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Hi Joe,
The mirror doesn't need fan cooling. What needs disruption is the air inside a solid tube instrument. It traps warm air that is inside the tube and the residual heat in the mirror, which can create convection currents (thermal currents) that can disrupt the image quality.
But I just said the mirror doesn't need cooling, and in the next sentence I say the mirror is warm!?!?
No, I'm not crazy!
You cannot use a fan on glass and expect it to cool uniformly. It will only cause a cold spot right where the fan is blowing, but the rest of the mirror is still warm. This creates strain within the mirror as it shrinks where it is cold, distorting the mirror surface.
Remember, glass is a poor conductor of heat, and borosilicate glass types (like Pyrex) even more so. You cannot force a mirror to cool faster than it will allow heat to dissipate from itself!
So why do scope manufacturers put fans on scopes? Two reasons 1: Misunderstanding of the thermal dynamics, 2: Marketing - people see a fan and they get all excited and think that 'this must be a good scope - it's got a fan!'.
These rear mounted fans will also not cool the inside of a tube. They can't - air will not go around a mirror if the cell holding the mirror is open in the first place!!! This is another misconception. You cannot force air to go around something if there is another place of least resistance for it to go - straight back out through the mirror cell!
If you look at professional observatories, they do use fans. But look closely at how they are used. Not one single fan is blowing onto the primary mirrors directly!!!! The fans are used to blow across the face of the mirror to blow away any dew that can form on it. But wouldn't the mirror get warm during the day when not in use? No, because these observatories are giant cool rooms, air conditioned to the temperature the evening is expected to drop to, so these huge mirrors do not get warm, ever.
An even worse mistake is if your scope is an open tube/truss type. There is no closed tube system then to trap warm air in the first place. The heat coming off the primary mirror is disrupted straight away, venting straight through the truss system.
Letting the mirror cool by itself not only does not create internal stresses and so figure distortion, but the mirror cools evenly. Any, any change in the shape and so tiny change in the focal length, this is negated as we constantly change eyepieces and so need to shift the focuser. Plus, the change in the focal length is so minute, our eyes won't see the difference. The only instance when it can make a difference is with imaging as this tiny, tiny shift in focus can be picked up. Also, keeping a fan on a mirror during the night will maintained the stress on the mirror as it will have a constant cold spot where the fan is focused, and the rest of the glass stays warmer. This will deform the image too.
Cooling of mirrors is more critical for imaging. But it needs to be a controlled cool. If you have a closed tube scope, reflector, refractor, SCT, Mak, etc, letting your scope cool to the ambient temperature is advantageous as I mentioned above if you are viewing under high magnification. If your scope is an open tube/truss one, a cooling period is not necessary.
If you would like your scope to cool prior to using it, this will take anything from 1/2 hour to a couple of hours, all depending on the temperature differential between the ambient and that of the mirror, and the size of the mirror - small mirrors will lose heat faster than large ones. A cooling period is also good for closed tube scopes. Just remember, you can still use your scope, open tube or closed, from the moment you've finished setting up!
It took me a while to rationalize the misuse of fans on scopes. Particularly now that I also build scopes, how glass behaves thermally has become more significant in my mind in order to best design the instruments I build.
Cheers,
Mental.
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