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Lunar

Beating the seeing

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I was just wondering, has anyone ever tried installing a fan on the side of the tube of a reflector as described on page 2 of this article?

http://skytonight.com/howto/diy/3304176.html

Seems a bit drastic to drill holes on the side of a new and shiny tube. Is it really better than putting the fan at the bottom of the tube?

Cheers

M.

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Air moving across the mirror is said to be better than the alternatives... Not sure why?

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Surly you would be circulating the air in the tube rather than around the mirror??

I would have thought it would be more important to keep the temperature of the mirror stable..Rather than the tube...

Need a thermal dynamics expert .....

I SAID WE NEED A THERMAL DYNAMICS EXPERT.....Chop Chop!

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Better than a fan is to keep your optics at the ambient temperature. A shed is the best astro accessory you can purchase....

My sheds are full of junk. I think my best astro accessory may be a yard sale.

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The pros use this technique on the big scopes. The idea is to draw air across the surface of the mirror to prevent stagnant air collecting at the surface. Stagnant air will of course be at different temperatures with altitude above the (usually very deep dish of) mirror and therefore different densities and will create its own very local seeing effects. For a smaller scope, you'd want to mount the fan on the outside of the tube such that it draws air more from the opposite side of the fan than the open top of the tube. The best way to assure this is to baffle the heck out of the bottom part of the tube so air moves easier in the axial direction that longitudinal. I doubt it would make much difference on a scope smaller than 16" to 20". Some of the guys around here with big scopes have done it and it does make a noticable, if minor, difference.

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Thanks Astroman, that was a great help. I might actually put that into practice one day if I ever build The 'Awesome Planetary Telescope' (APT). By the way, I'd need a garage sale as well to keep it in.

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I have a question:If you sell your garage, where will you put all the stuff? Seems counterproductive.... :insects1:

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OK here goes.

Firstly google Shlieren photography to get an idea of what the problem is. This is a way of imaging fluids by using the different refractive index of the fluid at different temperatures. The setup looks remarkable similar to a Newtonian telescope so no wonder the air currents mess the view up.

Nextly, now that we understand the issue with air at different temperatures we can understand that the problem caused by the mirror not being cooled down yet is less to do with the thing being all warped and twisted due to being at the wrong temperature, but everything to do with the puddle of air at the wrong temperature thats sat on it.

Convection is where a solid changes temperature by transferring heat to the fluid which surrounds it. The fluid then moves away through bouyancy as the density of the fluid touching the solid is different from that of the main body of the fluid. Therefore when something is cooling by natural convection it has to have lumps of air stuck to it that are at a different temperature to the rest of the air. This creates a rough and nasty lens effect which changes with the shape of the warm air touching your optics. The closest thing that I can think of to this as an analogy is a lava lamp. Globs of warm air will form on the optics and then slowly detach.

The usual method of getting rid of these effects is by allowing the optics to acheive the same temperature as the ambient air (by waiting and waiting) so that there is no significant convection going on.

Fan cooling can speed this up thereby shortening the cooling period. This attempts to equalise the temperature of the optics with the ambient temperature more quickly.

The cross flow system works by removing the air above the mirror before its temperature has changed significantly from that of the ambient air. Thus the mirror can be significantly different in temperature to the ambient air without blobs of warm air forming on it. This gets rid of the wobbly air and the instrument works a lot better. The method is favoured with larger instruments as the cool down time could be forever. The ambient air temperature usually drops during the observing session and a big thick mirror might never have time to cool sufficiently to work well under such conditions.

HTH

Captain Chaos (closet thermodynamicist)

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The cross flow system works by removing the air above the mirror before its temperature has changed significantly from that of the ambient air. Thus the mirror can be significantly different in temperature to the ambient air without blobs of warm air forming on it. This gets rid of the wobbly air and the instrument works a lot better. The method is favoured with larger instruments as the cool down time could be forever. The ambient air temperature usually drops during the observing session and a big thick mirror might never have time to cool sufficiently to work well under such conditions.

:idea1: :thumbup:

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I have a question:If you sell your garage, where will you put all the stuff? Seems counterproductive.... :insects1:

:D

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OK here goes.

>snip<

HTH

Captain Chaos (closet thermodynamicist)

That's what I said! :?

Nicely explained, CC.

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Better than a fan is to keep your optics at the ambient temperature. A shed is the best astro accessory you can purchase....

My sheds are full of junk. I think my best astro accessory may be a yard sale.

I have a question:If you sell your garage, where will you put all the stuff? Seems counterproductive.... :insects1:

Good point. If I sell my yard, I'll have nowhere to observe from. :D

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The method is favoured with larger instruments as the cool down time could be forever. The ambient air temperature usually drops during the observing session and a big thick mirror might never have time to cool sufficiently to work well under such conditions.

Captain Chaos (closet thermodynamicist)

Just one added note: Professional domes are air conditioned. They cool the closed dome during the day to approach the forcast low temperature during the night. This is particularly striking here in Arizona, where the ambient temperature on Kitt Peak at noon in July may reach 35ºC, but if you walk on the floor inside the Mayall 4 meter dome, it's a balmy 4ºC. Otherwise, CC's assertion that cool down time may be "forever" would be accurate.

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