fan cooling for tube newtonians

[Starkler]

Theres been a few threads lately relating to temperature control and mirror cooling in newtonian scopes.

Here is one solution that is reported to work well for some people.
It consists of a fan sealed at the back of the scope, with a baffle of inner diameter equal or slightly larger than the mirror diameter.

Such an arrangement will much more efficiently cool the mirror by providing air flow around the mirror, but the real benefit is from scrubbing the boundary layer of warm air from the front surface of the mirror and hopefully flushing it out of the tube.

P.S. I need lessons in making drawings i know :P

(edited title)

[janoskiss]

That looks good, Geoff. Installing the baffle would be the trickiest part. And when it's done you'll have people lining up to get a look through the fan. :lol:

[Striker]

This thread should be good for all with Newtonians...

Nothing wrong with your drawing skills Geoff....I know exactly what you mean.

[janoskiss]

Good point Tony. Dobbers tend to forget that there is other ways to mount a Newtonian. There I was at Star Camp, and to the horror of EQ-mounted Goto-Newtonian owners, I kept trying to tug at their OTA to centre an object in the field of view. :scared:

[Starkler]

A practical consideration is insulating the ota from mechanical vibrations from the fan.

I have made a plywood disk to sit on the back of the scope holding the fan. This will be held to the back of the ota with velcro straps and rubber weather strip will seal the disc and provide mechanical insulation.

[slice of heaven]

Finding the right spot will be trial and error
JohnB(ausastronomer) was working on the same setup but haven't heard what the final conclusion was.

[Starkler]

Even without the baffle it has to be a much better way to cool the mirror than the silly arrangement some GS scopes ship with.
A fan mounted directly to the back of the mirror cell will just bounce air off one small part of the mirror.

[Orion]

I'll be in the process of building another telescope soon and I was thinking of putting two small fans on ether side of the OTA just above the mirror surface, including a larger fan on the back of the mirror. All fans will be sucking not blowing.

Any ideas on how this system might work?

[Starkler]

This is a very long thread, but a must read with a lot of tips re fan cooling.
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/360923/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1

[slice of heaven]

Very true, that's why I changed mine to the sealed configuration after reading that thread from CN you linked to

[Orion]

Thanks Geoff, what a great read.
I think I'll go with the side fans to help the cooling down process and have the rear ones left on when observing.

The talk on curved spiders wasn't bad either!

[davidpretorius]

this is what asimov sent me, it looks great!!!

http://precons.com/iis/gallery/thermal%20fan%20copy.pdf

[Starkler]

The op of the CN thread advises that the suggested setup in that article isnt the best way to do it.
A fan blowing across the primary with exit holes on the opposite side cannot channel the warmer air out of the tube without it rebounding and bouncing around in the light path.
It would probably make more sense for the side mounted fan to be sucking rather than blowing.

[davidpretorius]

sucking from side or baffles like your ideas?


i wonder which will be best???

[Orion]

I believe so to.

[davidpretorius]

if you were sucking through the side, I spose it makes sense to draw air from underneath (hole there already), from above (10" hole already) and from holes drilled on the other side to the fan over the mirror???

[Starkler]

My new cooling fan arrangement, courtesy of Starklers dodgy workshop productions is having its first light (blow) tonight minus the baffle.
Im hoping the seeing settles down a bit later and Ill try some fan on/off tests.

Putting my ear to the open end of the OTA it sounds like a vacuum cleaner :lol:

[ballaratdragons]

I don't know what GS scopes have the fan mounted this way. My gs Factory fitted fan is set way back from the mirror and blows cold air over the entire surface. And it must curl around and blow across the mirror removing the boundary layer at the mirror surface coz it blows dew of the front!

[Nightshift]

I disagree Geoff, I have the 12" GSO and the volume of air passing out the top of the tube when the fan is running is very high, it clearly passes over the entire mirror and up the tube, if you have any doubt, try putting the cap on and removing the smaller aperture cap, the wind blows quite strong. Viva la GSO. ;)

[davidpretorius]

my fan that is in the side of my tube is from the top of a cpu in a computer. It could not blow out a candle and from about 30cm or more away, hard to feel the passing of the air. I may get a more powerful one!

[slice of heaven]

Change the configuration to a setup with a sealed bottom(as Starkler is suggesting) and the difference is quite noteable.

DaveP, the gs 12" fans are 75mm across the blades, might be a bit bigger than yours. Lately I've been using an 18v battery pack for the initial cooldown period, then reverting back to the 12v pack for normal operation after cooldown.

[davidpretorius]

thanks, mine is 56mm for the outer housing.


that is good, my hole is 57mm in the side of the tube, so i can always go bigger, but i can't go smaller!!!!

[slice of heaven]

I've been thinking of looking for a bigger one still to try, or maybe coupling 2 or 3. The initial cooldown period is the time consuming part for a quick setup, hence the using of an 18v pack to crank it up.

[asimov]

I usually stoke up a big pedestal fan & stick it at the back of the mirror after setup up. Turn it on flat out & go away for a couple of hours. It must help a bit? You can really feel the air coming out the focuser end dramatically!

[Starkler]

I wonder if a few finned heatsinks stuck to the exposed underside of the mirror with thermal goo would speed up cooling noticeably.

[asimov]

Yep. Been thinking along those lines too recently.

[davidpretorius]

at the end of the day, what is our ultimate aims here?

for viewing: 1 to 2 degrees difference would be ok
for imaging: less than .5 degrees for the pro's
are we wanting the mirror to be able to cool at say 4 degrees a minute so that within 15 minutes you have caught up to ambient, do we want it in less than 15 minutes???

[Starkler]

No boundary layer !
I dont see why stricter thermal requirements should apply to imaging only.
At snake valley when the temperature was stable and seeing was good, it was good for everyone, imagers and visual observers alike. When it was bad, it was bad for everyone.

[slice of heaven]

Neither do I, Your 1st thread opened my eyes to the possible improvement for visual viewing Dave. I,m now thinking Bird's peltier coolers might be the asset for visual as well,

[davidpretorius]

To get my brain straight.

Boundary layer is a layer of warmer air that adds to distortion and needs to be replaced with air that is ambient???

[Starkler]

Dave what we call the boundary layer is a layer of warmed air sitting on the face of a mirror which is warmer than ambient. This acts as a wavering lens and distorts the view.

There are two ways to attack this problem:
1: flush the boundary layer air out of the optical path.
2: make the mirror temp the same as ambient in the tube so there is no boundary layer.

[davidpretorius]

1: so optical viewing can still be done at say 1 degree if fan is left on with my current setup???

2: no temp diff = no layer


so at the end of the day, it would be great to be able to work out what temp difference create an unwanted layer.

maybe two identically collimated scopes, one cooled, one not in the same seeing conditions.

[rmcpb]

From what I have read it seems to me that even 1 degree will influence the views dramatically but it seems to be mainly due to the boundary layer that a warmer mirror generates.

From this I think the diagram at the top of this thread is a beauty as the fan will cool the mirror over time BUT disrupt the boundary layer immediately. I'm changing the setup on my scope to be just like this and give it a go, can't do any harm but will probably improve the performance of the unbaffled fan I have at present.

Problem is its the first clear night for ages and my scope is in pieces and the mount is a disaster :( Good thing I still have the binocs :)

Cheers