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Equipment Discussions >> Reflectors

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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: Bob S.]
      #5627800 - 01/17/13 10:49 AM

Bob S.,

Quote:

John Pratte of JP Astrocraft who has made several 25" scopes with 3 fans attached to an open backed primary cell 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.




There's still plenty of room for trial and error. Seems we're far from the day when we can just consult a book full of spec charts to look up what we should do to solve thermal problems for a specific telescope. But wouldn't that be great?

Mike


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5627822 - 01/17/13 10:59 AM

Quote:

There's still plenty of room for trial and error. Seems we're far from the day when we can just consult a book full of spec charts to look up what we should do to solve thermal problems for a specific telescope. But wouldn't that be great?

Mike




We need a program that will do for scope cooling, what PLOP did for mirror cells.


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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: Mark Peterman]
      #5627830 - 01/17/13 11:03 AM

That would be nice, as long as the programmer keeps it simple. It should be easy enough that any amateur who can measure accurately can use it. It should avoid hard-core math* and engineering terms and principles ... please. I'm sure most of us are just interested in getting a sharper image, not a degree in optical engineering or thermal dynamics.

Mike

* "Hard-core math" = anything beyond simple algebra. A university math professor used to live in my building. When I told him I'd gotten as far as calculus, he said, "Oh, you mean higher arithmetic."

None of that attitude, please, when writing the "Thermal Solutions for Dobsonian Telescopes" program or book.


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SteveG
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Reged: 09/27/06

Loc: Seattle, WA
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5628110 - 01/17/13 01:37 PM Attachment (21 downloads)

Thanks for the comments everyone. I'm interested in improving both cool-down and the boundry layer, so I'm open to new ideas. I need to run more tests and plan on trying the following:

Measure and time the cool-down with the current configuration.
Test for fan vibration with the current configuration (haven't noticed anything yet).

Reverse the fan so that it's "sucking" air from below, and I'll try the smoke test again and in addition, will measure & time the cool-down via an infrared thermometer.

Lastly, after reading through this thread, I'm interested in adding a 2nd fan, high-quality low vibration type, to either blow across the face of the mirror, or more likely just to exhaust the air from just above the mirror (exhausting outside the tube).

At this point I can't make many claims on these improvements, other than adding the rear baffle dramatically improves air movement around the mirror. Cool-down, here in the Pac NW, is still surprisingly long - at least one hour in winter!


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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: SteveG]
      #5628152 - 01/17/13 02:00 PM

Steve,

What did you make the baffle from? Is that plexiglass painted black or ...?

I like to construct baffles from black foam core painted flat black and doped with an outdoor sealant. An x-acto knife is sufficient to cut foam core and it's easy to make snug fit. I think it might be a better dampener of vibration than other materials DIYers are using. And it's cheap.

Mike


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Thermal Issues and Fans Successes new [Re: SteveG]
      #5628168 - 01/17/13 02:12 PM

Quote:

Thanks for the comments everyone. I'm interested in improving both cool-down and the boundry layer, so I'm open to new ideas. I need to run more tests and plan on trying the following:

Measure and time the cool-down with the current configuration.
Test for fan vibration with the current configuration (haven't noticed anything yet).

Reverse the fan so that it's "sucking" air from below, and I'll try the smoke test again and in addition, will measure & time the cool-down via an infrared thermometer.

Lastly, after reading through this thread, I'm interested in adding a 2nd fan, high-quality low vibration type, to either blow across the face of the mirror, or more likely just to exhaust the air from just above the mirror (exhausting outside the tube).

At this point I can't make many claims on these improvements, other than adding the rear baffle dramatically improves air movement around the mirror. Cool-down, here in the Pac NW, is still surprisingly long - at least one hour in winter!



If you get close to ambient in one hour with a single fan and a 10" mirror, you're doing VERY well.
Try different scenarios with this mirror cooling calculator and you'll see that 1 hour cooldown is quick! If you didn't have a fan, it might never make it to ambient.
http://www.cruxis.com/scope/mirrorcooling.htm


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SteveG
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Loc: Seattle, WA
Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5629929 - 01/18/13 01:07 PM

Yes - Don - I actually think it's more like 1.5 hrs! Still too long in winter.

