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Equipment Discussions >> Reflectors

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

Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5614947 - 01/10/13 06:50 AM Attachment (25 downloads)

Here is a picture of the rear sucking fan embedded in the mirror cell sandwiched in Sorbothane. The MagLev fan and back of mirror cell is fully enclosed in the rear of the mirror box so it creates a venturi effect. The combination of the front blowing fan and the rear sucking fan also seems to evenly cool the primary mirror. With the rear sucking fan running only at high speed, the primary tends to become overcorrected due to apparent improper cooling of the edges of the mirror without the center being sufficiently equilibrated.

Edited by Bob S. (01/10/13 07:31 AM)


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Sarkikos
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5614963 - 01/10/13 07:10 AM

Bob S,

Quote:

The only other really successful application that I have seen for fans is a recent encounter with Dan Kleppner's 10" Starmaster with a rear blowing fan and 4 holes strategically placed above the primary mirror to exhaust the flowing air.




This sounds similar to what I've done in my 8" f/6 Dob and plan on doing in my 10" f/4.8. I'm still waiting for an opportunity to take out the 8" in good seeing to determine what effect the venting holes will have on the image. I'd like to do that before I start drilling the 10" OTA.

Is there a thread or other information on the internet about Dan Kleppner's modifications to his 10" scope? I'd really like to get this to work without having to install a boundary layer fan.

Mike


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Scanning4Comets
Markus
*****

Reged: 12/26/04

Loc: Ontario, Canada
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5614978 - 01/10/13 07:35 AM

I just read this bit from the gentleman below:

Kitchener Waterloo Centre
Royal Astronomical Society of Canada

Thermal Management Confirmation

http://www.atmlist.net/pipermail/atm/2005-August/009756.html

From what he said, it looks like I will be adding a side fan to my reflector with some exit holes. I'll be leaving the fan on the back as well and have two of them. I'd love to have my scope operating that good!

Sounds like a plan!

Cheers,


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5615020 - 01/10/13 08:12 AM

That list has many testimonials to the efficacy of boundary-layer fans. And many did what I was thinking of doing, having two fans below the primary and one side fan, just as you're planning, too.

So far I've gotten good results in my 10" Dob with only one fan below the primary, no boundary-layer fan and no venting holes. As long as the seeing cooperates, I do get images of planets that look like "cardboard cutouts." But I feel the performance could be even better and more consistent.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5615025 - 01/10/13 08:16 AM

This is the three fan and eight 'D'-cell battery-pack system I picked up on sale from Orion last year. I planned on adapting it to my 10" Dob. One fan could be used for the boundary-layer, two for below the primary.

Orion Three-Fan Cooling System for Convex-Back Dobsonians

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]
      #5615083 - 01/10/13 09:03 AM

Mike, I think what I was trying to point out earlier was that fan utilization does not seem to be a slam dunk like some may hope. Many friends have reported to me that fans seem to have more efficacy in certain kinds of climates. I do not dispute or suspect this is incorrect but do not know what climatic variables work best with fans. However, there is not much data on how much fan pressure is needed and where the placement is most optimal. Even though I relied on the successes of others' installations, I remain mildly skeptical that I have made a huge difference in my scope's performance at this point. I would like to think that it will take me the better part of a year to get a good sense of what variables seem to have the most efficacy? I should have mentioned that I also used a 1.25" thick Lockwood primary mirror in my 20" f/3 which should significantly aid in thermal equilibration issues. What seems to be interesting is that even when my mirror is fully equilibrated, my fan system appears to be additive in terms of overall performance. I need lots more data to fully feel that I may have gone in the right direction. Bob

Edited by Bob S. (01/10/13 09:14 AM)


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Sarkikos
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5615257 - 01/10/13 11:03 AM

What this hobby needs is a comprehensive book on thermal control and equilibration for Newtonian telescopes. AFAIK, nothing like this exists. Maybe knowledge about the topic is still in too much of a state of flux and apparent contradiction to even attempt it?

Such a book should also thoroughly cover the use of heat to prevent dewing.

Mike


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johnnyha
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5615957 - 01/10/13 05:57 PM

Question for all: It seems like when you are installing a side fan or fans blowing across the mirror with vent holes on the opposite side... the vent holes on the opposite side might benefit from exhaust fans? Otherwise it seems to me like the side fans blowing across the mirror will hit the opposite side and break up around the vent holes, causing further turbulence inside the mirror box or tube.

Or is there pressure created by the side fans that causes the air to vent smoothly without exhaust fans?

Thanks for all the great info, another really informative thread!


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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: johnnyha]
      #5616012 - 01/10/13 06:25 PM

Quote:

It seems like when you are installing a side fan or fans blowing across the mirror with vent holes on the opposite side... the vent holes on the opposite side might benefit from exhaust fans? Otherwise it seems to me like the side fans blowing across the mirror will hit the opposite side and break up around the vent holes, causing further turbulence inside the mirror box or tube.

Or is there pressure created by the side fans that causes the air to vent smoothly without exhaust fans?



That would depend on velocity.
If the side fans are being used for cooling, then they are likely to push a lot of air, which would cause the problems you mention.
But if the fans are just there to break up the boundary layer (say, after the mirror is nearly completely cooled), they can operate quite slowly and the gentle push will easily exit the other side.

If there is sufficient clearance between the mirror and the inner wall of the tube or mirror box, then whatever turbulence is created will be outside the view.

In my experience, though, a mere breeze blowing up the tube is sufficient to break up the heat signatures of air currents in front of the mirror and that takes neither a particularly large fan nor a high cfm one.

It is for cooling that larger or higher cfm fans are needed, but once the mirror is close to ambient, the fans take on the task of keeping the mirror at ambient and breaking up any boundary layer that attempts to form.


