Quote:
Originally Posted by bird
Yes, you do :-)
I lived in Armidale for a few years Terry, and I'm sure if you measured the temperature outside from 5pm to midnight you'd see it dropping like a stone at maybe 2C per hour or more. In order to pull enough heat out of large glass mirrors to keep up with that rate of cooling it takes a substantial amount of power.
Baz's climate in Canberra is very similar to Armidale, and about the same overnight temp profile.
Peltiers generate a temperature difference between the hot and cold sides, and as the outside temp drops then so does the inside temp - and the faster you can cool the mirror the better to keep ahead of it.
Ideally you want to cool the mirror to maybe 1C or 1.5C below ambient while the temp is dropping, and then when it levels off you can switch off the cooling. After a few minutes the mirror will equalise throughout the glass to ambient and you're away. But this requires you to have enough cooling capacity to pull ahead of the dropping ambient.
When you look at the amount of poorly insulated metal in these scopes then you realise that they will act as a very low efficiency fridge. If you're lucky then the cold air temp inside the scope will be 4C below the outside temp. Compare this to a nicely insulated esky where you might get 30C difference.
But the low efficiency is a blessing - it means you can't accidentally go too far down the cold side - the worst you can do is overcool the mirror by a few degrees.
cheers, Bird
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Ah the joy of owning an open tube telescope.
Much less of a problem.