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Cooling fans

daveb

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brantuk

brantuk

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Absolutely - I have a fan on the back of the mirror of my 16" and it cools in roughly 30mins. The difference to the mirror surface is remarkable - I'm so sad I once sat constantly watching it visibly clearing lol. It also pushes air up the tube clearing out warm currents whilst keeping air moving over the mirror discouraging dew formation. Here's everything you need to know about newts and fans:

http://www.fpi-protostar.com/bgreer/fanselect.htm

Hth :)

 

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daveb

daveb

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I have a very small fan (40mm) built in to the mirror cell and blows air away from the back of mirror, i can't see that it would be powerful enough to cool  a 12" mirror down quickly or to clear the warm air from the tube. This is why i want to clear the warm air before i start to observe. What would be the best type of fan?

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brantuk

brantuk

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Lol Steve :grin: yes I should get out more hahaha - but I was very curious on that day and just had to see it for myself. I was actually amazed cos the scope is kept in the garage very near ambient and the fan still made a huge difference. :)

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Physopto

Physopto

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I would suspect a larger good quality ball bearing slow running fan so as not to introduce any vibration. Some of the better computer fans come with silicon anti vibration mountings. You can use a PWM voltage control as well if wanted.

Derek

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daveb

daveb

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4 hours ago, brantuk said:

Absolutely - I have a fan on the back of the mirror of my 16" and it cools in roughly 30mins. The difference to the mirror surface is remarkable - I'm so sad I once sat constantly watching it visibly clearing lol. It also pushes air up the tube clearing out warm currents whilst keeping air moving over the mirror discouraging dew formation. Here's everything you need to know about newts and fans:

http://www.fpi-protostar.com/bgreer/fanselect.htm

Hth :)

 

Hi Brantuk, this made very intresting reading. The fan i have fitted in the mirror cell of my oo 12" mirror is less than half the size it should be going by the text you sent me, which means i must fit another fan on the rear of the scope☺thanks 

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brantuk

brantuk

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I'm not so sure it's an exact science Dave - I run my fan for half an hour on full speed to clear - then turn it down to half speed to keep a steady flow whilst observing. And the exact speed needed depends so much on temperature and moisture conditions on each night. But it is a good article that gives a decent guide where to start from. Hope it helps. :)

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jetstream

jetstream

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Great link Brantuk, Brian Greer has done extensive testing. I might add that for large mirrors the placement of the fan or fans can be crucial to maintain the optical performance of the mirror, it has been said. The safest bet is one at the bottom as is usually seen. We are lucky to have access to all this info...

http://www.loptics.com/articles/starshape/starshape.html

 

fans_small.jpg

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jetstream

jetstream

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8 hours ago, daveb said:

I have a very small fan (40mm) built in to the mirror cell and blows air away from the back of mirror, i can't see that it would be powerful enough to cool  a 12" mirror down quickly or to clear the warm air from the tube. This is why i want to clear the warm air before i start to observe. What would be the best type of fan?

The mirror only cools at a certain rate, a bigger fan doesn't necessarily mean it will cool faster. Having the tube just above horizontal will let the heat out one side of the tube and the cool down the other (so to speak) speeding cooldown. Some use boundary layer fans but I wouldn't want dust or grit blowing across my mirrors face...

Once near equalized the interesting thing is what happens to the boundary layer... this is in Greers info.

If it were me I wouldn't change the fan, I shut mine off for periods when high power observing- the view will tell you when its time to turn it back on.

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CraigT82

CraigT82

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35 minutes ago, Ricochet said:

What do you use as a power source for the fan(s)? Presumably if you're not using any motors or dew straps you can get away within something smaller than a 12v jump start power pack? 

You can get holders for AA batteries (8 of them for 12v)

http://www.amazon.co.uk/Leads-Switch-Black-Battery-Holder/dp/B00CQKCXXE

or what I use is a cheap Chinese lithium poly 12v battery off eBay. These thing claim to be 6.8Ah but they're not... Stil cheap and handy for powering 12v stuff though like fans or dew strips though.

http://m.ebay.co.uk/itm/281908725680

 

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daveb

daveb

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On 28/02/2016 at 18:34, brantuk said:

I'm not so sure it's an exact science Dave - I run my fan for half an hour on full speed to clear - then turn it down to half speed to keep a steady flow whilst observing. And the exact speed needed depends so much on temperature and moisture conditions on each night. But it is a good article that gives a decent guide where to start from. Hope it helps. :)

I think its suck it and see, thanks

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