Direction of fan

[MarkInSpace]

Hi All!
Should the primary mirror fan on the back of a reflector blow onto the back of the mirror, or suck air from the back side of the mirror?
Cheers!

[mura_gadi]

Hello,

I have asked the same question and it seems most ppl blow. But I would have thought it would be cleaner to suck. I plan on chopping up some vacuum cleaner bags for my fans at the base of my dobs. A very cheap and effective way of dust control. Its an old trick from keeping my gaming towers clean, cover the intake fans with vacuum cleaner bags.


Steve

[luka]

There is a good article discussing the direction of the fan here (http://www.iceinspace.com.au/63-771-0-0-1-0.html).

[MarkInSpace]

Plenty of food for thought in that article - thanks!

[mura_gadi]

Hello,


Thanks for the link and thanks to Alex for the article.

Quick question, next to getting the mirror acclimated tube currents are the next biggest issue for newts.

But there are never any holes in the tube or fans on the side of the tube above the mirror to remove the thermal currents early. They are allowed to travel up the tube to exit. Pretty sure it would be easy to design a breather/extractor hole to allow hot air from the mirror to exit without allowing light in closer to the mirror cell.

Would this be useful? Or is the extra weight/time to add the feature in not worth it? By the time the mirror has acclimated enough to be used the currents have subsided anyway?


thanks
Steve

[RyanJones]

Hi Mark,

My suggestion and the way I am doing it is to suck. My main justification is that I have also made a primary mirror mask to cover the mirror clips and this is fixed inside the tube. It also then acts as a shroud so the fan at the back drawing the air out also draws air from the edge of the primary to remove the boundary layer. It’s all theory at this point as I am in the process of making a comparitor circuit to run the fan at full speed until the mirror is at ambient or just above and then step down to a lower speed to reduce vibration and perform the boundary layer removal.

My 2 cents

Ryan

[Allan_L]

Unless you have a collapsible Dob ;)

[glend]

Boundary layer fans are a thing, and they work. I had a closed back 12" GSO Dob, with just a cutout in the camp mat used to block off the back. The fan was then used to push air across the back of the mirror and exit up the sides. Just above the mirror surface I had a ring of rubber weather stripping attached to the side of the tube, the air would hit this weather stripping and be deflected over the surface of the mirror, ultimately making its way out the front of the tube.
I have seen other boundary layer fan setups where a small fan is suspended by wires in the blind spot below the secondary, it does not affect central obstruction and is invisible to the scope user. There are also solutions where small scroll type fans are used to blow across the surface, usually on custom made Dobs. I built a 10" Dob (Carbon Strut tube) where I used little pusher scroll fans from eBay to blow air over the mirror surface (they were attached to the carbon struts).

[Nikolas]

Making the hole light proof adds to the cost of the scope, even SCT's have vent holes behind the mirror.

[mura_gadi]

Hello,

Thanks for the relies, apologies for my late one.

I can see the advantage of removing the hot air over pushing it through the top of the tube, for both dust and heat. Having the right name for the fans is also very helpful when it comes to internet searching, so thanks for that.

I think some of my ideas for the scope can consolidate a little more. At this moment the mirror is lagging while I antagonise over the tube.


Thanks
Steve
Ps. I have just started reading "How to star test" by Harold Suiter and saw this, which is why I remembered the post:
"I installed a fan behind the mirror to pull air down through the tube and over the mirror. It worked wonderfully!"...