That's an interesting experiment that I'd try myself if it weren't almost continuously cloudy here in Bend right now. Here are a few questions and comments:
Thanks, I really appreciate your and others feedback!
1) Have you measured the temperature difference between the front element and the ambient air temperature of the glass itself? You'll need a black temperature sensor to do that.
The surface temperatures were measured by the sensor being bedded into thermal contact grease [http://www.ti.com/product/LMT70] "gooped" onto the glass/ can skin, same as I use between the peltier "hot side" and the heat sink [https://www.amazon.com/ARCTIC-Freezer-Xtreme-Twin-Tower-Heatsink/dp/B001X017G2/ref=sr_1_16?ie=UTF8&qid=1544920160&sr=8-16&keywords=cpu+cooling+tower+heat+pipe] .. the peltier "cool side" uses a thermal conductive pad [https://www.amazon.com/Arctic-4237-Efficient-Conductivity-Handling/dp/B00UYTTLI4/ref=sr_1_3?ie=UTF8&qid=1544920267&sr=8-3&keywords=thermal+conductive+pad] between it and the can bottom (same that I use for cooling my D5100) ... the air sensors were simply free floating ... what would be a better way to get the temperatures? I wasn't able to find any references to a "black sensor", does that refer to the coloration it should have or is there a part I need to find?
2) How open is your horizon? The results of your experiment can be significantly impacted if you have a lot of trees and buildings around. You'll get the best results if the horizon is unrestricted.
I placed the cans about 10ft apart from each other in the middle of an open area with about 100ft from the nearest trees, with the horizon being no less than 45 deg altitude from any can there ... I was also careful to place the wind screens far enough away so that I still had a 55 deg horizon from the can to the top of the windscreen for just that reason. I don't image at altitudes less than 60deg due to airmass, so for me that was an acceptable "cone". Note that the cans didn't have any kind of dew shield, just a flat sheet lying across the top ... dew shield testing is my next step, already in process ...
3) Are you recording the ambient temperature and relative humidity? The humidity makes a very big difference in what you'll measure. You'll get the biggest drop with the lowest humidity.
Yep, I use this sensor already to gather temperature and humidity levels [https://www.mouser.c...hdc1080-sensor/] to feed into the RaspPi to do the dewpoint calcuations already both inside the OTA as well as next to the corrector plate in the OTA ... looking back at my wall of text I (apparently) didn't mention that I put in this kind of sensor outside the can (2x), inside next to the plate glass 2" away from the top and 2" away from the side (4x) as well as next to the high precision temp sensors next to the weight/ thermal mass (4x) this to track the humidity ... would there be other places to put this for the experiment?
4) If you tell me the air temp (C ), the relative humidity (%) and the measured temp of the glass after a few hours under a clear sky, we can compare what you measure with what my model predicts. I can do the same thing for an unheated dew shield if you give me the diameter and length. It's best if you wrap the dew shield in Reflectix to keep it as close to the ambient air temperature as possible.
Sure, I've got that data for inside OTA and outside can temps, RH's inside the can next to the glass and outside the can during the runs of the experiments!
I have the ability to measure the ambient air temperature and the corrector temperature on my scope out at DSW. My model predicts a temperature drop at the front surface of the corrector of -2.3C and I two nights ago (when it was really clear) I measured a drop of -2.01C. That's with a 381mm long dew shield on a C14. My two sensors have not been carefully calibrated so that level of agreement is well within the accuracy of my measurement. I've made the measurement informally a number of times and the values range from about -1.8C to -2.5C. When I have some time, I'll have to get more methodical so I can get a good average value. It's hard to do everything I want to do since the scope isn't in my backyard.
(smile) You can repurpose trash cans like I did and be the new talk of the neighborhood!
Looking at my data, I am not seeing drops on my plate glass that I can attribute to the radiative cooling of more than about 2-3C (the outside glass sensor tracks almost precisely with the ambient air temperature, and the inside glass sensor tracks very closely with the interior OTA air temperature less about a 20% differential (so, if the outside air temp is 5C and the interior OTA temp is 10C, the outside glass sensor will read 5C and the inside glass sensor will read 8.5C which makes sense as glass is a thermal insulator) ... you probably has a smaller differential because you're running tempest fans to help stabilize the internal/ external air temps ...
Later this week I'm going to be running some more rigorous experiments with the cans to experiment with dew shields ... first, the can interiors right now are 'kinda shiny inside and that's not how a real OTA is inside so I've put down some coats of Rustoleum High Temp black paint on the interior of the cans to simulate the black interior of an OTA (even though I know that's going to dramatically increase the internal radiative cooling from inside the OTA through the glass). Second I've made four sets of dew shield mockups with some Reflectix at various lengths (I bought the 100ft roll, so I have a TON of the stuff here) with the shields at various lengths (I've cut some for 6", 8" and 10" ... that's about as long as I want to go without it becoming a sail) and have spray-adhesived Reynolds Wrap to two of the sets ... they will be velcro'ed on to the plate glass surface but should approximate a "dew shield". I've also spray-adhesived some black felt to two of the sets (one of the Reynolds wrap and one of the bare ones) so I can test to see how a black interior affects things ...
One of the things I'm excited about trying is the focusing ... you write a nice treatise several years ago here about the effect of temperature changes in the mirror on the nominal focus zone, and as I recall you found that very VERY slight changes in the mirror temperature would result in noticeable changes in the focus zone ... so the discovery of this "protective inversion layer" redarding mirror temperature changes is quite fascinating for me. I have no way to duplicate your testing regiment you used back then, so I'm going to be taking pictures of airy disks of target stars throughout the night with both a "protected" and an "unprotected" OTA and see how the focus changes for them ...