Fan questions

[Kevdan]

Hi all,

I have ordered a 12v fan for my 10" newt but is it better to suck the air from the mirror out or to blow frsh air in, and if blowing fresh air in does anybody filter the air to stop the ingress of unwanted dust particles. I cam get lots of filter material that we use at work on air conditioning units so it would just be a case of finding a way to fix it to the fan.

Kev.

[TheNorthernMonkey]

I would have thought it was easier to blow air in, and filter the ingress, than try to suck air out and filter all the tiny holes where air might get in

[sologuitarist61]

.... then again, would you be blowing muck about inside the tube, whereas with sucking it out should get rid of bits and bobs of muck, also sucking it out would disperse the warm air inside and let cooling happen quicker. Questions, questions, questions ..... lol

[JamesF]

There was a similar thread a week or so ago. I think the conclusion was that it is better to blow air onto the mirror rather than draw it away.

James

[umadog]

Blowing air is much more efficient for the purposes of cooling. Some stuff may get deposited on the area directly underneath the fan, but that's the back of the mirror so no real problem. A fan behind the mirror won't cause stuff to be blown up the tube or onto the mirror's surface.

[LukeSuppalot]

I

There was a similar thread a week or so ago. I think the conclusion was that it is better to blow air onto the mirror rather than draw it away.

That's what I've been led to believe too. I've also been told if you leave the fan running on very low RPM whilst viewing you will be rewarded with "clearer, sharper views".

[umadog]

You may or may get sharper views. It may make no difference. The views may be worse. Experiment and see. The views becoming worse is the least likely option, especially on a smaller scope.

[cs1cjc]

Warm air currents act as a distorting lens so the mirror needs to be at ambient air temperature to produce good views at high magnification. Goood ventilation and, particularly for larger mirrors, a fan help to keep the mirror at ambient. See for example:

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

[StarryBob]

I would have thought sucking to be better, most cooling fans on vehicles suck, even on rear engine cars. By blowing into the tube you will mainly cool one part of the mirror at first causing a distortion whereas sucking the air out will circulate air around the mirror better but you risk puling something into the tube.

[umadog]

CPU fans blow. Ultimately it doesn't matter which way the fan works. It's the air circulation that matters. Sucking may work if you seal up the back of the scope. Even then, it may take quite a long time to draw down cooler air. I think this is why blowing works better.

A cooling mirror doesn't cool homogenously (whether there's a fan or not) so they'll often show spherical abberation whilst they're changing temperature. Once the mirror is all the same temperature, this settles down. There was, however, an interesting thread on CN recently discussing how a fan may drive spherical abberation in thin mirrors even late into the night: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5290524/page/1/view/collapsed/sb/5/o/all/fpart/1

[Kevdan]

Hi,

I watched this

by Astronomy shed, where they have sealed the entire back of the scope used a slightly larger fan than needed to push air through the tube fron back to front, I like this idea and it looks fairly easy to make, just need to find a speed controller, the fans are very cheap less than £10.

Kev.

[LukeSuppalot]

Try here http://www.maplin.co.uk/search?criteria=potentiometer or here http://www.conrad-uk.com/ce/en/overview/0241710/Rotary-potentiometer

[Moonshane]

at the end of the day a newt is an open system; some dust is inevitable and it won't affect views generally at all unless really bad. I never clean my mirrors unless I am selling a scope as people get fussy about dust when buying.

all my fans blow onto the back of the mirror and seem to work well enough. I am sure there's a benefit but in truth I have never timed it. I'd sooner be looking down the eyepiece than at a watch timing the cooldown in reality you can start observing as soon as you have set up, collimated and found your star map at lower powers, increasing power as the mirror reaches equilibrium.

[Kevdan]

Hi guys,

So here is my version of the Astronomy shed Fan mount assembly, All I got to do when I get chance is to change the plywood plate for a bit of 3mm thick plastic.

It took a couple of hours to make including cutting the wood plus a ride to maplins. total cost about £20. and hopefully tonight i get first light with my 250pds cant wait...

First pic of assembly

then attached to scope

Kev.