Mirror cooling & Deformation, all you ever wanted

http://home.hetnet.nl/~artm/atm/articles/cooling.html
This link contains a study of mirror cooling and deformation with no fans, one fan on rear and a fan on both sides of the mirror.
The study shows that a fan on both sides is much better than just one on the back and that the amount of deformation is greatly reduced.
Interesting read for those interested.

Floyd:

A while back I made a "back of the envelope calcuation" that was quite similar, it showed that for an 8 inch diameter one inch thick mirror, a 1 degree C thermal gradient from one side to the other would cause something like a have wave deflection. I made the same assumptions and so this was also a spherical deformation.

As the link pointed this is not as bad as it sounds because it is spherical deformation superimposed on a parabola that is only slightly different from a sphere, if this were a scope like an SCT with spherical optics, the only difference would be a slight change in the position of the focal plane. Of course the thermal gradient will not be uniform or linear ...

Anyway, I exchanged Emails with Brian Greer on the subject. As you know Brian has been the one to do quite careful testing. I came away from that exchange with the belief that the thermal strain in the mirror is not the critical issue at all, rather it is the what I would term "the local seeing inside the scope" caused by the warm components including the mirror.

Jon

I guess my question would relate to a closed tube 12" dob. What would the ideal fan number, placement be for such a scope. I see your single enclosed fan assembly. Is this your recommendation?

This can all be avoided with a quartz or ULE mirror.

Rolando