Observing at the GMRT

Astronomers often get to travel to exotic, far-flung places all in the name of science. To non-astronomers these may seem like free holidays but there often isn't much chance to do any sight-seeing or sunbathing as you are usually miles from anywhere, up a mountain. Being awake through the night, and asleep during the day certainly doesn't allow much chance for tourist activities either, although it does mean that you can email people at a reasonable hour back home (at least if you go sufficiently far east/west). Doing radio astronomy doesn't help too much as you can usually find things to observe during both day and night.

Megan is currently on an observing trip to the Giant Metrewave Radio Telescope (GMRT) in India so I thought I would drop her a line and find out what she was up to and what the GMRT was like. I caught her just as a huge two-day science festival had started at the telescope, and thousands of people had descended on the site with mobile phones just to annoy her.

What is the GMRT?

The GMRT (run by the National Centre for Radio Astrophysics, or NCRA) consists of thirty antennas arranged in a Y-shaped configuration near the village of Khodad, India. The central square kilometre contains 14 antennas, randomly arranged, while the rest are distributed in three approximately equal arms. The antennas themselves do not have solid surfaces like radio telescopes such as the Lovell. Instead the reflecting surface is made up of many thin wire "ropes". This works because at the long wavelengths we are using (21 cm and longer) the wires have the same effect as a solid surface. The use of wire instead of panels makes the telescopes lighter, cheaper and more stable in high winds.

What observations are you doing and why do you need to use the GMRT?

We are observing the first half of a sample of nearby Seyfert galaxies, looking for absorption caused by clouds of neutral hydrogen gas in front of the core of the galaxy. The GMRT is designed to work at low frequencies (1420 MHz and below) so it is perfect for doing this sort of project. The GMRT is nearer to the equator (latitude 19°rees; North) so we can see some objects here that never rise above the horizon for a more northern telescope like MERLIN. There is also less man-made interference here than there would be with MERLIN, as the site is a long way from the nearest large town.

One of our first sources was NGC4594 (M104), which is also known as the Sombrero galaxy.

Is it a busy site? Are there many observers there?

There are a few other observers here at the moment. For the last few days there were two Spanish astronomers using the telescope in the early morning, and an astronomer from the NRAO observing after us in the evening. About 100 people work here at the telescope site, most of them as telescope operators, engineers, technicians and other support staff. Most of the GMRT astronomers are based on the University campus in Pune, two hours drive south of the telescope, and spend a few days a month at the telescope making observations or helping visitors with theirs.

What is a typical day/night like?

Because this is a radio telescope, an observing run can be scheduled at any time of the night or day, depending on when the objects you want to observe are above the horizon. The length of a run can vary too, our shortest run was only four hours while the longest was 13. Before the run starts you have to create a command file which tells the telescope what to do. It lists all the commands to tell the telescope how long to spend on each source before moving on to the next. You also have to select some other parameters such as the frequency band in which you want to observe, the frequencies of two of the local oscillators and the bandwidth over which you want to observe. Once the run starts there isn't a lot to do as the command file does most of the work, so most of the run is spent in the computer room adjacent to the main control room which, in order to minimise interference caused by the computers, is situated inside a giant vault.

What is there to do between observations or when the equipment/telescope breaks?

Luckily it is very unlikely that enough will go wrong with the telescope to make observations impossible! Occasionally there are power cuts which mean that you might lose one (or more) of the outer arms for may be an hour or so, as happened to us on Sunday evening. The central square antennas can be run on a backup generator though, so generally you can continue observing, just with reduced sensitivity and a lower resolution.

The site is a long way from the nearest town, but the landscape is quite impressive so there's plenty to explore. There is a track which winds around the central square which you can walk round in under an hour. It does get very hot during the day though, even in winter, and there are apparently panthers around in the evening, so it's best to go early. The village of Khodad is about three kilometres away and the road passes through some suprisingly green farmland. It's a good walk to do if you want to take some pictures of the central square!

Posted in astro blog by Stuart on Monday 28th Feb 2005 (19:31 UTC) | Permalink
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