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Project Galileo
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Reged: 11/14/07

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Re: Thermal Issues and Fans Successes new [Re: Project Galileo]
      #5626778 - 01/16/13 06:45 PM

Greater minds than mine have found this to be true as well.

Check this out.


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Pinbout
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Re: Thermal Issues and Fans Successes new [Re: Project Galileo]
      #5626867 - 01/16/13 07:43 PM

Quote:

I hate to pick at the issue but that is not what happens at all. I can say without exception I have never seen that happen with the sucking system I employ.




during another lifetime ago when I started designing black boxes, I had an engineer tell me its easier to pull a string thru the box then to push it, using his illustration to talk about how to move air over the hot electronics to help cool them.


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Bob S.
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Reged: 07/14/05

Re: Thermal Issues and Fans Successes new [Re: Pinbout]
      #5626878 - 01/16/13 07:52 PM

Pete, I too have to disagree with what you have said about sucking fan systems. I have not empirically experienced any of the potential problems that you suggest might happen. Additionally, Maura Da Lio's very helpful empirical work informed me and my scope builder early on for my new scope that is working quite exceptionally well with both a blowing front fan, annulus, and a
fully enclosed rear sucking fan. Bob

Edited by Bob S. (01/16/13 08:50 PM)


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Scanning4Comets
Markus
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Reged: 12/26/04

Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5626905 - 01/16/13 08:04 PM

Here's a video showing the three speeds of my fan at the back of the mirror HERE.

Cheers,


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Sarkikos
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5626945 - 01/16/13 08:22 PM

Bob S. et al,

Have the sucking fan systems been used successfully on solid-tube 8" or 10" Dobs? Or only on truss scopes of larger aperture? Apples and oranges...

Mike


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Scanning4Comets
Markus
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Reged: 12/26/04

Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5626979 - 01/16/13 08:40 PM

Quote:

For typical after-sunset temperature gradients around here (Ohio, USA), mirrors up to roughly 1-3/4" thick can track the falling temperature close enough with only a rear fan. Thicker mirrors need airflow over both surfaces. When the mirrors get extremely thick, you will never achieve a low enough delta T even with forced convection on all surfaces. In this case, you just rely on scrambling the front boundary layer as much as possible. The effect on the wavefront is to preserve high spatial frequencies (fine detail) at the expense of low spatial frequencies (veiling glare). This is a good trade off when doing high-resolution imaging, as veiling glare can easily be processed out.

Note that my results above won't apply if you live in a region that experiences different temperature gradients. The late Jeff Medkeff reported his 10" mirrors never settling down, and he lived in Arizona where they have large temperature differences from day to night. If you live along a coastline or a small island, you probably have a small rate of temperature drop, and your thermal problems will be less severe.

Bryan Greer






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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: SteveG]
      #5626995 - 01/16/13 08:49 PM

Steve,

Excellent series of videos! Very instructive. Evidently the important features to incorporate are a fan blowing onto the bottom of the primary, a sealed baffle around the fan, and a ring baffle above the primary.

I wonder what effect vent holes between the upper edge of the primary and the ring baffle would have on the system? That is what I've put in my 8" f/6 solid-tube Dob. I have a wide baffle around the fan below the primary, but the baffle is not sealed to the OTA. However, I do see a vortex in a defocused image. I think this indicates that air is being pushed up around the edges of the mirror and over its surface, even though there is no tight seal. This was before I drilled the vent holes. I haven't had a chance to test the 8" since then.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5627005 - 01/16/13 08:55 PM

Mark,

Quote:

Quote:

For typical after-sunset temperature gradients around here (Ohio, USA), mirrors up to roughly 1-3/4" thick can track the falling temperature close enough with only a rear fan. Thicker mirrors need airflow over both surfaces. When the mirrors get extremely thick, you will never achieve a low enough delta T even with forced convection on all surfaces. In this case, you just rely on scrambling the front boundary layer as much as possible. The effect on the wavefront is to preserve high spatial frequencies (fine detail) at the expense of low spatial frequencies (veiling glare). This is a good trade off when doing high-resolution imaging, as veiling glare can easily be processed out.

Note that my results above won't apply if you live in a region that experiences different temperature gradients. The late Jeff Medkeff reported his 10" mirrors never settling down, and he lived in Arizona where they have large temperature differences from day to night. If you live along a coastline or a small island, you probably have a small rate of temperature drop, and your thermal problems will be less severe.

