Visit to the Gauribidanur Radio Observatory
By akarshsimha; Published 07 Jul 2008
It was a really successful and wonderful trip. Probably one of the most fruitful trips organized by BAS so far. The attendees were mostly Engineers from Texas Instruments, which made the trip highly educative. The primary agenda of the trip was to learn, in detail, the functioning of the Radio Telescope, the concepts and electronics behind it - and this purpose was completely met.
We started off from Texas Instruments, Bagmane Tech Park, C.V. Raman Nagar at about 8:30 AM. With a few pick ups on the way, we made it to Gauribidanur pretty fast. On the way, we discussed about radio astronomy, antennas, interferometry, and GPS! We met Dr. Ramesh of IIA, with whom we had been in correspondence, and he extended us excellent hospitality.
We had our tea and sat down in a room which had been converted into a "classroom" by adding a few chairs. Dr. Ramesh briefed us about the basic functioning of a radio telescope and explained various concepts. It was a very fruitful discussion, with many of us coming back with questions after the discussion. Dr. Ramesh explained how radio astronomy is different from optical astronomy, how resolution is a problem at radio wavelengths, and how it could be overcome by interferometry, to become far more powerful than optical astronomy. There was a discussion on the spatial response and frequency response of a single antenna and how one could calculate the response of certain shapes of arrays, and why a T-Shape array was advantageous. Dr. Ramesh also told us how radio astronomy has developed through cases of serendipity in the past, starting with Jansky's discovery of extra-terrestrial radio sources, to Penzias' and Wilson's discovery of the Cosmic Microwave Background and Bell's and Hewish's discovery of radiation from Pulsars. Then, we broke for lunch.
Dr. Ramesh had organized for us, an excellent lunch. We were discussing about random topics like amateur telescope making, polarization of electromagnetic waves, synchrotron emission, software for radio signal analysis and port of certain drivers to Linux!
After that, Dr. Ramesh answered a question about the polarization array at the observatory, and the technique involved in measuring magnetic field strengths on the sun by using the period of circular polarization in synchrotron radiation.
Therafter, we took a "walk" along the telescope. Dr. Ramesh showed us the polarization array, the 34.5 MHz array (which looked very funny, and it was really hard to believe that it was a radio telescope!) and the Gauribidanur Radio Heliograph. He opened up a few of the devices - DC power supplies, an ingenious switchable delay device (used to change the direction in which the telescope is seeing), a 8-channel phase-preserving signal mixer that would preserve the input and output impedance (forgot what the exact technical name for it was), and a final stage amplifier for boosting the signals before sending them to the lab - and showed us their internals. I found the delay device really ingenious - it consisted of coaxial cables as delay devices and a diode based switching mechanism to include or exclude delay cables - and this is what changes the direction in which the telescope is looking!!! All of us were surprised to see the 34.5 MHz telescope - which defeated all our ideas of a telescope - what looked like a clothesline to hang clothes and some fencing were actually the 34.5 MHz telescope and a cleverly positioned "flat mirror" to reflect back 34.5 MHz signals in phase!! We were wondering how it would be if we were trying to sell this array to someone as a "telescope"! Of course, the construction of a radio telescope is entirely different from an optical telescope. While it is easier to relate a dish antenna to a telescope, a collection of dipoles turning out to be a telescope was something surprising for many people.
We then visited the control room and looked at the various instruments used to collect the data from the array and pass it on, finally, to a computer. This involved an RF to IF down converter, a shift register for "group" delays, a correlator and an industrial computer. Dr. Ramesh showed us the Gauribidanur Radiospectrograph in action, and various images taken by the Gauribidanur Radioheliograph (GRH) and frequency vs. time plots made using the Radiospectrograph showing a coronal mass ejection and it's after-shock. He showed us an optical coronograph image of a CME that was also imaged earlier in Radio by the GRH, which clearly showed the advantages of a radioheliograph in mapping the source of CMEs.
Dr. Ramesh then showed us a two-antenna radio telescope for use by amateurs and students, to be distributed as a part of their public outreach programme. It was a relatively simple instrument in comparison with the real observatory telescope, but it really showed cool interference fringes. The electronics were really good - the instrument had a very high signal-to-noise ratio, and the telescope is capable of seeing the brightest radio sources like Cygnus A, Cassiopeia A, Taurus A, Centaurus A etc. We were also considering the possibility of studying lunar occultations of these objects with such an instrument, if such an event occurs.
After some more discussion, some tea, and a photography session at the south array (which makes a really nice subject for photography!), we took leave of the observatory. Discussions on antennae continued between folks from TI and the observatory even on the way back. The journey back was rather silent with half the folks asleep.
Overall it was a wonderful trip!