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First Light - 50 years on

Fifty years ago today, the Lovell telescope made it's first scientific observations of the sky. The telescope, almost (but not quite) complete at the time, was left pointing at the zenith (straight up) and the signal was recorded as the sky, and the Galaxy, drifted overhead. This sort of observation is known as a drift scan. With a little bit of persuasion, I was given a night of telescope time with which to recreate this observation with the modern equipment we use now. Having spent the last few days getting both software and hardware working, everything was finally ready yesterday afternoon.

The telescope currently has an L-band receiver fitted in the focus box so we are observing at a frequency of 1.4 GHz, the right place to observe the important 21-cm line of neutral hydrogen. As well as plugging the signal from the receiver into a stripchart recorder in the (freezing cold!) observing room to get a plot similar to the original, I've also been taking spectra overnight, something that wasn't done during the first observation in 1957.

As I don't happen to have a handy spectrometer lying around, I ended up making a simple one from borrowed bits and pieces. The signal from the telescope is plugged into the scanner (an AOR5000) which has been in use as part of the meteor experiment for the last six months. The scanner is really good for this because it can be controlled via the serial port of a PC. My laptop is sat together with the scanner on the floor by the rack where the signals arrive from the telescope in the observing room. The laptop is running a program which tells the scanner to tune to a particular frequency and take a measurement (actually, it takes several measurements at each freqency and then averages them), then alter the frequency slightly and take another measurement, then do the same all the way accross the bandpass of the signal coming from the rack. The results of each measurement are then written to a file for later analysis.

The plot below shows one of the sythesised spectra taken with this equipment earlier. The signal due to hydrogen in the Galactic plane is the higher points around 1419.25 to 1419.50 MHz. (Honest. It looks far more obvious on the spectrum analyser which is also sampling the signal, but then that is a piece of equipment which was designed for that purpose...)

Spectrum of the Galactic plane

Spectrum of the Galactic plane (click to see it full-size) CREDIT: Megan / JBO

After spending quite a while trying to get this all working I've discovered a few things. Serial cables are not all the same. Some cables that look like serial cables are wired up very strangely. It took a while before I acquired a cable with the correct wiring to let the scanner and computer talk to each other! Software to do this sort of thing doesn't seem to exist either. Learning to use LabView in a period of 24 hours wasn't really going to happen, so the control software is written in perl. I discovered something else as well, when you send a write command to the scanner, you have to do a read (even if you ignore the response) before you do another write, otherwise it doesn't respond.

It was a lot of fun though, and I've learned a lot in a very short space of time. It's not often you actually get to observe overnight as a radio astronomer, and even rarer to get a night of Lovell time! I hope none of the pulsars that would have been looked at overnight did anything interesting...

When I'm a bit more awake (I've been up for nearly 24 hours now), I'll

add some pictures of the observing room and, when I've finished the

experiment, the stripchart recording.

Posted by Megan on Thursday 02nd Aug 2007 (05:53 UTC) | Add a comment | Permalink

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