Boundary layer fans?

[TheMightyKong]

Hi, does anyone have any experience of boundary layers fans in UK conditions?  Do they make a difference?  This is for a large (20in) thin (1.4in edge) fast mirror.  It should cool down quickly but I'm wondering about the boundary layer just above the mirror, I would think it won't go away until the mirror is in thermal equilibrium with the atmosphere.  Does anyone know how long that takes for a similar size mirror in typical southern UK conditions?  

If you found it made a noticeable difference, then what size fans are you using? How fast if the flow (CFM)?  Could you turn the fans off after a certain time or did you find it beneficial to run them all through the session?

Thanks in advance!

[Merlin]

You might find some useful info here, although a smaller mirror is involved: http://www.crossflowfansystem.com/docs/SkyArticle_01-2002.pdf .

[IanWatkins]

I'll have to read that PDF later as it doesn't work on this 'puter (must sort the Acrobat reader on this machine).

I think the idea is that once thermal equilibrium is achieved, the boundary layer will no longer be there.

Up until that point, you want to move the air above the mirror as smoothly as you can and by the shortest route. Most systems I have seen are filtered PC fans blowing air across the top of the mirror but these have been on open truss dobs. Not sure what you would do for a solid tube newtonian/Mak because I would assume blowing air up the length of the light path won't do much other than cause tube current effects. Maybe blow in from the side but suck out of the back?

Cheers

Ian

[TheMightyKong]

Thanks guys! I'll experiment when I have the scope. Re: rear fans, I've read about a baffle just above the mirror, called a 'mauro la lio' baffle after a poster on another forum, which seems to increase the effectiveness of rear fans sucking air down the telescope.

Does anyone know which fans are best for low vibration but still push a a large volume of air? I've heard noiseblockers are recommended, somebody mentioned the gelid fans, any others? And which is best? Thanks

[Lonestar70]

I'll have to read that PDF later as it doesn't work on this 'puter (must sort the Acrobat reader on this machine).

Cheers

Ian

Ian,

Slightly off topic... but...  

Try 'Foxit Reader' for your PDF's it is far less resource hungry than 'Adobe Reader'... and faster.

http://download.cnet.com/Foxit-Reader/3000-18497_4-10313206.html

Available for all versions of windows and Mac.

Best regards.

Sandy.