dust filters for mirror fans

[janoskiss]

Fans behind the primary mirror of a Newtonian are great for getting the mirror to thermal equilibrium and keeping it there throughout the night. They are also useful for keeping the optics from dewing up after bringing the scope into a warm house after observing in the cold outside.

But fans also suck dust and insects into the tube and much of this will land on the the mirror. A filter sandwiched between sheets of stiff flywire bolted over the intake side of the fan should keep these nasties out. I used 2-ply tissue paper for the filter but I'm sure there are better choices available. Of course the filter is replaceable.

Brief recipe: Cut the flywire to squares, allowing some overhang. Make room for screws, with a philips head screwdriver say, over the mounting holes in the fan housing. Place one square of flywire over the air-inlet followed by filter paper and another square of flywire. Affix to fan housing with screws, washers and nuts. Trim filter paper, tighten screws, and fold flywire over edges wrapping it around the fan housing. I made up three 80mm fans which are going behind my 12" mirror (because more is more :D). See photos.

[Dan31415]

Really stupid question time….but why don't you reverse the polarity of the fan (or turn it around) and have it spinning the other way? This way it would suck out air from the tube (obviously bringing it in from the top of the tube) rather then blowing everything in. I realise that for this you would need a ply cover like the other one on the forum (can remember which thread).

I ask not to criticise but I desperately need to put a fan on my Dob and am trying to figure out the best way to do it. Dust can be an issue in the NT particularly at Dark sites in the Dry season.

Thanks

Dan

[rmcpb]

Dan,

I have my fan blowing in from the bottom because it helps to keep dew formation at bay a bit longer than if it was sucking the air out. Otherwise pulling the air in from the top is a good idea but on dewey nights........

[janoskiss]

I would not like the air being sucked onto and hitting the mirror surface. Any dust around will get sucked right onto it. Also, while the scope is cooling the tube currents naturally rise because warm air is less dense than hot. A fan blowing air out the bottom of the tube will go against the flow and also gives the cool air a chance to warm up before reaching the mirror, as it travels down the tube. All manufacturers that supply fans with Newts have it blowing onto the back of the mirror AFAIK, and I'm not brave enough to start a new trend. ;)

[Photon]

With heatsinks on electronic devices designers say it is better to blow onto the surface being cooled. For the primary mirror in a Dob if we suck down the tube the air goes over the mirror edges and creates a vortex or slight vaccuum in the mirror centre which has less cooling efficiency than air being blown hard on the back of the mirror then finding its way around the mirror and up the tube. The rough surface of the mirror back is also an advantage, it effectively increases the surface area compared to the polished side, again increasing the cooling efficiency. A filter is a good idea, take a look at electronic equipment or your own PC and you will usually see a build up of dust around the fan grille and blades.

[Starkler]

Typical 80mm fans wont create a lot of air pressure so unless your filter medium is very loose weave, not much air is going to get through. Make sure you compare filtered and unfiltered fans for airflow.

[janoskiss]

Geoff, airflow seems very good. The "filter" is just a 2 ply unscented kleenex tissue.

[astro_nutt]

G'day everyone,
If I may, I would like to contribute an idea...for something other than 2 ply tissue..how about filterfoam, ( the type used in domestic A/C's)..or dacron...both of which can be washed and re-used. Secure a piece of flywire over the fan..then cut out a circle of dacron to fit over the bottom and up the side of the tube just enough to secure with a piece of elastic...this way the entire bottom of the mirror cell is filtered!
Dark skies everyone.

[janoskiss]

Geoff, you're just too damn clever!

I fitted the fans to a sealed baffle and to the back of the scope and there is bugger all airflow from three 2 watt 80mm fans on the 12" Dob, compared with the one 4 watt 90mm fan unfiltered on the 8" Dob. I'll probably have to ditch the filters and put up with the dust. :sad: I'll leave one layer of flywire to at least keep some of the bugs out. :shrug:

[avandonk]

Scotchbrite works very well as a filter and has very low air impedance. It also works very well on computer air inlets and is self supporting. Rinse first to get rid of any detergent even when new.

bert

[janoskiss]

Thanks Bert, but there is a range of 3M products (sponges, cloths ...) carrying the Scotchbrite name. Which one do you mean?

[avandonk]

The fibrous synthetic green scourer for cleaning pans I think.They are rectangular and come in various thicknesses.

Bert

[Astroman]

you can event use them to polish aluminium with a bit of WD-40 mixed in, comes up a treat...