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| boundary layer cooling ? http://webstertelescopes.com/forum/viewtopic.php?f=7&t=567 |
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| Author: | jonW [ Thu Sep 16, 2010 7:11 am ] |
| Post subject: | boundary layer cooling ? |
Just curious if anyone has tried this on a Webster scope ?? eg: with a clip-on fan pointing down at the primary from the top of the mirror box .....or the fan located adjacent to the reflective surface of the primary in the corner of the mirror box ??? Good idea or bad idea ? I've seen it with the fan suspended above the centre of the primary held by 4 wires but would never contemplate that way for fear of an accident. Jon |
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| Author: | Eric [ Fri Oct 01, 2010 1:42 am ] |
| Post subject: | Re: boundary layer cooling ? |
The idea of the fan in each corner works good on small mirrors, but on large mirrors it removes the heat from the mirror in a "V" shape and appears astigmatic at the eyepiece. The fan suspended above the primary has been used successfully, even if it is unnerving at first. |
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| Author: | jonW [ Thu Oct 07, 2010 3:56 am ] |
| Post subject: | Re: boundary layer cooling ? |
Interesting... I'd overlooked the possibility of uneven cooling. A 3rd option could be to build a + shaped wooden frame that sits securely ontop of the mirror box with a fan in the middle blowing at the centre of the primary. Maybe take ~30 min off cooldown time?? Then take it off. The suspended fan using wires that is left on during observing might have downsides of diffraction effects of the wires and the fan's shadow increasing the central obstruction. Eric wrote: The idea of the fan in each corner works good on small mirrors, but on large mirrors it removes the heat from the mirror in a "V" shape and appears astigmatic at the eyepiece.
The fan suspended above the primary has been used successfully, even if it is unnerving at first. |
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| Author: | lightyear [ Thu Oct 07, 2010 6:35 am ] |
| Post subject: | Re: boundary layer cooling ? |
Hi Jon W. At Cherry Springs Star Party 4-5 months ago, I spoke to a guy with a 32" Webster. He had a suspended fan with 4 wires. Said he maid sure the fan was smaller than the central obstruction (6" I think), and used the same wire-thickness as the spider-vanes. I never got to look through it myself, but he said it was a worthwhile project. Everybody has their own idea of what is a good thing, and what is not. Myself, I'd just go with the removeable fan idea, and keep the usual one below the mirror, on all night. Then again, upgrading to the multi-fan option seems like a good idea to move lots of air. -David. |
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| Author: | dave [ Thu Oct 07, 2010 7:12 am ] |
| Post subject: | Re: boundary layer cooling ? |
jonW wrote: The suspended fan using wires that is left on during observing might have downsides of diffraction effects of the wires and the fan's shadow increasing the central obstruction. I've installed boundary layer fans on both my 25" and 22" scopes with only positive results. I used small picture hanger wire and lined the wires parallel with the secondary supports and never saw additional spikes on stars. The fans were of course smaller than the secondary and so remained in the shadow not adding any additional obstruction. A rheostat installed on the UTA allow fan speed adjustment at the eyepiece. The boundary layer fan was not a permanent install and could easily be attached and removed. To ensure proper collimation the fan was clipped on first and with a laser the secondary would be aligned exactly with the center of the fan (the fan being centered in the structure). After moving the fan out of the laser path the primary would be adjusted to match the laser location. This ensured the primary was centered in the structure (added benefit of the fan). Collimate as usual, clip the fan back on and observe! dave |
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