From mmd@star.ucl.ac.uk Tue Sep 21 15:02:47 2004 Date: Mon, 27 Nov 2000 15:26:19 +0000 (GMT) From: Michael Dworetsky To: Roger Wesson Subject: Re: the FAQ for guides These are both FAQ questions; responses below. Mike Dworetsky, Department of Physics | "Television? The word is half & Astronomy, University College London | Greek, half Latin. No good can Gower Street, London WC1E 6BT UK | come of it." email: mmd@star.ucl.ac.uk | --C. P. Scott (1846-1932) On Mon, 27 Nov 2000, Roger Wesson wrote: > My guiding session last Friday threw up a couple of questions which were > rather tricky to answer. Firstly, when I told the first group that CCDs > have to be cooled to -70C for maximum efficiency, someone asked me how > they were cooled down. I couldn't really answer that. I see from the > webpage it's a '2-stage Peltier cooling device' but I'm really none the > wiser for that! The cooling is by a 2 stage (I think!) Peltier device for the ST6 and ST8 used on the Meades, which reaches 30 C below ambient temperature. The Radcliffe and Allen CCDs have a 3 stage cooling device that reaches -70 C (about 80 below ambient). (This is one reason they are much more expensive to purchase; size is the other reason). A Peltier device is simply a solid state heat engine and running current through it transfers heat from one end to the other. This reduces T until the amount of energy being consumed to run the heat engine can no longer compensate for the amount of heat leaking in to the cooled end. In a multistage device, extra power is used to cool the hot end of the innermost chips in the heat cycle, allowing them to reach lower temperatures. It isn't a very efficient way to cool anything but it is very compact and reliable, which is why we use it. The reason for cooling is to reduce thermal noise (agitation of the lattice which injects electrons into the photosensitive wells). Ideally we would like to work at liquid nitrogen temperatures around -120 C but LN and students are not a safe mix and the provision of a constant supply (filling twice a day including weekends) is expensive in manpower and liquid. This is normal procedure at mountantop observatories. In our Radcliffe/Allen CCDs we get about 1 noise count per minute. If we could use LN, we could get down to 1 count per hour. > > Also, one of the ladies in the second group asked me if the observatory > video would be OK to give to a 10-year old. I know she asked you when I > didn't really know, but I don't know what the answer was? It probably would be all right technically, as there isn't much more involved than basic astronomy, but the video is meant to show prospective university students what practical astronomy work is like at ULO. A 10-yr old might find the recruitment parts of it boring, but much of it would be interesting to a general audience. Always explain that it is a recruiting video sent to sixth forms but that much of it might be interesting to a general audience. We have a satisfaction or money back policy, and it never hurts to mention this, in case they feel that the video isn't what they expected. -- Thanks, Roger. Mike D.