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I hate to pick at the issue but that is not what happens at all. I can say without exception I have never seen that happen with the sucking system I employ.

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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. When the mirrors get extremely thick, you will never achieve a low enough delta T even with forced convection on all surfaces. 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.

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.

Bryan Greer

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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. When the mirrors get extremely thick, you will never achieve a low enough delta T even with forced convection on all surfaces. 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.

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.

Bryan Greer

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So ... sucking or blowing? I think this snippet has more to do with whether a bottom and side fan are needed or not, and it was assumed that the fans are blowing onto the mirror.

Mike

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Well then we're seriously at odds based on our first hand experiences. Alas we also have different designs on a few levels. Obviously its working for you so it makes no sense to change but it was a disaster with my setup.

It'd be interesting to do a side by side with your set up. Lol neither of us are budging though.

Pete

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I hate to pick at the issue but that is not what happens at all. I can say without exception I have never seen that happen with the sucking system I employ.

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during another lifetime ago when I started designing black boxes, I had an engineer tell me its easier to pull a string thru the box then to push it, using his illustration to talk about how to move air over the hot electronics to help cool them.

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There's pros and cons and a black box isn't a telescope assembly. Pushing the air has the flow more focused and directional while suction is spread out across a greater area without the same directional energy. There are ways a blowing fan can be made to better cool than suction on that merit alone. I'm not getting into the dickerings of electronics cooling though I'm sure your methods were best.

Pete

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As I mentioned previosly, many attempts at blowing fans from behind and across the mirror in smaller closed tubes have not born a lot of positive fruit. However, recently, a friend brought a 10" Starmaster down to the Chiefland Astronomy Village that had a blowing fan from the back in what is essentially a pretty closed mirror box and then drilled three large holes just above the front surface of the primary. The primary mirror is a new 1" thick Zambuto 10" f/6. The results of this cooling strategy was nothing less than spectacular. The contrast and thermal stability of the scope was significantly enhanced. The large vent holes provided a means for the air blowing up from behind to escape out of the mirror box without too much apparent bouncing around inside the mirror box.

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You know, this sounds oddly familiar ... Isn't this the very strategy that I've been talking about? Allowing the warm air to escape as soon as possible from the optical system just makes good sense to me. But so far I've only drilled vent holes for my 8" Dob, and haven't even had a chance to test it.

The 10" Starmaster had a "mirror box." So it was a truss scope, not a solid-tube? If so, I wonder how this might change the efficacy of the venting holes vs a solid-tube 10".

How large were the three "large vent holes?" Mine were only 1/4" - that was the largest bit I had available - but I drilled seven of them, about 1" apart, at the top-most edge of the primary. It would be an easy thing to drill more holes or widen the ones I have.

Mike

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I hate to pick at the issue but that is not what happens at all. I can say without exception I have never seen that happen with the sucking system I employ.


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during another lifetime ago when I started designing black boxes, I had an engineer tell me its easier to pull a string thru the box then to push it, using his illustration to talk about how to move air over the hot electronics to help cool them.


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There's pros and cons and a black box isn't a telescope assembly. Pushing the air has the flow more focused and directional while suction is spread out across a greater area without the same directional energy. There are ways a blowing fan can be made to better cool than suction on that merit alone. I'm not getting into the dickerings of electronics cooling though I'm sure your methods were best.

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what's up with the tude, dude?

fluids are fluids.

of course there are benifits to blowing and benifits to sucking

people still blow their fans on heat sinks.

just taking a fan and blowing on the mirror is a brute force tactic, primatively unsophisticated.

it works but if you don't want to have an open discussion about a more elegant solution...

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contacted my buddy about his 10" Starmaster. The box has about 4 holes in the rear of the enclosed mirror box. He reported to me that he installed a 4.5" fan with no speed control. He also cut 3 2.5" holes equally spaced across the side of the mirror box just above the front surface of the primary.

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I contacted my buddy about his 10" Starmaster. The box has about 4 holes in the rear of the enclosed mirror box. He reported to me that he installed a 4.5" fan with no speed control. He also cut 3 2.5" holes equally spaced across the side of the mirror box just above the front surface of the primary. Now this is a relatively small scope and so the air dimensions seem to support the strategy.

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Which would be better, a larger number of smaller holes or a smaller number of larger holes? Of course for me, a larger number of smaller holes would be easier. Just take out the drill, put in your largest bit and have at it. No need for a hole saw. The seven 1/4" holes I drilled could be OK for my 8" f/6. I'll have to see. But I might want to do it differently for my 10" f/4.8.

Mike

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