Bryan Greer









So ... sucking or blowing? I think this snippet has more to do with whether a bottom and side fan are needed or not, and it was assumed that the fans are blowing onto the mirror.

Mike


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Scanning4Comets
Markus
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Reged: 12/26/04

Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5627020 - 01/16/13 09:03 PM

No.

If it is only a rear fan, it would be blowing. That's fairly obvious, lol.



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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: Scanning4Comets]
      #5627046 - 01/16/13 09:23 PM

So then pretty much irrelevant to my question, "Have the sucking fan systems been used successfully on solid-tube 8" or 10" Dobs?"


Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Project Galileo]
      #5627050 - 01/16/13 09:27 PM

Well then we're seriously at odds based on our first hand experiences. Alas we also have different designs on a few levels. Obviously its working for you so it makes no sense to change but it was a disaster with my setup.

It'd be interesting to do a side by side with your set up. Lol neither of us are budging though.

Pete


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Bob S.
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Reged: 07/14/05

Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5627496 - 01/17/13 06:38 AM

Quote:

Well then we're seriously at odds based on our first hand experiences. Alas we also have different designs on a few levels. Obviously its working for you so it makes no sense to change but it was a disaster with my setup.

It'd be interesting to do a side by side with your set up. Lol neither of us are budging though.

Pete




Pete, These issues are not as transparent as they seem. As I mentioned previosly, many attempts at blowing fans from behind and across the mirror in smaller closed tubes have not born a lot of positive fruit. However, recently, a friend brought a 10" Starmaster down to the Chiefland Astronomy Village that had a blowing fan from the back in what is essentially a pretty closed mirror box and then drilled three large holes just above the front surface of the primary. The primary mirror is a new 1" thick Zambuto 10" f/6. The results of this cooling strategy was nothing less than spectacular. The contrast and thermal stability of the scope was significantly enhanced. The large vent holes provided a means for the air blowing up from behind to escape out of the mirror box without too much apparent bouncing around inside the mirror box. We had the scope next to a non-cooled 30" f/3.6 scope all night and the 10" with active cooling was simply more stunning than we would have expected or have experienced without the active cooling system. The problem is that this is one of the first times that I have seen this strategy perform at the levels we were experiencing that night. I cannot be sure what variables were contributing to what? Bob


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5627529 - 01/17/13 07:18 AM

I've got the same set up with an array of holes for thermal exhaust just above my mirror. This helps beyond a shadow of a doubt. It's primarily for the boundary fan but it also helped even when it was just the rear fan blowing.

Pete


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Pinbout]
      #5627542 - 01/17/13 07:33 AM

Quote:

Quote:

I hate to pick at the issue but that is not what happens at all. I can say without exception I have never seen that happen with the sucking system I employ.




during another lifetime ago when I started designing black boxes, I had an engineer tell me its easier to pull a string thru the box then to push it, using his illustration to talk about how to move air over the hot electronics to help cool them.




There's pros and cons and a black box isn't a telescope assembly. Pushing the air has the flow more focused and directional while suction is spread out across a greater area without the same directional energy. There are ways a blowing fan can be made to better cool than suction on that merit alone. I'm not getting into the dickerings of electronics cooling though I'm sure your methods were best.

Pete


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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.]
      #5627567 - 01/17/13 08:05 AM

Bob,

Quote:

As I mentioned previosly, many attempts at blowing fans from behind and across the mirror in smaller closed tubes have not born a lot of positive fruit. However, recently, a friend brought a 10" Starmaster down to the Chiefland Astronomy Village that had a blowing fan from the back in what is essentially a pretty closed mirror box and then drilled three large holes just above the front surface of the primary. The primary mirror is a new 1" thick Zambuto 10" f/6. The results of this cooling strategy was nothing less than spectacular. The contrast and thermal stability of the scope was significantly enhanced. The large vent holes provided a means for the air blowing up from behind to escape out of the mirror box without too much apparent bouncing around inside the mirror box.




You know, this sounds oddly familiar ... Isn't this the very strategy that I've been talking about? Allowing the warm air to escape as soon as possible from the optical system just makes good sense to me. But so far I've only drilled vent holes for my 8" Dob, and haven't even had a chance to test it.

