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The tests that Anthony and I did many years ago show that active cooling is the only way to get optics to within 0.5 degrees of ambient.
You need the optics to be at this temperature to reduce thermal distortion of the object being imaged. Boundary layer controls are a poor attempt at managing thermal distortion and do have an effect but no where near that of cooled optics.
Mirrors cooled naturally will take many hours to come to within 2 degrees of ambient. My C14 takes until near dawn for natural cooling to take effect and even then there will be some thermal distortion.
Mirrors can be rapidly cooled and will performed flawlessly once cooled. They may need further cooling and then after each cooling there will be a period of settling usually 5-10 minutes and then you will be able to undertake imaging or high resolution viewing for an extended period.
I have used a hair dryer on my C14 corrector plate for 8 years and never had a single crack. So I am dubious about causing a fracture. That said, I do wave the dryer around vigorously to prevent me heating one area more than another. In dewy conditions I will get around 15 minutes of imaging done before I need to hit the corrector with more heat.
Anyway, these are just my views and having spent a lot of time discussing this with Anthony over the years I know his views too.
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