It's common knowledge that for the best views/images, the mirror has to be at the same temperature as ambient.
People like Bird, and later David Pretorius, have gone to great lengths to
actively cool their mirror using peltier coolers and fans to blow cold air onto the mirror to help the temperature come down quicker than just blowing ambient air.
I've had the
cooling fan on my scope for about 2 months now, just blowing ambient. I've also got the temperature sensor to measure ambient vs mirror temperature, and even just a normal fan blowing air onto the mirror does make a difference and helps the mirror cool quicker than just leaving it out on its own.
But if the ambient temperature keeps dropping, the mirror will never catch up. I've only ever seen the mirror get to within 0.8° of ambient once ambient has stabilised, but it usually sits 1-1.5° above.
Last night, the ambient didn't drop for quite a while - it was sitting on around 12°. I started imaging at around 6:30pm and as usual, the seeing was ordinary with large "waves" going across the preview screen. Over the next half hour, the mirror temperature actually came down to within 0.3° of ambient - the first time it's been that close.
And then it happened - the image stabilised incredibly. The seeing still caused the image to vibrate and distort a bit, but the large waves had completely disappeared. The image was much steadier and there was a much larger percentage of crisp frames.
It showed me for the first time, without a doubt, how important it is for the mirror to be
at (or as close to) ambient temperature. What had been blamed on bad seeing was in part due to the boundary layer causing distortions in the image.
I had thought that a difference of around 1° was close enough, but now realise that it has to be even closer. Last night the change happened at around 0.3-0.4° difference, when the air in the tube stabilised and the image became incredibly still.
This experience has now fast-tracked my plans to actively cool my mirror, as Anthony and DP have done.