moving around enough that the beam is shifting more than the resolution (which is about 11 arcminutes for the Lovell at a typical wavelength of 21 cm) then your image will appear blurred, just as in an optical telescope. A (very rough) back-of-the-envelope calculation shows that for the Lovell telescope a movement of about 15 cm at the focus box would be required to shift the beam by one full beamwidth on the sky. This is quite a big shift, and not the sort of movement that is allowed to occur.

Another issue arises when you link up telescopes as interferometers. If the separations between your telescopes are large enough then, even if they all point at the same object, you have different bits of atmosphere above each antenna. This causes problems as different amounts of atmosphere at different temperatures (for example) affect the signal differently. One effect of this is that observations at certain frequencies can only be made near solar minimum since solar activity affects the ionosphere which, in turn, affects the path of radio waves passing through it.

Does heavy rain and in particular hail cause a problem?

In short, yes. Snow can cause problems as well, for more than one reason. As it settles on the surface it prevents some of the radio waves from reflecting up to the receivers, so the signal drops. If the telescope is pointing low enough and enough snow collects on the surface it can also put strain on the gears. During heavy snow some years ago, a telescope that had been in one position for a while was moved to point at another patch of sky. As it moved, the snow that had collected on what now became the upper side of the surface fell of and hit the bottom edge. The combined mass of all the snow was too much for the gears and the telescope fell over, destroying part of the mount in the process.

Do passing trains effect the results?

Not by vibrations, no. But the increased radio frequency interference (RFI) from all the mobile phones is a problem.

With thanks to Andy Howson