17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha. Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
Quote:Note that the mirror does not need to be exactly at air temperature. I have yet to find the tolerance though.
Quote:I have the feeling that thick mirrors can now chase the air temperature close enough to maintain good views during the whole night.
Quote:For typical after-sunset temperature gradients around here (Ohio, USA), mirrors up to roughly 1-3/4" thick can track the falling temperature close enough with only a rear fan. Thicker mirrors need airflow over both surfaces.
Quote:In this case, you just rely on scrambling the front boundary layer as much as possible. The effect on the wavefront is to preserve high spatial frequencies (fine detail) at the expense of low spatial frequencies (veiling glare). This is a good trade off when doing high-resolution imaging, as veiling glare can easily be processed out.
Quote:Note that my results above won't apply if you live in a region that experiences different temperature gradients. The late Jeff Medkeff reported his 10" mirrors never settling down, and he lived in Arizona where they have large temperature differences from day to night. If you live along a coastline or a small island, you probably have a small rate of temperature drop, and your thermal problems will be less severe.
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