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Equipment Discussions >> Reflectors

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tnranger
super member


Reged: 01/14/12

Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5621306 - 01/13/13 05:39 PM

Not trying to highjack this thread, but I'm trying to understand it all.

I have a XT10i and 2 scavenged computer fans. One fan's mounting holes line up exactly with the fan holes on the mirror mount. The other fan is slightly bigger (~8.5-9cm). I do not want to cut holes in the OTA for side fans.

Will a single fan at the bottom of the mirror improve cooling enough to noticeably improve seeing? Is the consensus view that the bigger fan mounted/suspended further away will do a better job of cooling than one mounted directly to the mirror mount? Should the fan blow or draw?
Thanks for any help.


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Starman1
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Reged: 06/24/03

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Re: Thermal Issues and Fans Successes new [Re: tnranger]
      #5621319 - 01/13/13 05:45 PM

Quote:


1. Will a single fan at the bottom of the mirror improve cooling enough to noticeably improve seeing?
2. Is the consensus view that the bigger fan mounted/suspended further away will do a better job of cooling than one mounted directly to the mirror mount?
3. Should the fan blow or draw?
Thanks for any help.



1. Yes, it will.
2. Probably, but it depends on the conditions of use, too.
3. blow. This is the most effective way to cool the mirror, force hot air out of the tube, prevent heat from the observer from being sucked in, and improve the nature of the tube currents in the tube.


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rlmxracer
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Reged: 11/09/11

Loc: Motocross Mecca , CA
Re: Thermal Issues and Fans Successes new [Re: tnranger]
      #5621337 - 01/13/13 05:58 PM

I felt the mounting the fan on the mirror mount was too close to the mirror an would impeade the air flow reguardless of fan direction.

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tnranger
super member


Reged: 01/14/12

Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5621362 - 01/13/13 06:06 PM

That makes sense. A ceiling fan moves more air if it's away from the ceiling instead of flush mounted.

Thanks guys!


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Scanning4Comets
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Reged: 12/26/04

Loc: Ontario, Canada
Re: Thermal Issues and Fans Successes new [Re: tnranger]
      #5621497 - 01/13/13 07:34 PM

This thread seems to be chock full of mass confusion.

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nevy
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Reged: 02/07/12

Loc: UK
Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5621543 - 01/13/13 08:02 PM

The best way is just to suck it and see , as the saying goes.

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Bob S.
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Reged: 07/14/05

Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5621553 - 01/13/13 08:09 PM

Quote:

Quote:


1. Will a single fan at the bottom of the mirror improve cooling enough to noticeably improve seeing?
2. Is the consensus view that the bigger fan mounted/suspended further away will do a better job of cooling than one mounted directly to the mirror mount?
3. Should the fan blow or draw?
Thanks for any help.



1. Yes, it will.
2. Probably, but it depends on the conditions of use, too.
3. blow. This is the most effective way to cool the mirror, force hot air out of the tube, prevent heat from the observer from being sucked in, and improve the nature of the tube currents in the tube.




Don, Blowing does not seem to be as effective as sucking at least for larger mirrors. I am not sure where all of this talk of sucking in body heat has come from. My 20" scope with an AstroSystems shroud does not seem to suffer at all from sucking in body heat. It also uses a blowing fan on the front face of the mirror about 9" above the mirror which is quite effective when used with the rear enclosed sucking fan and annulus that surrounds my 20" mirror. Bob


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Sarkikos
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Reged: 12/18/07

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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621576 - 01/13/13 08:20 PM

Bob,

I wouldn't be surprised if there's a substantial difference in what will work best to control thermals for solid tube 8" and 10" Dobs as compared to larger truss scopes ... especially much larger 20" scopes. I can't imagine that the optimal solution would be the same for both optical systems.

Mike


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Bob S.
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Reged: 07/14/05

Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5621583 - 01/13/13 08:24 PM

Quote:

Bob,

I wouldn't be surprised if there's a substantial difference in what will work best to control thermals for solid tube 8" and 10" Dobs as compared to larger truss scopes ... especially much larger 20" scopes. I can't imagine that the optimal solution would be the same for both optical systems.

