Scope Cooling Fan, Do I need to use it if I leave my 16" dob out all day?

[barnstorm]

My 16" Meade Dob Starfinder has been modified by a previous owner to include a 12v computer fan to cool the mirror.

I don't like the setup because it seems to be added in such a way that it will introduce a great deal of dust.

If I simply leave the scope out in the back yard all day (with a cover of course) will this eliminate the need for a cooling fan?

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[Daniel-K]

if you leave it out all day it will probably heat up and take even longer too cool down. try and keep your optics in cool dry place. your 16inch mirror should take about 90 mins to cool down to ambient temp when taken from your house, thats roughly how long mine takes. i cant see how then fan would introduce dust unless it was cut into the tube some how most fans are put under the primary to bring the mirror down to ambient temp faster.

[barnstorm]

It was cut into the tube, above the mirror.

What are the indications that your scope is not at OAT?

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[pajr777]

Sunset is a good time to set up

[decomo50]

Maybe I got it wrong but I own a 10" Celestron starhopper dob and as well as using its built in fan to cool the primary mirror - When I bring it back into the kitchen where it is stored, I put on the fan again (10 min) to prevent condensation appearing too quickly on the surface of the glass, this seems to prevent it happening at all...my theory being the longer thier is moisture on the surface of the glass - the more chance of attracting air-bourne dust

I also own a 16" truss dob without a fan But i do have a lined case to store the mirror in seperate to the scope, this scope is allowed to cool outside for 30 minutes and the mirror and eyepieces are left to reach room temperature afterwards before being put away

[AWR]

Could you add a filter to the fan?

I read a review of the benefits of adding a fan above the mirror some time ago but the reviewer had included filters in his design. Even just a butchered vacuum cleaner bag might help.

Andrew

[cs1cjc]

It is easy to see if you need the fan on. The proper way is to do a star test. If the diffraction rings pull away at the top (just move the scope slightly up and down to see which is the top), then you have tube currents and the mirror is not yet cool. The easy way is to look at Venus. If you see a perfect crescent then your mirror is cool, but if there is any any smearing, it is not.

The mirror is a large piece of glass with a large heat capacity and it needs to be kept within a degree © or so of ambient for there to be no tube currents which will degrade the image. At night with the temperature falling in UK conditions, the mirror will take an age to reach ambient, if it ever does.

Indeed I think that cooling is more important than precise collimation. Imprecise collimation (without a coma corrector) simply moves the sharp sweet spot of a netwonian slightly off centre. However a mirror which is not cool has a degraded image over the whole field. Based on my experience, you will need a fan that is on!

There are a couple of interesting articles here:

Telescope Optics Topics

Attaching a cooling fan to Newtonian - Deep Sky Watch

[barnstorm]

A filter sounds like a good idea. Thanks for the article links.

Is there distortion if you view with the fan on?

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[pajr777]

My projector has in intake fan which has a foam filter inline - it's amazing how much dust accumulates on that piece of foam. If I ever decide to attach a fan to my 'scope I would try to emulate that system.

[pajr777]

A filter sounds like a good idea. Thanks for the article links.

Is there distortion if you view with the fan on?

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The guy in the second link above used a three speed fan from a 'silent' pc and he says the vibrations are minimal at the lowest speed.

[AWR]

A filter sounds like a good idea. Thanks for the article links.

Is there distortion if you view with the fan on?

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I seem to remember that the article I read suggested that there is less visible distortion with the fan on. Try it and let us know!

I've not tried it - I'd like to but I'm not brave (or skilled) enough to cut a hole in my OTA!

Andrew

[barnstorm]

The foam filter seems like a great idea! Thank you!

Maybe I can find a cheap airfilter system and take the filter assy out?

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[cs1cjc]

Powering from the back is the simplest way and it may then be possible to mount the fan on something high tech, like a piece of cardboard which is good fit to the tube. On my 14" the fan is a fixture, but a cardboard inset is a help in ensuring that air actually goes up the tube. I normally observe with the fan on, on the 14". My first try at a fan for the 8" is not satisfactory (too noisy and too little air), but a mark II version is in hand. I expect to write it up when I have finished.

In passing, the flight case foam is porous so might be of use as a simple filter.

[pajr777]

The foam filter seems like a great idea! Thank you!

Maybe I can find a cheap airfilter system and take the filter assy out?

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You're welcome and another little thought - the replaceable pollen filters fitted to cars these days may be adaptable to the task in hand

Edit or what about the replaceable HEPA filters on Dyson vacs - about the right size and already in a rigid plastic cage