I’ve done a fairly careful calculation to answer this question but I still haven’t made any measurements to confirm the results. This is a tough calculation because it has to rely on a sky model to determine the temperature of sky—and there are a LOT of different sky models (around 100.) The folks working on passive cooling systems have made a lot of measurements to determine which models work the best and the data doesn’t always agree. Still, there are some fairly widely accepted models and I used one of those. Assuming that your system is completely passive (meaning no heat is being applied,) the answer depends on a LOT of factors including clarity of the sky, ambient air temperature, relative humidity, dew shield length and azimuth angle. In general, my calculation shows that the temperature of front surface of a corrector pointed straight up without a dew shield can fall by as much as nearly 7C relative to the ambient air temperature. Again, I won’t claim high accuracy to that number but it seems reasonable.
BTW, I looked through a lot of papers to try to confirm this number and I couldn’t find one that answered this relatively simple question. That doesn’t mean that it’s not out there but it’s actually a hard thing to compute with any accuracy and no one seems to have addressed this specific question. I have in mind a simple experimental configuration to make some measurements to confirm my results but I’ve never gotten around to gathering all the equipment and getting it done—but maybe I will one of these days.
I’ve handled the question that you’ve asked about when to turn on the heaters by building an Arduino controlled “dew predictor” to automatically activate my heaters when conditions exist for dew formation. The Arduino measures the temperature of the front of the corrector plate with a black temperature sensor positioned right next to the corrector surface along with the ambient air temperature and humidity using a “precision” RH/temperature sensor. When the temperature of the corrector sensor is within 6 deg C of the predicted dew point temperature, the system automatically turns on a Dew Buster temperature controlled heater system. The system is “latched” so that the temp-dewpoint spread has to fall to 7 degrees before the Dew Buster is switched off (once it’s been turned on.) I use two heater straps on the dew shield and one behind the corrector plate on the OAT—mounted under Reflectix. I also circulate air in the OTA with Tempest fans. This whole arrangement has worked extremely well to prevent dew under some really tough conditions at my remote location. Furthermore, it minimizes air local turbulence to allow imaging while the system is running.
Edited by jhayes_tucson, 26 June 2019 - 01:29 PM.