installing a primary cooling/scrubbing fan in the shadow of the secondary

[themos]

I am very tempted by this idea. Last night I could clearly see the moon shadow of the secondary mirror on the primary and it just brought home to me that there is a substantial area of the primary that is just not used. I could even mount a fan ON THE MIRROR ITSELF if it wasn't for the need to see the centre spot for collimation. Then again, it is understood that "real men" collimate with a star test and not with the centre spot. What do you think?

[Astro_Baby]

I quite like the idea of a fan in the centre of the primary to bust up any thermals. Not sure if I;d ever be able to see a difference but the techie side apeals to me BUT I have the same issue - you;d have to remove the fan each time you wanted to collimate. On an open truss design its would be easy enough but on a solid tube desogn it could be a real problem

[Whippy]

Funnily enough, I was thinking about this for my Orion Argonaut as the centre of the primary is missing and from what I can tell, it's just a plastic plug so I guess it's certainly do-able but I reckon the mirror cell would have to be fairly well built to take the extra weight, possible vibration from the fan and keep collimation. My problem is that because the scope getting on a bit and information on it on t'web is scarce to say the least I'm worried that I'll strip it down and break something and never be able to find a replacement part!

Tony..

[johninderby]

Hi

There's a great article on the subject of cooling fans behind, at the side and in front of the primary here that's well worth reading.

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

John

[Astro_Baby]

Thanks John,

I saw that link ages ago but didn't bookmark it. I made some notes at the time though. I have a poroject on to do a basic fan assembly for my SW200. Which - if I ever get round to I'll post some pics of.

I not on the article the front fan is in fact on some kind of open truss assembly. If I couod come up with a neat way of getting one into the SW200 that could also be removed easily for collimation I think I'd do it.

[calibos]

Heres what I'll be implementing for my 16" Lightbridge

Garden hose for the ducting. Qualpex plastic pipe for the plenum. Front of mirror sealed from rear primary cooling fan.

[Astro_Baby]

Thats impressive - silly question though - wont the air rise in different directions as the scope moves about ?

[dark knight]

I would have thought this will create more air currents across the primary. I Like the idea by Calibos, but would only use it to blow light dust etc off the mirror. Don't also forget the fan will create heat that will then rise up in the OTA. Would you cool the fan?

[calibos]

The boundary layer problem is not with air currents per se but actually a homogenous layer of warm air sitting on the suface of the primary with a different refractive index than the rest of the air in the OTA tube. ie picture a glass with a little bit of water in the bottom of the Glass. Put a straw into the glass and look at the end of the straw in the water. Now imagine that Glass is your OTA tube. The bottom of the glass is the mirror and the small amount of water in the bottom is the boundary layer sitting on the mirror and the straw is the beams of light.

Mirrors need to cool to the ambient air temperature for 2 reasons. One is that until the mirror stabalises at ambient different areas of the glass will cool at different rates leading to small stresses in the glass and some deformation, ie astigmatism. This is a very minor problem however. The other reason is that until a mirror is at ambient it is radiating heat, some of which collects in the paraboloid of the primary mirror 'dish'/bottom of the OTA. ie Boundary layer air. This is in effect internal OTA 'bad seeing'. Invariably the actual atmospheric seeing is never as bad as people think, its actually boundary layer bad seeing they are experiencing.

So...We either leave our mirrors to cool naturally to ambient which involves leaving the scope out hours before we want to observe or else spend the first few hours observing through really 'bad seeing'. We all know how one is lucky to be able to plan a session even a few hours in advance, so this really is a non starter. This is why so many of us look for outside storage solutions like sheds of garages where our scopes are already much closer to ambient which shortens the waiting time.

Or....We fit cooling fans to our scopes to hasten the cooling to ambient and shorten the waiting time before we can start observing. I don't think its a choice of either/or though. Even if one was guaranteed clear skies and could put the scope out hours in advance without worrying about it getting rained on, one should still fit and use a fan. This is because unless one has a mirror less than a half inch thick, your mirror will never reach ambient. The temperature drops for most of the night and a naturally cooled mirror over 1/2" just can't naturally radiate enough heat to track ambient. So what you want is a fan that hastens initial cooling but also stays on to help the mirror track ambient. This is why so many of us go to the trouble of coming up with solutions to vibration isolate the fans and to controll their speed. To stop a constantly on fan transfering vibration to the OTA and intorducing microvibration to the view through the scope. ie by mounting the fans on vibration isolated baffles and wiring up fan spped controllers so that we can run the fans full tilt for the initial cooling period and then lower their speed to just enough to track ambient while minimising vibration.

Why do we install boundary layer fans then when a rear cooling fan will eliminate the boundary layer when the mirror reaches ambient?

Because while a rear fan hastens full cooling to ambient it hastens it from 4 or 5 hours to 1 or 2 hours. Some of us don't even want to wait that 1 or 2 hours till our scopes start delivering their best views. We want them to deliver from the off!

