I observe doubles with a 10” f/20 Maksutov which has a big, heavy corrector. I have the scope permanently mounted in a RoR observatory which is not too far from ambient temperature at any time (max 5-7 degrees) since our winters are not as harsh as in other countries and summers are warm not hot, there is no need for air conditioning, etc.
I used to need to leave the roof open at least 1 hour before observing if I was to get any stable images during the nights of most significant change in temperature (spring, autumn). Since I am looking at very close doubles (<5 arc-sec separation), I need good observing conditions. Atmospheric seeing is the main determinant factor, but image stability at the eyepiece is a close second.
I installed insulation around my OTA this summer (material similar to Reflectix; just standard radiator wall insulation) and the improvement has been noticeable. Star images are stable from the start of my observing sessions even if I have not managed to open the roof of my observatory in advance.
This effect is similar to the very beginning of some observing sessions when, not having cooled the telescope, star images would be very good for a short 5-10 mins before thermals appeared inside the OTA. A pre-thermal shock nirvana kind of scenario.
An added bonus of the insulation, which has been mentioned in other posts above, is that my corrector doesn’t dew. I don’t use, and have never used, a dew shield and typically rely on a good dewheater band just behind the front glass. Humidity is high in England (average >60% and in some nights >90%). With the insulation material wrapped in a double layer around the OTA, the dew heater band is not very effective, yet my corrector is free of dew. I again, suspect this being down to the much slower cooling rate of the whole OTA. Note the insulation material doesn’t extend more than 1 inch beyond the corrector; ie no Reflectix dew shield.
To all considering this solution, I say, go for it!
Hi Roberto, etal,
I'm wondering (and suspecting) that the radiating ambient and residual heat within your roll-off observatory, and the fact that you're observing within a walled structure (despite it being open to the sky), is helping mitigate or forestall the conditions leading to the formation of dew. I'm curious if your "RoR" is surrounded by grasses or low-laying vegetation and, if so, have you've ever noticed dew (or frost) forming outside the observatory? Also, I'm curious if your RoR is located on a rise or hill or elevated slope vs being located in a slump or valley or other geographic depression?
The reason I ask is that regardless of whether I'm observing with my C9.25 or my thick-meniscus SW180 Mak, my (club's) observing location from a paved parking lot edge observing across an expanse of meadow directly to the south and west which is a flat half-mile from a local reservoir, pretty much dictates that I will be contending with either dew or frost with either scope if conditions permit. Invariably when the temperature dips near the dew/frost point the cold, sinking, moisture-laden air pours across the small valley (sometimes in the form of a visible rolling fog) and despite any best counter-measures (dew shields and wraps and heaters) it's pretty much "game over" for the glass-fronted scopes. Meanwhile on the surrounding hills the dew point comes later in the night (or not at all) and its effects are less severe.
I bring all of this up to this discussion as there are geographic ground-level climate factors to consider in trying to solve the, um, "dew problem" beyond merely wrapping and/or venting and/or heating one's scope. A moist, dew-prone valley will defeat any best counter-measures.
Edited by Jimmy462, 30 November 2018 - 09:04 AM.