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Re: [ATM] Fan locations & function for a 32
Bob,
ULE should have a better thermal conductivity than say Pyrex but it is
still 200 lbs of mass. If at all possible, do NOT encase the mirror at the
bottom of an OTA structure. I always leave the primary mirror as open as
possible because a slight breeze from nature will move much more air past
the mirror than small fans. When there is no moving air from nature, I
create some with a 3-5k CFM fan in the observatory that pushes air out of
the observatory and therefore gets air moving again. This moving air will
help the primary equalize and then stay equalized. The fan is in the same
room as the telescope but it is not pointed at it but rather straight
up. One of the many benefits of such an external fan arrangement is that I
am blowing any fan heat directly out of the observatory, not onto the
telescope/optics. It also helps everything in the observatory
equalize. Plus the vibration from the fan is inconsequential because it is
not on the telescope or sitting on the isolated pier.
If this is for a Dobsonian and not a mounted OTA meant for imaging, then
the suggestions by others are the most prudent. Air across the face to
reduce the boundary layer as much as possible and fans blowing air at the
back of the mirror. Just remember that if the OTA base is very close to
the ground when looking within 20 degrees around the zenith, those back
fans are going to have static pressure and will not operate at their rated
CFM's. In extreme cases the fans will start to push slightly warmer air at
the back of the mirror because the motors are heating up.
For filtered air I've used a car air filter (Chevy I think). It has
enormous surface area and will allow maximum air flow while still allow
filtering of the air that enters the back of the OTA. This pushes clean
air into a solid tube OTA so that no "dirty" air can enter. With a good
volume of air flow, this means even the secondary is being bathed in
filtered air. This also helps reduce dew formation on the secondary. Plus
the filter's good for 40k miles! You can see the filter in the lowest
photo on this page:
http://www.dreamscopes.com/pages/cells/y-BP%26cell-010-07.htm
The inside back looks like this:
http://www.dreamscopes.com/pages/cells/y-BP%26cell-010-02.htm
Then a cap over the filter bolts to the back of the OTA and seals the
fan(s) chamber.
200 lbs of glass is difficult to work with, in every respect. Just
remember that for roughly every 1C of difference between the mirror and
ambient temperature you will have 0.3 to 0.5 arc second degradation of
image quality. If you underestimate thermal issues, you'll spend too much
on the optical quality that an optician might provide you...
Thank you.
Sincerely,
Shane Santi - President
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833
At 12:19 PM 1/30/2007, you wrote:
>I accept the concept of sweeping away the boundary layer from the primary
>mirror surface. And I'll use some fans under the mirror back.
>
>I'm doing final design of the aluminum tube mirror box for my 32" alt-az.
>So it's a good time to block in the space needed for the fans/vents. Plan
>to use 3 or so fans to blow across, and same to draw out and vent the air
>on opposite side.
>
>My questions to you expert, particularly those who use them, are:
>
>1. Should the flow be across the mirror front be from one side to the
>other side? Or should it be vertically? (From the lower side of mirror
>box, exiting on the top? or vice versa? ) The scope will have a full
>shroud as a minimum or possibly a rigid tube later.
>
>2. Since its operating in conjunction with the above (and without starting
>a debate) should the fans (imbedded in the cell underneath the
>mirror) suck or draw the air down and vent out the back of the box, or
>blow on the back of the mirror.
>
>3.Are there 3" diameter fans that would be able to maintain flow across a
>box about 40" wide, or would a 4" be nec.? Brand names and models also
>appreciated
>
>The mirror is a 32" f4, 3" thick weighing about 200 lbs. Its Corning ULE,
>so thermal deformation won't be an issue. But I observe in an area with
>rapidly dropping temperatures, I'd like to exploit the best current
>knowledge we have on optimizing and managing tube currents and minimize
>image degradation.
>
>Would really appreciate your advice real soon, as I'd like to cut some metal.
>( email privately if you prefer) Thanks
>
>Bob
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