Cooling AND Thermals in SCT's

As a long time user of SCT's I (like many) am constantly aware of two thermal issues. Firstly, cooling the optics to a state of equilibrium with the ambient temperature, and secondly, dealing with currents INSIDE the tube.

The first problem is solvable, either by patience or by fan cooling (Lymax e.g.), but the second seems more problematical. Many times the SCT seems more prone than other designs to thermal problems inside the tube: I can only surmise it has to do with the fact that not only is the tube uninsulted, but also that there are two operating reflective surfaces, the light passing a total of three times inside the tube before it reaches the eyepiece, and that the secondary magnifies the image (and perhaps the instability?) by a factor of around 5X. Whatever the cause or causes, I am constantly nagged by the feeling that the system is operating under par.

I am contemplating two attempts at a solution: lining the inside of the tube with cork, blackened of course, and an aggressive fan or fans, most probably mounted at the rear, but maybe at the side of the tube, so as to "scrub" the primary mirror. Peltier cooling seems an interesting alternative also.

Has anyone attempted, successfully or not, to do these things? I would love to hear from others with their experiences.
P.S. The tube in question is a 9.25.

Nicholas Seiflow said:

As a long time user of SCT's I (like many) am constantly aware of two thermal issues. Firstly, cooling the optics to a state of equilibrium with the ambient temperature, and secondly, dealing with currents INSIDE the tube.

Honestly, there is one factor that is probably more important than anything else (once the scope has initially acclimated and assuming temperatures are not dropping rapidly throughout the night):

Seeing. This is a long focal length telescope. If your seein' ain't what it oughta be, get the focal length down with an f/6.3 reducer corrector and you will be happier.

I've had some success mounting fans inside a C14
to mix the air and speed-up cooling. I did not
want to make any permanant mods to the scope (no holes
in the rear cell or tube) so the fans only mix
the internal air. I put the fans on a metal bar that runs
from the corrector back to the rear cell; it attaches
via factory screw holes in the tube. I've got a
Fastar model, so the assembly inserts through the
secondary hole -- no OTA disassembly required. There
are 3 small fans on the bar between the primary and
secondary, and 2 more that extend behind the primary.
The fans blow in the direction tangent to the tube surface,
which minimizes turbulence -- basically setting up
a high-velocity tornado-like effect inside the tube,
with its rotation axis on the optical axis. The fans are
left at maximum speed during observation; they
are mounted on sorbothane pads to eliminate vibration.

This also helps to cool the scope, to the extent that
high speed air is passing over the entire inner surface
of the metal tube. But cooling also becomes less
important since there are no thermal air currents
inside the tube.

It works extremely well. I would go as far to say
that it is a completely different scope with the fans
on vs. off. In good external seeing at 1000x, there
is usually a broken undulating Airy disk from internal
air currents. Switching on the fans converts this to
a reasonably stable textbook Airy pattern in a few seconds.

I tried varying the power to the fans to see what
this would do, but so far maximum speed gives the
best images.

A related issue is warm air currents coming off
the corrector. I've also installed a fan in
the dewshield that blows at the corrector, to
break-up the boundary layer there. This also helps
a lot, especially if you need to use a heater
to prevent dew.

There are many sites where people have added fans
to SCTs. These are a few that guided my initial thinking:
http://www.soleburymountain.com/telescopes/c14Surgery/c14_surgery.htm
http://www.shbinstruments.com/C14-Rebuild-1.jpg
http://www.shbinstruments.com/C14-Rebuild-2.jpg
http://www.rothritter.com/C11_Project/
This one had the greatest influence, though I
aimed the fans in a different direction:
http://www.wilmslowastro.com/tips/c14.htm

Some time ago I tried insulating the tube exterior
with foam sheeting to reduce the internal
air currents (no fans). This did not do much, however.
As I see it, the problem is that the big glass
primary (most of the warm thermal mass) is at the
bottom of the tube, and will still drive warm currents
up the tube, even if it is insulated from the exterior.
But it probably also depends on your climate and
situation.

I also tried using a Lymax CAT Cooler, and this
did help. But tube currents would quickly
reappear once you removed it for viewing. If the
night time temperatures are falling, you need
to repeat the CAT Cooler pretty often.

Active TEC cooling also seems interesting, but I
have not tried it yet.

- John B.

Nicholas,

I have been down the "cooling" road quite a few times. Some solutions worked and some did not. I found active cooling to work the best. Re-circulating is the best of all. With re-circulating you do not introduce any outside air (no dust and increased temperature exchange) into the tube. I will post some of my projects and answer any questions you have concerning them.

Here is my first re-circulating system. A C-9. There is one fan in the "nacelles" on each side. The other posts have the fans in the nacelles also. One in each nacelle. Not a easy solution but worked quite well.

Keith

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Here is a C-8 with a re-circulating system.

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Here is a re-circulating cooled C-11.

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Here is a passive cooled C-11 w/removable panels. Panels are removed in this shot.

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I have more but I will end here with a "stirred" cooled C-9.

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