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I actually believe the mirror was at or near ambient.

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That's probably the case. We have so few nights where the temperature is stable that I can't imagine not running the fan. I've done on/off tests on high power images and the difference is obvious. A clear high power planetary image will turn to mush in 3-5 seconds after turning off my fan. It snaps back clear about 1 second after turning it back on. My fan blows on the back of my mirror. The back of the tube is closed off around the fan so that the air has to travel up out the tube. Of course tube currents probably aren't as much of a problem on your trusser.

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Tom (Pegster)
DSH-8 (GSO Dob)
15x70 Oberwerks
SVP 100 f/6 achro
WO 66 Petzval
Sears Discoverer EQ 60/900
8x42 Regals




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