I have decided to start a new thread for the continuation of this little experiment, as the first thread got overrun by other interests.
The goal of this project is to have my new 7" Mak producing sharp, detailed views from start to finish during the hours I typically observing in the winter months, roughly 7pm to 10pm.
The challenge: When skies are clear, outside temperatures during (and before) this time period are always falling quickly. In stock form, the scope's mirror loses its otherwise-excellent optical figure in response to the falling temps. I'll explore using Reflectix with an insulating layer to minimize this deterioration.
Conditions consistent for all observations: The scope is stored in an unheated room, whose temperature in the mid-afternoon is roughly in the 48ºF range. It is moved from there to its mount on the outdoor observing pad at 4pm. Setup/alignment is done at 6:45pm, and observing starts at about 7pm. Humidity has been relatively high lately in the evening, in the 65-75% range. Temperature excursion has been typical for this time of year: peaks at 3pm in the low-mid-40's; 38º at 4pm; 30º when observing starts at 7pm; low-20's when observing ends at 10pm.
The first observation was with the scope in stock form (no dew shield). I saw that tube currents were not a major issue, but the optical figure deteriorated and the images were soft. A star test showed asymmetry on either side of focus, which is not the case when the scope is thermally stable. This lack of detail persisted until the very end of the session, when the thermal acclimation apparently caught up and the excellent optical figure returned.
On the second evening, I wrapped the cylindrical part of the OTA in a single layer of Reflectix before starting to observe (still no dewshield). On this evening, the optical figure was never restored, the images were soft through the entire session. Star tests confirmed that correction never occurred.
I have 10" metal tube and 8" Sonotube dobs. Their cooling systems are identical (an enclosed rear fan). I have noticed over time that the Sonotube dob not only has less tube current issues, but its primary mirror maintains its figure better during the transitional hours. It is reasonable to think that the insulative value of the Sonotobe vs sheet metal plays a role in this.
Applying this to the behavior of the Mak, it suggests that perhaps the OTA is cooling the outer edge of the mirror faster than the middle, which leads to its deformation, until it equalizes.
When taking a scope from a "warm inside" to a "cold outside" to observe, there are two different phases that it goes through. And each have to be dealt with differently.
Phase One is the major cooldown that happens when the scope is first moved out into the Great Outdoors. This is the biggest adjustment and there's nothing subtle about it. The scope's mass is puny compared to the cold atmosphere and objects around it, so equilibrium is completely on the Outdoor's terms, and the scope gives up its stored heat via every available means; radiation, convection, and conduction. Insulating the scope would only slow this process down. So best to just take it outside and let it "acclimate".
Phase Two basically begins when you want to start using the scope. Here things get more subtle. We can no longer talk about the scope as "one thing"; we have to deal with its component parts and their different thermal properties, mostly made of glass or metal. The challenge in Phase Two is to get the various parts of the scope to cool down at the same gradual rate, while immersed in a falling-temperature environment. This is where insulation might be useful, to "tune" and/or "smooth out" the various parts of the scope's response to temperature transitions.
As most of us know by now, Reflectix is a decent radiant barrier but a poor insulator, about the same R value as a single pane of glass. The solid plastic provides a conductive path to whatever it is in contact with. But when used to cover or enclose a good insulation material, it enhances that material's R value by stopping the air from infiltrating and cooling it (vapor barrier). Good insulators are generally "fibrous tangles" made of non-conductive material.
In numerous threads on CN, it has been said that the primary goal is to maintain the telescope mirror "at ambient". So, what is "ambient"? For most scopes, "ambient" means the temperature of the air that the scope is immersed in. This POV falls flat when applied to a closed optical system, such as the Mak-Cass I'm using. The mirror is enclosed and is never directly exposed to the atmosphere. Air temperature variations only reach it indirectly through the corrector plate, the OTA body, and the backplate/cell assembly. They also cool the air inside the OTA. For an enclosed mirror, "ambient" is a complex mix determined by its proximity to the OTA and the back plate/cell assembly, which are typically metal, and predominantly by radiative cooling, and secondarily by the air temp inside the OTA.
Mulling all of this over, it seems to me that what is needed is to present temperature changes to the mirror more uniformly over its entire surface. Which means, lessening the influence of the OTA due to its outsized impact on the mirror edge. I'll try accomplishing this by insulating the cylindrical portion of the OTA, and allow the backplate/cell assembly to dominate the temperature transfer to the mirror.
So that is the reasoning framework behind this approach. Lessen the influence of the OTA, and allow the back plate to be the dominant "heat exchanger" to the outside world. It presents temp changes to the mirror across its whole surface much more uniformly than the OTA does.