Mike - Its 1/4" thk black Plexiglas. Shiny on the outside, flat on the inside. I'm very lucky to have a good plastics shop doing custom work for my business, so I can just draw up an astronomy part and get it made. I just received a really nice eyepiece tray that I designed for my Vixen Porta II.


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5630471 - 01/18/13 07:33 PM

Thanks to the work of all those who have gone before us, I think that we can all agree on the following for a solid tube telescope design with a single, rear fan:

A sealed fan baffle is required for optimum effect.

An annulus in front of the mirror is necessary for a rear fan to effectively scrub the boundry layer on the front of the mirror.

The questions that still I have are:

For a fan located behind the mirror, which is better, blowing or sucking? It appears the annulus will direct air over the mirror with either method but there must be advantages/disadvantages to each method.

What size fan (CFM) is adequate for a given mirror size? Is the bigger the better or do you get to a point of diminishing returns?

Is there an optimum distance above the mirror to mount the annulus?

Mark


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azure1961p
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Reged: 01/17/09

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Re: Thermal Issues and Fans Successes *DELETED* new [Re: Mark Peterman]
      #5630658 - 01/18/13 10:06 PM

Post deleted by azure1961p

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

Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5630708 - 01/18/13 10:25 PM

Pete, It would be nice if you could tone down the heat a little bit and allow others to have opinions or better yet share their empirical knowledge. The work of Mara Da Lio, myself and many others should not be strongly discounted. If you have a lot of experience and maybe 10-20 scopes that you have experimented with, then you join the club of many of us who do have this amount of experience with closed and open tubes. It appears that a sucking fan with an annulus might be one of the most efficacious strategies for small closed tube telescopes? I am not saying it as an absolute or with ultra strong push but based on the experiments to date, it seems like one of the better strategies. Bob

Edited by Bob S. (01/18/13 10:40 PM)


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Chucky
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Reged: 04/16/10

Loc: Dublin, Ohio
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5631107 - 01/19/13 07:24 AM

<< I think that we can all agree >>

Will never happen <g>


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5631295 - 01/19/13 10:39 AM

Quote:

Quote:

Thanks to the work of all those who have gone before us, I think that we can all agree on the following for a solid tube telescope design with a single, rear fan:

A sealed fan baffle is required for optimum effect.

An annulus in front of the mirror is necessary for a rear fan to effectively scrub the boundry layer on the front of the mirror.





No it's NOT necessary and a side mounted fan has stronger directional airflow. The annulus works but its hardly the last word. Frankly your finds are a little puzzling, particularly on a thread this large.

Pete




Pete, first thanks for starting this thread, it has been a very interesting topic and I have learned a lot from everyone's experiences.

Regarding my post, have you viewed SteveG's videos? Did you see the difference in a sealed baffle versus unsealed?

My second point, about the annulus, relates to my scenario of a SINGLE fan, mounted on the REAR of the scope. I have no desire to cut a hole in the side of my scope (at this time). While I don't doubt that without the annulus, there is some 'pulling' of the boundry layer as the air flows linear up (or sucked down) the tube, I doubt it is as effective as when using the annulus.

Do you have any knowledge to share regarding the questions I asked in the same post?


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Mark Peterman
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Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: Chucky]
      #5631307 - 01/19/13 10:47 AM

Quote:

<< I think that we can all agree >>

Will never happen <g>




LOL, yeah, what was I thinking! Take a two minute video, have two people watch it, and they COULD tell you two different stories about what they saw.


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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: Mark Peterman]
      #5631353 - 01/19/13 11:13 AM

Mark,

Quote:

Quote:

No it's NOT necessary and a side mounted fan has stronger directional airflow. The annulus works but its hardly the last word. Frankly your finds are a little puzzling, particularly on a thread this large.

Pete




Pete, have you viewed SteveG's videos? Did you see the difference in a sealed baffle versus unsealed?

My second point, about the annulus, relates to my scenario of a SINGLE fan, mounted on the REAR of the scope. I have no desire to cut a hole in the side of my scope (at this time). While I don't doubt that without the annulus, there is some 'pulling' of the boundry layer as the air flows linear up the tube, I doubt it is as effective as when using the annulus.

Mark




Keep in mind that Steve has a 10" truss scope, while Pete is working with an 8" solid-tube f/8, I believe. Steve's scope has a relatively short mirror box, while Pete has a long solid-tube to deal with. If Steve does not wrap his scope in a shroud, he already has a very large hole in the middle of his "tube" (though admittedly not at the level of the mirror face). Pete has an enclosed solid-tube OTA about 5' long. These differences might be important.

