Mirror cooling fan direction. Help please

[Rocketron]

Sorry if this is a silly question but can anyone tell me which way the cooling fan behind my primary mirror should be running, should it force air onto the back of the mirror or exhaust it away:confused:.

The scope in question is an Orion Optics 14" dob.

Many thanks

[Astronut]

I'm toying with the idea of a DIY project to fit a fan and baffle to my dob and have been doing much research on the best approach.

The most common approach is to blow air onto the back of the mirror. A big improvement can be made by fitting a baffle around the fan housing to make the fan more effiecient ie. air hitting the back of the mirror flows toward the mirror edge and ideally into the tube and over the mirror surface to break up the surface thermal layer (but this usually requires an internal tube baffle just above the mirror surface).

[Doc]

I agree my fan is positioned beneath the primary mirror and blows onto it to cool it down.

[barkis]

Of cousre I'm sure you already know that a cooling fan is only to assist the mirror in reaching ambient temperature, and once acheived, should be switched off. any air currents flowing over the mirrors surface, whether cool or warm, will distort images.

I don't mind you saying,

"Of course we knew that stupid":D

Ron.

[Rocketron]

Thanks for the advice folks,

My fan actually sucks air away from the back of the Primary at the moment, only thing that worries me about having it blow onto the Primary is that you could be blowing dust into the mirror cell.

[johninderby]

Yes it won't do much unless it is blowing on the back of the mirror. Dust doesn't seem to be a problem, but if you're concerned buy a fan filter which is only a pound or two (PC case fan filter) and stick it over the fan.

While the main point of a fan is to cool down the mirror quicker it can also be left running while observing if the air temperature is dropping faster than the mirror is cooling. So long as it doesn't cause vibration.

One thing that is becoming more popular in the US now with larger mirrors is to have two small fans in the side of the tube that blow across the mirror's surface while observing. Apparently it keeps the air just above the mirror's surface mixed and stops a thermal boundary layer from forming that can cause distortion. Another approach is to suspend a fan just above the front of the mirror that blows down into the centre and again stops a thermal layer from forming. A bit tricky as you have to line up the supporting wires with the secondary vanes to prevent additional diffraction spikes.

Hope I haven't confused you but there are a lot of ways to use a fan on a dob or newt.

John

[Rocketron]

Thanks John,

I must admit that is it does make sense that you blow on an object to cool it down.

I will look into one of those fan filters.

Don't think I will bother trying to suspend a fan in front of the mirror, that sounds like a receipe for disaster.

Cheers

Ron

[johninderby]

I've got an OO 14" dob as well.

It doesn't actually take much airflow to cool the mirror down, and in fact too much airflow doesn't work that well. If the cooling air moves too fast it doesn't have time to pick up heat from the back of the mirror. Hence the small size of the fan that OO fits.

Yes some of the Americans do go to crazy lengths to cool their mirrors down. Still I suppose it gives them something to do when it's cloudy

John

[Rocketron]

Cheers John,

Sent you a PM

[calibos]

Yes some of the Americans do go to crazy lengths to cool their mirrors down. Still I suppose it gives them something to do when it's cloudy

......and some crazy Irish too!!

Ya wanna see what i have planned for my LB16!!

Vibration isolated suspended Baffle with 120mm fan. With a gap around the rim to let the air escape instead of being forced up the tube.

I'll be sealing around the mirror to isolate the inside of the tube from the rear fan.

Don't need the air blowing up the Tube to clear cold tube induced tube currents because the inside of my LOTA and UTA will be lined with an insulating sandwich of felt, corrugated cardboard and cork with a layer of flocking paper on top. Its a Notice board with the frame removed and glued inside the tube

I also don't want rear fan air going up the tube because it would interfere with my boundary layer fans which will also be mounted on the rear vibration isolated baffle....behind the primary mirror

How is that possible??

I am making a kind of plenum cooler. ie. I have 4 3/4" pipe T junctions with a length of 3/4" qualpex plastic pipe sealed at the ends coming out of the top parts of the T. 2 on one side of the mirror and 2 on the other. I will run 4 pieces of 3/4" bendy garden hose down and around the back of the mirror from the bottom of the T's to the 40mm boundary fans mounted on the rear baffle.

Along the legths of qualpex I will drill out several holes, larger holes farther away from the T junction to equalise the air pressure coming out all the holes. There will be positive air pressure inside the pipe, hence why its like a plenum which should make for a steady uniform flow of air. This will mean that I won't have just a corkscrew of air blowing across just a slice of the mirror, I'll have an even flow across the entire width of the mirror.

2 of the fans will be blowing air in and across the mirror from the qualpex plenums on one side of the mirror and the other 2 fans will be sucking air out through the plenums on the opposite side of the mirror.

I have indeed had a lot....A LOT! of cloudy nights to plan all this

I really believe its worth doing though. The amount of times I have been on observing sessions and the rest of the lads said the seeing wasn't very good at all and I had to say, "I beg to differ!!" They'd be sticking to observing a tiny Saturn at 100-150X and I'd be viewing a crisp Saturn at 300x +. Might have been something to do with them viewing through uncooled, non-boundary layer modded Newts and non-lymax Coolered SCT's and me viewing through a modded Orion Dob with baffled rear cooling fan and the deflecting ring baffle boundary layer remover mentioned above. (redirecting/reusing the rear fan air across the back of the mirror and then forcing it up around the mirror into the tube where a ring baffle re-directs it across the surface of the mirror to scrub off the boundary layer.) The only reason I didn't re-use this simpler idea with the LB16 is I don't think it would work anywhere near as well on the much larger surface area of the 16in mirror like it did on the much smaller 12in mirror of the Orion.

Hey John, Maybe you didn't need to drop from your 16 to a 14. Maybe it wasn't that your local seeing didn't support a 16". Maybe it was just boundary layer air. (not that a loss of 2" makes much difference anyway...and now that I think of it, you went mass produced to Premium mirror at the same time so it was probably an 'Upgrade' regardless)

Anyway.....keeps me out of trouble while planning and modding whether it improves my views or not

[johninderby]

Looks like you have some very interesting mods planned. Pictures to follow maybe ?

The OO's 14" borosilicate mirror (99 Strehl) cools down really quickly and works just fine with the standard cooling fan, and has no problem in keeping up with changes in air temperature. The mirror seems to cool evenly all over. The quality and consistency of the glass seems to make a difference.

The Lightbridge's 16" mirror is fairly thick plate glass and takes a long, long while to cool down, even with cooling fan mods. While plate glass mirrors work fine once they've cooled down, they can struggle to keep up with changes in air temperature so modding is essential. I've since also found out that many 16" Lightbridges came with poor quality secondary mirrors, so if your Lightbridge doesn't perform as well as it should the secondary mirror could be the culprit.

John

[sublime_mick]

any pics/step by step guidelines?

[jamespels]

I found the spider makes a great place to mount the fan, and you can get some nice large diameter fans that don't need to spin so fast. Cools the mirror down quick, but the view can be a bit flickery...

Sorry - I'll get my coat.

[LukeSkywatcher]

A cooling fan blows air, an extractor fan sucks air.

Simple as a meerkat.