spot on vespine!.
for normal viewing, you might get away with a 1-2 degree difference, but for imaging at 1000x - 1600x, then you must have your difference at 1/2 degree.
Leaving your scope out for a few hours is a good start, but if the temps keep dropping, then the mirror has to catch up.rough rule of thumb my 10" cools at 2 degrees an hour unassisted.
take out a thermometer and just watch the ambient drop and how quickly it drops, when it settles, allow time for mirror to catch up and bob's your aunty.
I actively cool mine based upon birds method
http://www.acquerra.com.au/astro/equipment/13/ and
http://www.acquerra.com.au/astro/cooling/4/
Here is mine
http://www.iceinspace.com.au/forum/s...ead.php?t=8805
I get at least a 5 degree an hour loss of heat from the mirror this way.
I pull it out at sundown, plug it in and then come out to a settled temp scope