Mike




Mike, I do not see why not? The issue is to uniformily cool the primary mirror on all sides, reduce hot air masses rising off the ground, bring in cool air to the primary and mitigate thermals from surrounding scope materials. I am not sure what would be different in any scope? Bob


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demiles
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Reged: 11/07/06

Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621607 - 01/13/13 08:33 PM Attachment (12 downloads)

I use the a 80mm front and rear on my 15 in. And used a 70mm on my 10.

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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621609 - 01/13/13 08:34 PM

I had it affirmatively demonstrated to me that:
1) you don't need to seal the back end of a tube to get the mirror to blow up the tube
2) blowing up the tube was sufficient to establish a laminar flow of air out of the tube and disperse the boundary layer. It wasn't the most effective cooling regimen, but it was adequate for a 12.5" full thickness mirror in a tube.
3) the experiment was fan on/fan off. The difference was immediate, and profound.
I do not believe that sucking air toward the mirror will be as effective at cooling as blowing on the gloss from up close.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621611 - 01/13/13 08:35 PM

Bob,

Differences between large truss Dob and 8" or 10" solid tube Dobs for thermal issues:

- Shroud or open truss vs solid tube: allows easier access to install and maintain a front-blowing fan directly above the primary (I don't see this as practical for a solid-tube scope)

- Larger mirror: requires more time to thermally stabilize and more difficult to maintain thermal stabilization over change in delta during the night

- Longer distance from primary to sky end for larger scope: might require different solution in which way to exhaust the warm air from the optical system

There are probably other differences but these popped immediately into my head.

Mike


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demiles
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5621645 - 01/13/13 08:47 PM

Mike if you look at the specs on an 80mm fan it could range from 8cfm up to 60cfm plus. And my opinion not just any fan will do, fans from Noctua, Noiseblocker are of higher quality than most. I have bought many fans and those two are the best for having minimal vibration. BobS I've tried those Sunon maglev fans and you can do much better, the one I have had lots of vibration.
Lots of fans

Edited by demiles (01/13/13 09:16 PM)


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Bob S.
Carpal Tunnel
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Reged: 07/14/05

Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5621709 - 01/13/13 09:19 PM

Quote:

I had it affirmatively demonstrated to me that:
1) you don't need to seal the back end of a tube to get the mirror to blow up the tube
2) blowing up the tube was sufficient to establish a laminar flow of air out of the tube and disperse the boundary layer. It wasn't the most efeective cooling regimen, but it was adequate for a 12.5" full thickness mirror in a tube.
3) the experiment was fan on/fan off. The difference was immediate, and profound.
I do not believe that sucking air toward the mirror will be as effective at cooling as blowing on the gloss from up close.




Don, If you go over to the thread I started on the 20" f/3 Lockwood/Starmaster, you will find that indeed, the blowing face, sucking rear fan has been extremely effective. I termed it a CBLMS which is an acronym for Comprehensive Boundary Layer Mitigation System. I have tried blowing fans from the rear in an open system with very limited success in many different scopes (both small and large closed/open tubes). I have tried side fans just above the primary's surface coupled with rear blowing fans with limited success. The problem I have with these strategies is that they seem to create some turbulence with limited efficacy for my North Florida/South Carolina viewing conditions. When a fan blows from the back, I am not sure how the air reliably disperses the boundary layer that is just above the mirror's front surface? When I saw Joe Wambo's 32" scope with a fan placed in the secondary shadow gently blowing air onto the front surface, I witnessed extremely sharp images. I then talked with Jimmy Lowery who advised that 10 sucking fans in an enclosed structure were producing amazingly good images in his 48" scope. Having combined those two strategies together, I have found that the CBLMS that also incorporates an annulus surrounding the primary with about 1/2" larger diameter is providing exceptionally good boundary layer mitigation with views that are consistently better with both blowing/sucking fan strategy operating in tandem. The combined strategy also seems to be uniformily cooling the entire primary which is turning out to be a big deal. If you only cool one side, you can tend to get unequal cooling of the primary which can lead to suboptimal primary mirror substrate performance and subsequent impact on the mirror's figure. Bob

