South Pole Diaries 1993/94

3rd February 1994

From Michael Burton.....

Another day of astronomy action at the Pole. The IRPS saw its first astronomical source, the Moon! We scanned up and down from zenith to horizon while waiting for the Moon to transit our scan, and sure enough a strong signal appeared for about 15 minutes in the 4 degrees around declination -13 (ie 13 degrees above the horizon). We had to observe at L because there is simply to much scattered sunlight at shorter wavelengths - we saturate the detector. Even at L there is a 10 fold increase in sky brightness from zenith to horizon, so there is a strongly varying background to contend with, but the Moon came in at about 4 times the local background, so there was no doubting it. A few panics occurred while doing the scans; as the Sun moved round we would get occasional reflections off pieces of metal onto our mirror, and the signal would shoot up. But only when we were pointing near the zenith! I guess we haven't really though about baffling our dewar against sunlight; astronomers aren't usually accustomed to thinking about such considerations!

The most confusing thing about our observations was trying to predict the time. On the vax here there is a program which gives you the Moon's declination and azimuth, but when trying to convert this into a time to observe I'm convinced the program not only has grid north and south the wrong way around, but measures angles north through west rather than east! Maybe the programmer thought they were at the North Pole!?

We also tried hitting Jupiter, about 45 min later than the Moon, but no luck. However that really isn't surprising given that we have a 2mm aperture looking out into perhaps the brightest background you will find on the Planet right now! Incidentally realising this last point has solved our last major problem, the fet-balancing of the detector. We were worried we were still getting excess and variable noise levels on the detector. In one experiment I noticed that the noise seemed to be dependent on the elevation angle of our mirror, something which is clearly impossible! We are getting flux into the dewar, and there is so much radiation if you are pointing to the horizon (as we often did as its easier to check on positions of filter wheels etc). So the secret is to point to zenith, put in the CVF to the long end of the K window and the noise drops to the right levels! It won't be a problem in the winter, but I guess the designers of the IRPS didn't envisage it being used in our current conditions!

Time is starting to run out for me now, and I'm having to get going with the microthermal experiment in earnest. I seem to have got the calibrations done for that, and now come the fun part of tying all the cables up the mast! There are still things to do with the IRPS and mcba keeps sending new software patches and coming up with ideas of how to improve things, but I'll probably have to put them on low priority if I'm to get things finished!

I even took time out to write a few postcards! Its important to get a South Pole postmark, rather than a McMurdo one! The weather is still holding up, and the temperature keeps dropping. -39 right now. Soon it will be that magical number -40 and I'll stop having to always tell my American colleagues the units I'm using when discussing the weather!

Michael