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Old 23-06-2019, 01:04 PM
Stefan Buda
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Stefan Buda is offline
Join Date: Jul 2013
Location: Melbourne, VIC
Posts: 636
Originally Posted by multiweb View Post
Hi Stefan, good to see you again up and about.
You have a heating pad for the secondary and fans only for the primary, correct?
Heating pad for the secondary and also heaters in the air stream for the primary, but only about 6 Watts, just enough to provide dry air in the morning. See earlier pictures.
Originally Posted by gregbradley View Post
I am curious. What does overcooling the secondary do? I thought the cooler the better when it came to mirrors or is it more the cooler the better so long as the secondary and primary are in sync with regards to ambient?

Does it make the secondary shift shape slightly compared to the primary?

By overcooling I mean getting bellow ambient or very close to it. It happens because the secondary assembly is radiating away its heat to a very cold sky and can easily drop below ambient. The same reason you get ice on top of your car when the ambient is still a few degrees above zero. The secondary itself is only 14mm thick in the center and will not develop much of a thermal gradient that would cause shape shifting. If there is a small change in focal length, the auto focus should deal with it. I will set the controller to maintain the secondary at about 1.5 degrees above ambient. That may not be worm enough to prevent dew in some conditions but it will not produce a heat plume either. A humidity sensor would allow the calculation of the dew point, but at this stage I want to avoid using one.
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