Quote:
Originally Posted by peter_4059
I agree with you Adam - if you keep the mirror above the dew point temperature then it isn't a problem.
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This is correct. Any part whether it's glass or aluminium left uncovered open to the night sky (clear sky) will drop in temperature quicker than the ambient air. The sky will suck up any heat out of it, the same as the sun would heat it up if left uncovered during the day.You'd burn your hands touching an alu tube that's been left in the full sun although ambient temp might be mild. A fan will blow ambient air on the surface to minimise the temperature difference. A dew shield will in most case also cover the secondary and primary unless you're imaging right up at the zenith and then it's pretty much back to square one. I also wrap my mount with rags at night so it it doesn't get too cold because as soon as the air warms up dew will form on it as the aluminium is freezing.