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Cooling a (currently) closed eye-end Cass

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#1 AlphaGJohn

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Posted 27 June 2013 - 12:08 PM

Dick Parker's photos in this thread show fans on the end-cap of his Cass scopes:

Thread on Cass Focusers

Until I had a go-to mount (which requires power), I never would have considered doing anything that required fans (regardless of the effect on image quality), but now I've gotta have juice available. I'm using a turned wooden piece for the end-cap and it currently has no extra holes (just primary adjustment knobs and focuser). Presumably I will need some airflow through the back.

The 10" mirror has a conical back so it's not going hold as much heat as would a full-thickness one, but that's not going to solve the heat problem w/o some way to get some circulation, right? I'd considered just some baffled holes to allow passive airflow. (The tube is fiberglass w/ an ID of (very) approximately 12" and is about 29" long.)

What have others done for cooling a Cass?
How do you block light from entering along with the air?
What fans have been appropriately quiet and reliable?
Blowing or sucking?

(I have been reading the long thread on visualizing air currents w/ Dobs; so I'm mostly interested in specifics for Cass-type scopes.)

Thanks!

John

#2 Mirzam

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Posted 27 June 2013 - 01:26 PM

My 12.5" Parallax instruments cassegrain has 6 ventilation ports around the otherwise closed backend of the scope. These ports are about 3" in diameter and can be closed using plastic plugs. I obtained two fans (from Radio Shack) to fit over two of the ports and use these prior to and during observing. There are certainly better fans available but these do seem to work. The mirror, like yours is conical. There is a marked improvement in the views through the scope with the fans turned on provided that the atmosphere is itself fairly stable. This improvement in performance persists during the night. The improvement in viewing with the fans turned on has been noted many times by making critical observations of close binary stars. Because the scope is kept in an observatory it is maintained fairly close to ambient temperatures. The benefits of fan usage would seem to indicate that the mirror never really achieves thermal equilibrium with the surroundings. Contrary to expectations my limited experience suggests that large conical mirrors do not have great thermal performance, probably because the center thickness is typically quite high.

JimC

#3 AlphaGJohn

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Posted 27 June 2013 - 03:59 PM

Thanks JimC,

Are the ventilation ports all on the end-cap or are some around the circumference of the lower part of the tube?

I'm assuming there's some kind of baffling inside the ports to keep out light?

I'm assuming your fans are blowing, rather than sucking?

I'm sure you're right about the thermal properties of conical mirrors--they might even distort more than a slab-type mirror because of the difference in thickness from center to edge and therefore be more subject to differences between the mirror's and the ambient temp.

John

#4 dweller25

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Posted 27 June 2013 - 04:06 PM

Here's what I do ......

Attached Files



#5 dvb

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Posted 27 June 2013 - 04:25 PM

My 12.5" Parallax instruments cassegrain has 6 ventilation ports around the otherwise closed backend of the scope. These ports are about 3" in diameter and can be closed using plastic plugs. . . .
JimC


Is it like this Dall-Kirkham, with the orange plugs on the back end, over the cell?
http://www.parallaxi...cassegrain.html

#6 Mirzam

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Posted 27 June 2013 - 06:03 PM

Yep. Exactly like that picture.

I close 4 of the 6 holes when using the two fans. From the viewing position of the secondary there is not much opportunity for light leakage from the rear holes. The mirror support plate and the mirror itself "baffle" most of the opening.

JimC






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