Edited by Bob S. (01/13/13 09:42 PM)


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tnranger
super member


Reged: 01/14/12

Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621822 - 01/13/13 10:21 PM

Don,

I assume you meant "you don't need to seal the back end of the tube to get the FAN to blow up the tube"?


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Datapanic
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Loc: Tucson, Arizona
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621831 - 01/13/13 10:26 PM

That's why I made my design reversible - blowing for cooling the mirror down and as needed, sucking to bring cooler air at the end of the 8-foot tube on down inside to equalize the temps all around. Most of the time, I've found that once things have near equilibrium, best results for imaging have been from sucking the air in from behind the mirror. When imaging, I'm nowhere near the eyepiece end of the tube so there's no problem with body heat.

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azure1961p
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Reged: 01/17/09

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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5621947 - 01/13/13 11:55 PM

Well I'm guessing a 100mm for the ten inch tho the 80mm is perfect for the 8". If 80s too small just cut a bigger hole and modify. Just remember the 80mm or 3" hole needs to have an inch of that hole intercepting the SIDE of the mirror. It looks wrong to see the fan overlap but that's what's need to put the fan wash across the mirrors face. Sure some will hit the side of the mirror and really do little good but the rest of the flow is doing fine.

In the sky and tel article by Adler the magazine published a very misleading illustration. It shows the fan hole cut ABOVE the front of the mirror and the airflow inexplicably cascading down upon it . Folks it does no such thing - ignore this bad illustration. I even tilted the fan to vector the air downward which looked utterly stupid from the outside of the tube and it did no good still. To reach the correct angle would have meant constructing air duct tubing to seal the flow of an angled fan. The whole thing was a mess.

Overlap an inch of the fan coverage on the side of the mirror and it becomes a neat tidy effective affair.

Pete


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azure1961p
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Reged: 01/17/09

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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5621953 - 01/14/13 12:03 AM

Quote:


3. blow. This is the most effective way to cool the mirror, force hot air out of the tube, prevent heat from the observer from being sucked in, and improve the nature of the tube currents in the tube.




I had a night of 7/10 drop to periods of 3/10 that came in these horrendous bouts of soft seeing followed by the secondary dewing. All because I tried having the air sucked down the tube. Exhalation doesn't belong inside the tube!!!!

Pete


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johnnyha
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Reged: 11/12/06

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Re: Thermal Issues and Fans Successes [Re: Bob S.]
      #5622017 - 01/14/13 01:09 AM

Bob let me try to get this straight - I'm confused... so the bottom fan under your primary is not blowing air up onto the back of the primary, but rather is exhausting air out the bottom, away from the mirror? I just want to get this straight as all this talk of "blowing" and "sucking" is getting confusing and I think some people are using them to describe the opposite thing.

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rlmxracer
sage
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Reged: 11/09/11

Loc: Motocross Mecca , CA
Re: Thermal Issues and Fans Successes [Re: johnnyha]
      #5622067 - 01/14/13 02:37 AM

Johnny, it seems different people are having success both ways. Several have tried both and seem to have good results eather way. I currently have a baffled 120mm fan behind the primary sucking air out the bottom. There are pics a page or two ago. I have only used it two nights but I've had great results. I honestly think its mostly because the mirror is cooled much faster than the standard 80mm fan. If you follow the air flow the cool air flows down the tube hitting the front of the mirror then wraps completely around the mirror. I have cut my time to get good high power views by 2/3 with this set up. It could be because the air flow is disrupting the boundry layer but i think the wrap around flow cools the mirror very efficiantly. Next week I plan to reverse the fan and see how that works. Rob.

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