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[ATM] fans




Hi all,

There's been a lot of posts about ventilating Newtonians lately...

>Sounds like mirror temperatures can be all over the place. Above ambient - below ambient -  

I spent many months testing my nearly-closed-tube 22", for example. I have three fans, the main one blowing on the back, the one blowing across the front, and the one in the top. I monitored the seeing in the light path with a bright star well out of focus, and compared the mirror temp with ambient using digital thermometers. 

I had very little trouble keeping the 2.2" Pyrex mirror within one degree of ambient. Here's a secret: Close up your tube with a shroud and so forth. Do not expose your mirror directly to the ground or the sky. And also protect your ambient-air thermometer from the ground and the sky. 

For example, if the thermometer is exposed to the sky and the mirror is not, the thermometer will generally read a few degrees colder than the mirror. You can make the mirror warmer than ambient by exposing it to the ground. Experiment, and you will see what I mean. Exposing a mirror to both the ground and the sky sets up a continuous thermal gradient in the glass and makes controlling the temp very tricky. It might even give astigmatism to your image. Try to measure the temp at the bottom and at the top of the glass.

>Surprising how many are still fixated on 
>cooling the mirror. 

My fan blowing across the front of the mirror is almost a waste. Since the front of the glass is insulated with aluminum the fan takes forever to cool the mirror. Sure, it does a wonderful job of homogenizing the boundary layer, but this is only a benefit while the mirror is cooling. It is better for me to turn on the main fan blowing on the back of the glass. This cools the mirror more quickly. And guess what? Since the back of the tube is almost closed off for light baffling, the back fan has little trouble homogenizing the boundary layer on the front of the mirror. 

I also have a small fan which sucks warm air from inside the top of the mirror box and expels it out a hole in the side. This helps a fair bit. Not only can it also stir up the boundary layer, but it also takes away warm air that would otherwise drift all the way up the tube. Just having a hole for ventilation is beneficial.

I use the big fan at sunset. By the time the sky is fully dark the mirror is normally very close to equilibrium. I can then turn the big fan way down in speed or use the little fan. I rarely use the front fan. Most of the night I use no fan at all.

And here's another secret: All that talk about fans recently, and no one (that I saw) mentioned what might be the most important fan of all!! I like to use a fourth fan sometimes for high-resolution observing, say a 12" fan or larger. The fourth fan is not connected to the telescope, it sits on the ground or a chair on the other side of the scope. It blows under and past the scope and into the observer in such a way as to take the observer's body heat and blow it away from the light path. Just try to not vibrate the entire scope.

Body heat in the light path can be bigger problem to open tubes, if you require a high-resolution image. Even with a closed tube, body heat is a problem any time the scope is near the zenith. The plume of heat rises off your body and spreads out, damaging your light path. So it is better to have a fan blowing your heat to one side. (Yes, I have seen this effect many times - also, a fan blowing UP the tube helps to blow the plume away; a fan sucking down the tube, well, ...?)

>If the air in front of the mirror is sufficiently homogenized, 
>is a large deviation of mirror temperature from ambient moot 
>with respect to seeing effects?

Yikes! Hot, homogenized air IS a little better than allowing the boundary layer in front of a hot mirror, yes, because thermal damages are averaged out over the aperture. 

But, a cooled-off mirror does not have a boundary layer! No thermal problems at all. From my experience with numerous scopes, the image will be much better when the mirror (the entire telescope) is at equilibrium with the ambient and you turn the fan off. 

Happy observing,

John




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