One big issue is the temperature delta between the mirrors and internal air. Rapidly exchanging the air inside the tube with ambient will result in the the mirrors being warmer than the internal air. They will then shed heat and plumes can result, just like in a newt. The bigger the temperature delta between the internal air and the mirror, the more heat that will be shed from the mirror. So, it's best to have the mirrors at or very close to the internal air temperature to mitigate heat shed from the mirrors. Insulating the tube and back plate considerably slows the heat loss to ambient with a sealed system. Yes, there is loss via the corrector and that will cool off the interior but that will occur at a much slower rate with the insulated tube. But as John points out, the back of the corrector will cool quickly to below the internal temperature of the internal air. Fortunately, the correctors don't have much optical "power" so any distortions resulting from changes in the corrector's figure with temperature are small. Again, as John points out, the real problem will be stable thermal gradients in the air and especially on the back of the plate. That cool air will fall and settle in the tube, creating other gradients. And that has been my experience as well with my C11 and TEC 7. But the Reflectix stuff really does help nicely from a usable visual perspective and really mitigates or pushes way out the time it takes the corrector to dew up.
Now the arm chair speculation. To me, it then makes sense to mix the internal air up enough to break up stable temperature gradients. So, do this mixing with well placed internal fans. Yes, it will cool the internal air volume quicker with the increased convection cooling via the corrector (and yes, a little bit via the tube even with the reflectix) but it would break up that cold air layer on the back of the corrector and keep pools of cooler air from forming elsewhere in the tube.
However, this cooling could lead you right back a bit towards the very issue to avoid in the first place which is the mirror shedding heat to the internal air because the interior air temperature drops too quickly.
So add heat to the tube's interior to replace, at least in part, the heat lost when mixing the interior air around
Maybe a heater strip wrapped around the tube's exterior, under the Reflectix? And, BTW, blow ambient air over the corrector front face to bust up any pesky stable gradients there too.
This way you can directly control, manage or modulate a uniform thermal state inside the tube while also minimizing performance robbing stable temperature gradients and plumes and pools(hot or cold). An added bonus is that there would be no need for a dew shield as there will be no more dew.
Sounds like a project brewing for me or others maybe?
Edited by Jeff B, 01 December 2018 - 01:03 AM.