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Mirror fan and size

Daniel-K

By Daniel-K
in DIY Astronomer

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John

John

Posted
Posted

The one on my 12" is only about 40mm in diameter so twice that seems a pretty good size to me. I don't use the fan though so I don't know about relative efficiencies of sizes and all that.

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Daniel-K

Daniel-K

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The one on my 12" is only about 40mm in diameter so twice that seems a pretty good size to me. I don't use the fan though so I don't know about relative efficiencies of sizes and all that.

thanks John im just looking for an excuse to mess around with the scope :)  even if it only gets used to help cooling down i still like the look  of them

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Daniel-K

Daniel-K

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thanks for the link Shane  i shall read that in work tonight :( ( clear forecast). would a bigger fan running slower give a better result though, being able to blow air more freely around the base of the OTA?

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Moonshane

Moonshane

Posted
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Dan I suggest you read the article. it explains clearly that the release of heat from the bulk of glass is what determines the cooldown not the size of the fan to some degree. the gladd can only cool so fast so at some point (quite a small fan) there's no point increasing fan size.

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Daniel-K

Daniel-K

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took a trip to maplin yesterday and picked up one of these fans. they never had any 80mm fans so i got this and ordered a 80mm fan from ebay yesterday and i can try both. i loved the idea that was in the link shane posted about using hairbands to attach the fan to the scope to eliminate vibrations. i have 2 young girls so there are lots of hairbands about, there not black but i will pick some up when im next out. im planning on putting 3 screws in the holes i have put arrows  next to to hold the fan in place at the moment its attached via the collimation screws but its solid when its on.   a bit of solidering was required to make up the cable all good fun though :D

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MoosBros

MoosBros

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That seems like a pretty trick idea for mounting....My fan is always on from setup until I'm done...My experience has been that it improves the performance of the scope for whatever reason...

The point about the glass having a fairly fixed rate of thermal adjustment is a good one to consider as well...

fans serve several functions...obviously to aid in reaching outdoor  thermal equilibrium..reducing tube currents at the mirror<solid tubes> and also somewhat helpful with dewing issues..the last thing they do  is create some issue with power requirements...batteries aren't free..bigger fans use more current.....and portable power<for fans, dew heaters, laptops, etc..> is a whole 'nother step up in cost and complexity...

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MoosBros

MoosBros

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Don't know what size the fan was on my 16" lightbridge but I used it once and didn't bother again as it cooled the mirror too quickly and it just dewed up quicker.

My secondary is the bane of dewing....the primary did  it once so far....that night everything got soaking wet...one of "those" kind of nights....I'll cross that bridge I'm  sure <primary dewing> eventually when I give up the 12" solid tube scope for a larger truss dob....

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michaeldurban

michaeldurban

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Not sure why you opted for that mounting..

It seems like that construction will cause more trouble and vibrations than it's worth

...wind..bumping etc.

Did you arrive at your construction because (presumably) a lot of people told you horror stories

about vibrations which can be so bad that you can no longer discern between a star and a house ??

nothing is further from the truth..

I bought a ball bearing (smooth) computer fan.

made a template and had it cut out in simple plywood...

I made sure that the diameter of the template would fit just inside the OTA...

Then I mounted the fan on the template making sure each point of attachment

(between fan and template) was separated by a soft rubber washer

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as you can see...the black template is not held in place by anything other than its own

pressure (flexibility).

Don't worry too much..

These micro-vibrations people speak about will only be noticeable at very high speeds (or low) when your fan

is not properly aligned in its own housing...

My fan template clamps itself inside the OTA at multiple points..

I have switched it on and off during viewing and I promise you...there is no difference.

just make sure you have little shock absorbers between the fan and the mounting part of the template.

And use a variable adapter..mine can switch between 3, 4.5 , 7 , 9 and 12 Volts.

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