Primary mirror cooling fan

[Olsonsview]

I am considering installing a small fan on the primary mirror cell of my Orion 8inch dob. ANy ideas on how to minimize vibrations? SHould I try to use rubber pads to dampen the fan vibes, or hang the whole thing on a rubber band suspension? What direction should the air flow? I see the cell has 4 evenly spaced holes with machine threads 2 3/4 inches apart. I was thinking of looking for a fan with that mount hole spacing to mate it up.

[KeithBC]

My factory-installed mirror fan is just bolted to the back of the mirror cell. Vibration shouldn't be an issue unless you get an el-cheapo fan that is out of balance. The air flow is blowing onto the back of the mirror. The air spreads out across the back of the mirror, flows around the mirror cell into the tube, and exits the front of the OTA.

[Olsonsview]

Thanks for the information Keith. Another question: any idea how high a flow rate I need? I have an electronic surplus store a couple miles from my house. They have maybe 24 different small 12Vdc fans. any idea what your fan is rated at? any label info on the fan? How many blades on the fan you have?

[skaven]

I use a dual ball bearing 80mm computer fan, suspended on rubber bands between the primary mirror lock screws. I direct the air toward the mirror. In my case, with the C10, if I were to bolt the fan directly to the cell (mine has the same holes you're talking about) there would be nowhere for the air to go. The rubber band suspension is really easy, and I get zero vibration.

I run the fan on a 9V source from my Power Tank. This seems to produce a nice amount of airflow without the fan making a lot of noise or vibrating.

The most interesting (and welcome) thing I've noticed since adding a fan to my C10 is that dewing has (so far) been completely eliminated. It seems the airflow coming from behind the mirror goes up into the tube and runs along the walls as well. This keeps the tube and the secondary at the same ambient temperature, which unless you're sitting in a cloud bank, should be above the dewpoint.

[skaven]

dual ball bearing 80mm computer fan

I should emphasize this, because most cheap computer fans use bushings, not bearings. Or a single bearing and one bushing. You want dual ball bearings: it'll last longer, run quieter, and use less power. You can pick up quality fans at a computer parts place like newegg.com.

[Olsonsview]

Thank you KeithBC and Skaven for sound advice. I did mount a fan that I ad laying around my workshop. It is a 12VDC fan, quite small compared to most. It is most likely sleeve bearings, but had it handy, small: about 2 inches square and no more than 1/2 inch thick. Not much weight to suspend with skinny rubber bands. Tested it, and it seemed very smooth running. I tried it out last night, and definitely see improvement to the view. I ran it for an hour before using the scope, and was greeted with good views of some familiar Messier objects. Thanks for the time and advice to all!

[Olsonsview]

fan.jpg
This is what it looks like. Thanks again.

[balint]

Well done!

[shannong]

I found these specs below at the site referenced with a lot of great information about cooling fans, why you use, how you use them, and how much you should be using. It's written specifically for Newtonian scopes. I was surprised to learn that the initial cool down is the minor benefit and that ongoing cooling for the viewing session was just as important if not more.

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Telescope Optics Topics

[shannong]

One thing to consider about continuous cooling fan use that effectively diverts air around the cell into the tube is that it will increase the amount of debris landing on your mirror especially if you live in an arid location and view in fields, etc. I have this nightmare scene in my head of my highly efficient fan system moving air across the face of the mirror into the tube and a car pulling up next to me in a viewing field with a cloud of dust following them.

Since it quit cooling down here at night I'm not sure I need a cooling fan to track the ambient temperature. <sarcasm> Last night it was 94% at 930pm at night.