I try to put the dew strip right at the corrector with its bulk behind (toward the back). But this is different, I had never heard anyone say go WAY back. I'll give that a try.
@choward94002, won't quote the whole thing, but that's... really different. Nothing I had read indicated a goal to heat the OTA that substantially, I always had the impression the goal was basically to restore it to ambient or a degree or so above, rather than let radiative cooling bring it below ambient. "Hot Tube" is a new concept to me.
How much power does it take to keep it that warm?
You've got two essentially unrelated things going on that folks conflate ... dew prevention and thermal management
Dew prevention means keeping the corrector plate warmer than the dewpoint temperature .. and note that dewpoint temperature is a function of both temperature and humidity. That's why you can be doing great on temperature and when a fogbank rolls in you get dew ... you've got to keep the corrector warmer than that dewpoint even when the humidity shoots up.
People handle that in two ways ... first they try to slow down the passive cooling of OTA compared to the cooling outside air temperature with dew shields, a reflectix coat, etc. and second they try to warm up the corrector plate with dew heater (actually warming up the air around the corrector plate, since the cork gaskets holding in the corrector plate make terrible thermal conductors). Keep the corrector plate warmer than the dewpoint temperature and you'll never have dew
Thermal management means keeping the OTA temperature higher than the primary mirror temperature ... if it's lower then you will get heat plumes from the primary up the focus baffle tube. The bigger the primary mirror the slower it cools, but the interior OTA temperature will cool at the same rate as the outside air. That's why it takes so long to acclimate a C14 compared to a C6, that's a big chunk of glass that needs to cool. That's also why you will sometimes get thermal plumes if there is a cold front that moves in ... the colder outside air will cool down the OTA to below the primary temperature and once again, thermal plumes.
People try to handle this in two ways; first, they try to force the primary to cool faster by setting up some serious convection currents in the OTA with outside air ... like blowing air on your soup to cool it down. That's the whole idea with fans, really blow that cold air onto the primary and cool it down faster than it would otherwise. You'll still get thermals, but they will be much briefer until the mirror cools down ... that's the tempest fans. Another approach is "crash cooling" which is what the planetary folks use (where thermals REALLY make a difference), essentially cooling down everything to below what it would ever get at night. With this approach the cooling night air doesn't cool down the system, rather it's actually heating it up. I've written stuff about that, and Kokatha has posted about it as well.
Unfortunately if you try each separately you run into issues ... using Reflectix and dew shields and heater is great for dew management, but doesn't help with the primary mirror cooling down so you're stuck with thermals. Putting on tempest fans to cool down the system, or crash cooling, is great until the humidity rises and you get dew.
The "hot tube" approach came out of my experiments with thermal management a year or so ago, and so far I'm quite happy with the results. I haven't had dew problems, even at my "wet site", for over a year now and that's in the face of a 4am fog bank at the "wet site" and cold fronts moving through during monsoon season at the dark site. I also haven't had thermal problems (after John Hayes showed me what to look for) using the hot tube approach
For your other question, each heating strap draws 2A [https://www.amazon.c...e?ie=UTF8&psc=1], four straps total so it's an 8A draw per OTA ... those aren't powered from a USB hub obviously, I have some power relays on the thermal management computer that handle this ... and those straps are under Reflectix, so they definitely keep the OTA toasty!
Edited by choward94002, 01 February 2021 - 05:50 PM.
