Primary mirror boundary layer

[Acrab (Victor Martinez)]

Hi,
Does anyone know if the side fans that are installed in the big Newton telescope to sweep the boundary layer, are more effective in extracting or introducing air? Actually, in my 16" dobsonian telescope I have three 120 mm fans, one in the back venting the primary, and other two side drawing air from boundary layer.
Anyway, this of thermal acclimation of mirrors of a given thickness, it is difficult to solve
Víctor

[mental4astro (Alexander)]

Ola, Victor,

My big 17.5" newtonian doesn't have fans as it is so open that it cools quickly for me as it is.

All the fans do is move air to cool the mirror to ambiant temperature. I would have thought that introducing cool air into the tube would cool the mirror faster, also pushing out the warm air in the process.

If your scope is totally open, I really don't see the need for cooling fans. They may, however, help reduce dew forming because of the air movement. Any ideas on this, Victor, anyone?

Alejandro.

[Acrab (Victor Martinez)]

My telescope is a Kriege design, close box.

[Alchemy (Clive)]

http://www.pha.jhu.edu/~atolea/WAS/t...newtonians.pdf

[mental4astro (Alexander)]

If it is an orthodox Kriege design, it will still be quite open, top and bottom. If your scope was a solid tube, I'd say a fan might be a good idea. I don't see the advantage though as there won't be interanl air thermal currents, and unless you are taking photographs within minutes of completing set-up, again I don't see the advantage of having a fan.

However, the choice is yours.

[Acrab (Victor Martinez)]

Thanks Alchemi, I already knew this article. Asked about the experience with people dobson owner of large telescopes and the most effective method to cool the big primary mirrors usign fans, outside the use of Peltier cells.

[sasup (Stacey)]

He estado leyendo un poco sobre el tema y parece que el consenso es que sí es una mejor manera de controlar la temperatura sobre el espejo. I hope that's right may Spanish is rusty.

[skies2clear]

Hi Victor,

I know what you are talking about. I too have a 16" Dob (Kriege) and have a few problems with the same thing. Even though the mirror box is open at the top and bottom, it still manages to trap air in it quite well!
I haven't installed side mounted boundary layers fans yet, but seriously looking at doing this. So I'm waiting to hear from anyone with experience too,

Clear skies

[Acrab (Victor Martinez)]

Originally Posted by skies2clear

Hi Victor,

I know what you are talking about. I too have a 16" Dob (Kriege) and have a few problems with the same thing. Even though the mirror box is open at the top and bottom, it still manages to trap air in it quite well!
I haven't installed side mounted boundary layers fans yet, but seriously looking at doing this. So I'm waiting to hear from anyone with experience too,

Clear skies

If it's help you, I had the best results with the side fans pulling air of the box

[tnott]

Boundary layer fans useful only if you want to use high power while you are waiting for the mirror to cool down, though it will cool down quicker if both the front and back surfaces of the mirror have air blowing on them.

Any fan will make a big difference over no air movement. Doesn't matter how open the structure - if the mirror is over a certain thickness, it will not keep up with the average drop in temp. during a night with radiation and natural convection alone.

But, there is a law of diminishing returns here, partly because we are limited by the ability of the inside of the glass to conduct heat to the outside. So - one fan good, lots not a great deal better, unless front and back.

Sucking is omnidirectional and blowing is unidirectional so sucking will not bring as much cool air in contact with the mirror. Try sucking out birthday candles!

Having a baffle (or sealed tube/mirror box) around the fan will make it much more effective because the air from the front of the fan is not just being sucked back in behind.

For these reasons, I just have just used one fan blowing on the back of the mirror for my scopes, with either a baffle or sealed mirror box.

Robert Houdart has made a mirror cooling calculator to get an idea of different scenarios:
http://www.cruxis.com/scope/mirrorcooling.htm

Peltier coolers work well but are probably only really necessary for the much higher powers of imaging planets.

[Acrab (Victor Martinez)]

The idea is to evacuate the hot air that rises from the surface of the mirror, not cool the mirror.
Look at this post:
http://www.cloudynights.com/ubbarchi...art/all/vc/1II

[tnott]

The idea of boundary layer fans is to replace the chaotic boundary layer of warm air clinging to the face of the mirror that is ruining the view into a smoother, laminar one, reducing distortion. This phenomenon is the primary cause of what people used to call "tube currents".

If the mirror's temp. is within a degree of ambient then you won't have a substantial boundary layer to remove.

So, if you turn the mirror cooling fan/s on when you are setting up, collimating, waiting for it to to dark, deciding what objects to view for the night etc. like I usually do, then one good fan on the back should be enough according to the data and modeling I have seen.

Also, while the mirror is cooling the figure is distorted anyway.

Robert's mirror cooling calculator is approximate, but can give you a ballpark idea how long it takes to cool if you play around with the variables.

But, if you want to view things like planets and double stars in a hurry and don't want to wait for the mirror to cool down then boundary layer fans may help somewhat for this specific situation.

[tnott]

Just had a thought - would little Dyson air multipliers be better for boundary layer fans, because they allegedly don't chop up the airstream like regular fans?

[tnott]

Another couple of good links for those who are interested in this issue:

http://www.fpi-protostar.com/bgreer/fanselect.htm
http://www.cloudynights.com/ubbthrea...fpart/all/vc/1