From: USER2@aaocbn.oz.au (David Mar)
Newsgroups: alt.callahans
Subject: DM's Telescope Postcard - 4
Date: 29 Aug 92 10:12:00 GMT

**NB**
Please do NOT send mail to this account. If you want to mail me, my address 
is:  mar@physics.su.oz.au
******

Mike Callahan simply begins reading, knowing that those interested patrons 
will listen:

Friday, 28 August, 16:22.

Heya all,

It has been an afternoon of playing with the spectrograph and the data 
collected over the past two nights. Jason has located something which looks 
like a problem with the spectrograph, although not too serious.

One of the standard stars we observed last night turned out to have a 
strange sky profile. To explain: we place the image of our object on to a 
narrow slit, with the spectrum dispersed out perpendicular to the slit. 
This way we get a spectrum of the object and also a spectrum of the sky 
next to it. Contrary to common sense, the night sky is not absolutely dark 
away from stars. There are always stray photons scattered off air 
molecules, and it is important to measure how much light is coming from the 
atmosphere so that it can be subtracted from the spectrum of the object 
proper.

Anyway, the sky spectrum near this star seems to drop in intensity towards 
one end of the slit, which is strange. Jason thinks that it may be caused 
by a partial obstruction of the light beam somewhere in the spectrograph. 
What it means in practice is that we have to be careful not to position a
quasar at that end of the slit when we are observing (the object is placed 
at one end, not in the middle), otherwise we will lose some light.

It's now half an hour to dinner. A hearty breakfast followed by dinner four 
hours later is a little hard to cope with sometimes - especially after only 
six hours sleep for three nights running. But the dessert tonight is 
chocolate pudding, which is always a treat when you come observing!

As for the weather, it actually looks good for once. Just a few clouds to 
the east, and clear skies the rest of the way. It's so changeable, though, 
that you have to be careful at all times. I've heard that it's 
thunderstorming in Sydney at the moment. I can smile because the weather 
here blows in from the other direction.

19:47 - The dinner was yummy. Fish and chips, like every Friday up here. 
But that's cool, since I like fish. The conversation was even better.

There were two older astronomers at dinner with Jason and me tonight: Don 
and Vince. They were regaling us with stories of observing runs up here on 
Siding Spring from the "good old days". There are half a dozen or so 
telescopes up here, most a good deal smaller than the AAT. Don told us 
about the time he arranged some liquid nitrogen to cool down one of the 
detectors at one of the smaller telescopes (most detectors work better if 
they are made very cold). There was an enormous vat of the stuff in the 
telescope dome and, as it evaporated, it displaced most of the rest of the 
air in the building. The result: several astronomers actually passed out 
from lack of oxygen that night! They've been more careful ever since then.

And Vince related the story of an astronomer who used to observe naked in 
the summer. As if that wasn't funny enough in itself, this particular 
astronomer was responsible for the most amusing logbook entry Vince had 
seen. What the fellow did was to improvise a cooling device with large tins 
of dry ice strapped to various parts of the light detector. Now remember, 
the guy observed naked. One of his logbook entries reads:
3am - stopped observing because of excessive condensation on exposed parts.

But my own observing is proceeding right now. Jason took some test 
exposures of bright stars during twilight to help diagnose the problem 
which has been plaguing this spectrograph for some months now. ANd now that 
it i fully dark, I am running on one of my quasars. I'll spend about 9000 
seconds on it, then move to my main target, which hasn't risen just yet. It 
looks like being an excellent night.

Saturday, 29 August, 02:37 - I've been quite busy tonight. Not with 
observing, although we've been doing a good deal of that, but with 
processing the data collected over the first two nights. It's a fairly 
tedious process, and I may as well get some of it done while I'm up here.

We've observed about 18 000 seconds but are now looking down the barrel of 
a rather nasty cold front. It is sweeping thick cloud in from the west. The 
outside temperaure has plummeted to about 4 degrees Celsius. It is hard to 
tell if we can still see the quasar, but we are still taking data. We'll 
probably be stopped by cloud for the rest of the night fairly soon.

03:52 - Tiredness is setting in once again. The weather has actually held 
out so far. We are still observing, and hopefully we will make it all the 
way to twilight.

I've reached an impasse in the reduction of last night's data. The next 
thing I have to do is the wavelength calibration, which requires some 
concentration over a fair bit of time. And I don't feel like concentrating 
on anything right now.

04:40 - Well, we're still observing, but we could be cut off by cloud any 
minute. We may already be - it is very hard to tell because the quasar is 
so faint that you can't see the photons arriving until you integrate them 
over tens of minutes. And going outside to look is unreliable, since you 
can't tell exactly where the telescope is pointed.

We're definitely giving up as soon as we hit twilight. It is useless trying 
anything then with these clouds around. Here's looking forward to another 
six hours sleep.

- Danger Mouse.
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