Thats why larger newt owners install boundary layer fans.

Smaller newt owners generally don't need to. The reason being that

A. the mirror is smaller and thinner and thus reaches ambient quicker.

B. If the smaller newt has a baffled fan then the mirror is small enough in area that the air blowing up around the mirror up the OTA creates vortices near the mirror edge that are strong enough to pull the entire boundary layer up off the mirror and quicky mix it up and homogenise it with the tube air around the rim of the OTA on its way up and out.

This doesn't work quite as well for Mid size Newt owners like myself with my Orion 12" Dob. The vortices pull off the outer half of the diametre of the mirrors boundary layer but leave the innor circle area un disturbed. One would now have made things worse, ie more of the mirrors light being refracted differently to the rest. We can ensure the full boundary layer scrubbing by installing a ring baffle around the inside of the OTA just above the surface of the mirror. This redirects the primary fan air blowing up around the mirror across the surface of the mirror, blowing off the boundary layer.

It is not a problem now having this turbulent air starting to rise up the middle of the OTA within the lightpath. A. it will quickly rise to the upper surface of the OTA tube edge and mostly out of the lightpath .

B. one has to remember that boundary layer seeing and atmospheric seeing is not so much turbulence that you are seeing. (Otherwise one couldn't observe if there was any wind) It is instead looking through layers of air with different refractive indexes. ie as long as the wind is air of the same temperature with the same refractive index...you don't see the wind. Bad seeing is looking through a column of air between you and the target with many different bubbles of air with different refractive indexes between you and the target location.

So one doesn't 'see' the turbulent air rising in the lightpath that you just blew off with the ring baffle. One only sees it if the fan air and vortices didn't churn and mix up the boundary air with its different refactive index with the cooler OTA air quickly and completely enough. On my 12" Mirror my baffled rear fan and ring baffle does accomplish this. It scrubs the entire boundary layer and mixes it up quickly enough that I don't see it.

Which brings me to one of my last points. With a 16" mirror or larger, neither a rear fan and its vortices, nor a ring baffle which redirects the fan air further into the area of the mirror is enough to scrub the entire boundary layer air. Again you'd be scrubbing a portion of it off but leaving a portion of it with its different refractive index in place. You also wouldn't be churning and mixing it up enough to homogenise it quickly enough that the effects were eliminated. You'd actually be making things worse.

So large Newt owners fit fans that blow directly across the surface of the mirrors. The thing is though that after reading about some tests that were done, it seems that although a fan directly blowing across the mirror can easily clear a wide path of boundary layer right across the mirror to the other side, its still a lane of air that its clearing. So the guys that just fit two boundary fans blowing across their big mirrors are really just clearing 2 lanes of boundary air leaving 'lanes' uncleared. You've now got stripes of air across the mirror with different refractive indexs, (cleared and uncleared lanes) Again this is almost worse than if you had done nothing.

This is why on some big dobs you will see an array of 5 or 6 boundary fans blowing across the full diametre of the mirror leaving no 'lane' unscrubbed. Now thats a lot of air blowing across the tube. One doesn't want all that 6 fan dust swirling around the OTA and settling back on the mirror. So these guys create a door or flap on the opposite side of their mirror boxes that they open when the fans are on to let all this air out.

Finally. You can't internally mount 5 or 6 fans and cut a slot and create a flap in a round steel Meade Lightbridge Lower OTA!!

Hence, my solution above. Boundary fans don't have to be mounted inside my OTA but are ducted inside with the hose pipe. I don't need 5 or 6 fans to scrub all the 'lanes' the full width of the mirror. I only need about 2 which blow through the ducting and qualpex out holes drilled in the qualpex. PLenum means higher pressure inside the pipe and the holes are smaller closer to the duct from the fans which means an equal amount of air blowing out of each hole. Think of each hole as a separate fan blowing across the mirror and added together the cover the full diametre of the mirror. Kind of like 5 or 6 fake fans powered by only two outside mounted real fans.

Instead of a slot and flap opposite the fake fan array I will be using another 2 ducted fans and qualpex plenum but with the fans reversed and sucking air out.

Is it a lot of hassle to come up with and implement? Its actually not going to be too much hassle but certainly a lot more hassle than 99.99% of people would/are going to.

Guess what though. My scope will deliver the best views it is capable of delivering given the prevailing atmospheric seeing that I can do nothing about, right from the moment I set up. No 4 hour wiat, no 1 hour wait, no 5 minute wait. Soon as the scope is collimated after I set up, bang, the scope is delivering the best views it is capable of that night!

[dark knight]

Wow, that's a great explanation, I just won't pretend to understand it all.

[dark knight]

I suppose the air blown through the small holes is not much different to a garden irrigation system, the hole at the end of the line allows as much water through as the first hole.