Steve's video agrees with my own experience so far. My primary fan is blowing onto back of the mirror, and for my Z8 I have an annulus above the mirror. I'd probably be doing better if I sealed the baffle around the fan. And I'm anxious to see the results in Steve's next video when he reverses the direction of airflow from the fan.

But things might be somewhat different for a solid-tube scope beyond the length of a mirror box. A boundary layer fan and / or venting holes do make sense for a solid-tube scope.

Now, if we don't want to drill or saw-cut holes in our OTA's, that's another matter altogether. I don't like doing that, either. But not wanting to do something is not a valid argument per se for not doing it ... though I do try to get away with that rationale here at home.

Mike


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5631403 - 01/19/13 11:43 AM

Quote:

...
What size fan (CFM) is adequate for a given mirror size? Is the bigger the better or do you get to a point of diminishing returns?




Bryan Greer suggests some sizes here: Fan_Select. It is also worth looking at the articles by Gary Seronik: Beat_the_heat and Alan Adler: Cross Flow Fan.

I currently have a small, quiet 40mm fan on the back of my solid tube 8", blowing. It has lowered cooling time to under 1 hour from 2 hours to never, when it had an almost completely closed back. Further improvement is needed and I have found this thread both interesting and useful.


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5631424 - 01/19/13 11:53 AM

Mike,

Yes, I am aware of the scope SteveG was using. The lower end of that scope is a 'solid tube' construction (maybe 24-26 inches tall). I believe the video was good eveidence of what is happening around the mirror in a round, solid tube, configuration.


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: cjc]
      #5631437 - 01/19/13 12:01 PM

Quote:

Bryan Greer suggests some sizes here: Fan_Select. It is also worth looking at the articles by Gary Seronik: Beat_the_heat and Alan Adler: Cross Flow Fan.

I currently have a small, quiet 40mm fan on the back of my solid tube 8", blowing. It has lowered cooling time to under 1 hour from 2 hours to never, when it had an almost completely closed back. Further improvement is needed and I have found this thread both interesting and useful.




Thanks cjc.

It has been a while since I read Mr. Greer's article so I will revisit it but I believe it was more or less centered around cooling the mirror and not so much the 'scrubbing' of the boundry layer so I am wondering if the stated CFM's per mirror size still applies.


Mark


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Sarkikos
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Thermal Issues and Fans Successes [Re: Mark Peterman]
      #5631537 - 01/19/13 01:01 PM

Mark,

Quote:

Yes, I am aware of the scope SteveG was using. The lower end of that scope is a 'solid tube' construction (maybe 24-26 inches tall). I believe the video was good eveidence of what is happening around the mirror in a round, solid tube, configuration.




I agree. I can't - well, shouldn't - argue against the evidence presented to my eyes by Steve's videos. And I'm not. But I'm thinking about what additional measures may be needed to solve thermal problems in a completely solid-tube Dob. I'm aware that we may be comparing apples and oranges. They are both fruit, but they are still different in some ways.

Mike


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demiles
professor emeritus
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Reged: 11/07/06

Re: Thermal Issues and Fans Successes [Re: Sarkikos]
      #5631663 - 01/19/13 02:20 PM

Bottom line is this, a simple rear mounted fan blowing against the back is effective in cooling your mirror. Boundary layer solutions are more complicated to execute and may be too difficult for some in that means modifying your tube or mirror box. There are so many variables involved , what works for one person may not for another with different climate conditions. Do your own testing and find out what works for you, there are alot of good ideas put forth in this thread. I'll be interested in your findings

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Sarkikos
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Reged: 12/18/07

Loc: Suburban Maryland, USA
Re: Thermal Issues and Fans Successes [Re: demiles]
      #5631715 - 01/19/13 03:03 PM

The ancient Romans had a saying that sums up how we should go about solving our own thermal issues: Festina lente. Make haste slowly! I've been trying to follow that advice, but it's not easy.

Festina lente

And I imagine it is a bit easier to mount a boundary-layer fan in a mirror box than to saw-cut a hole in a solid-tube OTA and attach a fan with a rubber baffle.


Mike


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