The logic is obvious if you don't make the wrong assumption that fans are for cooling.
The furnace in your house puts out heat and in forced air systems, or with ceiling fans, their purpose is to redistribute the heat, not cool your house. Insulation is used to retain that heat! Have you ever observed over the roof of house on cold night? Thermals or "bad seeing" is all you see despite the insulation which only slows the release of heat from your warm house if insulation wasn't there at all.
That probably is the most important detail that most still seem to overlook !
Lets go somewhat deeper into 'your house': Heaters produce heat, fan's (or natural circulation) redistribute the heat, insulation keeps that heat inside your house..
In that situation, there is an important detail: The fan's only purpose is to circulate, NOT ! to blow outside air inside, or blow inside air out, as they do in many scopes!
Only when you have a optically closed scope, big or tiny, insulation does work best. And adding (low power) fan's inside, for circulation, can in many cases contribute
somewhat, in better thermal homogenity of the air inside your scope. But still, the biggest 'win' comes from the insulation itself
When your insulated scope is inside your warm house, all optics inside have the same temperature. When you put your scope in the colder outside air,
the insulation and IR reflectivity of the Reflectrix slow down the cooling of the inside in such way, that even the most massive optic (primary mirror) can 'follow' the cooling
process without the creation of thermal deformations. And when the primary has no thermal issues, the remaining optics definitely will not have thermal issues!
But: If your house is not insulated properly, you immediately will be confronted with those nasty cold drafts, turbulence !! Which exactly is the same with telescopes.
So be sure that you insulate properly, before posting a verdict about insulation! You probably notice me using the words 'Proper insulation' quite often.. That's because
it is important to do this properly, or else you still end up with thermal issues. (Edit:) More importantly, you will probably end up with the conclusion that insulating a scope will not ever work, which is not fair towards a new insight which is not yet tested fully in all situations! (End of edit..)
Of course, there is thermal loss through that thin schmidt corrector plate if we talk SCT, but that is much, much less than the thermal issues you encounter when
you don't insulate your scope! Don't forget that glass still has quite some insulation capacity, and the deformations caused in the schmidt corrector plate, by the
warm air inside, working against the colder air outside, never were big enough to spoil even the first 10 minutes of my stargazing night!
My 16inch SCT has a extremely thin corrector plate for its size: only 5.7 millimetres (0.22 inch).. But with its proper insulation, that 16" scope still gives immediate sharp
images, right after put in a 15 degree colder outside temperature. I think that most here who tried it will agree: "The proof of the pudding is in the eating!"
My 16" has no internal fan's, but right next to it, I have two modified C11's (also insulated of course) that do have internal circulation fan's, and although I have a tiny bit
more that feeling of stability with those C11's, looking at the differences in a 'objective' way, makes me conclude that there is no difference.. So from that point of view,
I think that internal circulation fan's are not very much needed when your scope is insulated in a proper way.
Some think that they have the right (theoretical) answers, without even having tried this fairly new insight in a proper way, but trying to read those posts with theoretical
views, mostly takes more time than buying some insulation stuff, and try this yourself in the field !!
......My 2 cents...
Edited by yellobeard, 12 January 2019 - 12:47 AM.