The 10" Starmaster had a "mirror box." So it was a truss scope, not a solid-tube? If so, I wonder how this might change the efficacy of the venting holes vs a solid-tube 10".

How large were the three "large vent holes?" Mine were only 1/4" - that was the largest bit I had available - but I drilled seven of them, about 1" apart, at the top-most edge of the primary. It would be an easy thing to drill more holes or widen the ones I have.

Mike


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

Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5627701 - 01/17/13 09:38 AM

I contacted my buddy about his 10" Starmaster. The box has about 4 holes in the rear of the enclosed mirror box. He reported to me that he installed a 4.5" fan with no speed control. He also cut 3 2.5" holes equally spaced across the side of the mirror box just above the front surface of the primary. Now this is a relatively small scope and so the air dimensions seem to support the strategy. John Pratte of JP Astrocraft who has made several 25" scopes with 3 fans attached to an open backed primary cell and somewhat open rear part of the mirror box and what appear to be 5 large holes cut into the rear side of the mirror box just below and above the primary has mentioned that the strategy has had limited efficacy in terms of use after the primary is equilibrated. What this tells me is that there are a plethora of variables interacting with each other and the solutions to effective thermal/boundary layer management in all different kinds of scopes are pretty elusive. Anybody that makes hard and fast statements that such and such solutions absolutely work is likely misleading others into believing that the solutions are fairly simple when I suspect that they really are not. Some of us have been actively mucking around with these concepts for a decade or more and many of our attempts have been in vain or have had limited efficacy. Bob

Edited by Bob S. (01/17/13 10:50 AM)


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Pinbout
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5627702 - 01/17/13 09:39 AM

Quote:

Quote:
--------------------------------------------------------------------------------


Quote:
--------------------------------------------------------------------------------

I hate to pick at the issue but that is not what happens at all. I can say without exception I have never seen that happen with the sucking system I employ.


--------------------------------------------------------------------------------



during another lifetime ago when I started designing black boxes, I had an engineer tell me its easier to pull a string thru the box then to push it, using his illustration to talk about how to move air over the hot electronics to help cool them.


--------------------------------------------------------------------------------



There's pros and cons and a black box isn't a telescope assembly. Pushing the air has the flow more focused and directional while suction is spread out across a greater area without the same directional energy. There are ways a blowing fan can be made to better cool than suction on that merit alone. I'm not getting into the dickerings of electronics cooling though I'm sure your methods were best.





what's up with the tude, dude?

fluids are fluids.

of course there are benifits to blowing and benifits to sucking

people still blow their fans on heat sinks.

just taking a fan and blowing on the mirror is a brute force tactic, primatively unsophisticated.

it works but if you don't want to have an open discussion about a more elegant solution...


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Pinbout
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Reged: 02/22/10

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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5627708 - 01/17/13 09:42 AM

Quote:

contacted my buddy about his 10" Starmaster. The box has about 4 holes in the rear of the enclosed mirror box. He reported to me that he installed a 4.5" fan with no speed control. He also cut 3 2.5" holes equally spaced across the side of the mirror box just above the front surface of the primary.




for the air to move efficiently the other side should have twice the area openning, unless you want to develop pressure and creat a plenum.


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Sarkikos
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Re: Thermal Issues and Fans Successes [Re: Pinbout]
      #5627776 - 01/17/13 10:34 AM

Danny,

Twice what area opening? What "other side?" Where? Rear of the box or across the side? Should those three 2.5" holes be 5" holes? In a 10" Dob? Please be specific.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes [Re: Bob S.]
      #5627788 - 01/17/13 10:44 AM

Bob S.

Quote:

I contacted my buddy about his 10" Starmaster. The box has about 4 holes in the rear of the enclosed mirror box. He reported to me that he installed a 4.5" fan with no speed control. He also cut 3 2.5" holes equally spaced across the side of the mirror box just above the front surface of the primary. Now this is a relatively small scope and so the air dimensions seem to support the strategy.




Which would be better, a larger number of smaller holes or a smaller number of larger holes? Of course for me, a larger number of smaller holes would be easier. Just take out the drill, put in your largest bit and have at it. No need for a hole saw. The seven 1/4" holes I drilled could be OK for my 8" f/6. I'll have to see. But I might want to do it differently for my 10" f/4.8.

Mike


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