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johnnyha
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Reged: 11/12/06

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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5616025 - 01/10/13 06:34 PM Attachment (12 downloads)

Thanks Don, great answer. Another potential thermal problem with my Obsession mirror box is the top hole, the round baffle hole at the top of the square box, which also holds the lid in place and helps prevent stray light from getting in the mirror box. In the photo you can see I have covered the baffle with black protostar flocking. I'm tempted to put holes in the top around the edges of the baffle, because it seems like the baffle could be preventing the air from exiting to the side of the mirror and might actually be re-directing thermals back into the light cone. Any thoughts?

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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: johnnyha]
      #5616181 - 01/10/13 08:02 PM

The round baffle 6-8" above the mirror does more to block stray light from the optical path than nearly any other baffle. I definitely wouldn't drill holes in it.

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Pharquart
sage
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Reged: 11/11/09

Loc: Southwest Minneapolis Metro
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5616416 - 01/10/13 10:34 PM

Quote:

You can use what they call a "hole saw". It's like a cup with teeth on the rim and it attaches to a drill. There's a center bit to guide it in straight.
You can get these saws up to fairly large sizes on-line, but most larger hardware centers typically have them up to 3".
I did this to create a focuser hole in a tube I had and it's how carpenters put holes in doors for door handles.

<Author switch>
Thank you. I had seen these, but always thought they were meant for wood, not metal. They have no problem cutting a hole in a metal tube?





If you get the "bi-metal" kind (usually a red metal), they will quite happily cut through metal. A friend that owns a machine shops uses them to saw holes in thick stainless steel, a very tough metal to drill through. Take it slow, at a lower RPM on the drill.

Brian


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5616751 - 01/11/13 07:13 AM

Johnny,

Quote:

Another potential thermal problem with my Obsession mirror box is the top hole, the round baffle hole at the top of the square box, which also holds the lid in place and helps prevent stray light from getting in the mirror box. In the photo you can see I have covered the baffle with black protostar flocking. I'm tempted to put holes in the top around the edges of the baffle, because it seems like the baffle could be preventing the air from exiting to the side of the mirror and might actually be re-directing thermals back into the light cone. Any thoughts?




It seems like virtually all of the warm air would escape out that huge hole above the primary.

If you drill any holes at all to vent trapped air, I think they should be along the upper side of the mirror box (when the scope is positioned near horizontal), directly beside the edge of the baffle. Certainly don't drill any holes in the baffle itself! Maybe some holes along the edges of the adjacent sides would be good ... wherever common sense tells you that warm air would want to leak out of the system. But you don't want to weaken the box. I'd think about this awhile before getting out the drill.

Mike


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Pharquart]
      #5616754 - 01/11/13 07:15 AM

Brian,

Quote:

If you get the "bi-metal" kind (usually a red metal), they will quite happily cut through metal. A friend that owns a machine shops uses them to saw holes in thick stainless steel, a very tough metal to drill through. Take it slow, at a lower RPM on the drill.




Thanks. I'll keep this in mind when I look for a hole saw.

Unfortunately, though, I don't think my drill has a lower setting. I'll have to check. (I don't use it that often.)

Mike


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5617143 - 01/11/13 11:57 AM

I use a 6volt flashlight cube batter to my 5" DC fan behind the primary. Matching the voltage to the fan rating is overkill in my experience. Six volts pushes that fan fine and quiet and vibration free. Even the 80mm boundary fan is running at half its rating or less. There's a lot to be said for quieter fans too.

Pete


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azure1961p
Postmaster
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Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5617151 - 01/11/13 12:04 PM

Quote:

Here is a picture of the rear sucking fan embedded in the mirror cell sandwiched in Sorbothane. The MagLev fan and back of mirror cell is fully enclosed in the rear of the mirror box so it creates a venturi effect. The combination of the front blowing fan and the rear sucking fan also seems to evenly cool the primary mirror. With the rear sucking fan running only at high speed, the primary tends to become overcorrected due to apparent improper cooling of the edges of the mirror without the center being sufficiently equilibrated.




I reversed my rear fan so it too was sucking instead of blowing air up the tube. For me with its closed tube it was a disaster. Every single exhaled breath no matter how calm would rise up and fe sucked down the tube killing the images terribly. It was awful. Too the secondary fewer up n thirty minutes. What little heat is a ails me after the fan blows the air up the tube never allows secondary dewing. Residual heat at this low per centage now is actually benefitting my system - and to no ill effects. Boundary is running too.

Pete


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5617207 - 01/11/13 12:39 PM

Pete,

Quote:

I use a 6volt flashlight cube batter to my 5" DC fan behind the primary. Matching the voltage to the fan rating is overkill in my experience. Six volts pushes that fan fine and quiet and vibration free. Even the 80mm boundary fan is running at half its rating or less. There's a lot to be said for quieter fans too.




So you have one 5" fan blowing onto the bottom of the primary and up the OTA, and one 3" fan blowing across the boundary layer? And both fans are running well below full rpm?

From what I've read, it does seem better to keep the boundary layer fan running slow and easy. As was suggested in this thread, if it runs too fast, the air might become turbulant against the opposite wall and not vent through the holes optimally. One little 6v battery should do the trick, or a pot can be set up to vary the speed.

For below the primary, though, I'm thinking of two 3" fans instead of one larger fan. Maybe run them slow and easy, too, to prevent vibration. I'd rather have all the fans run all night rather than have to switch them off because of vibrations after they've cooled the primary. Probably smaller Dobs - with smaller, thinner primaries - have more latitude in this than the larger ones.

Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5617217 - 01/11/13 12:46 PM

Mike try the blocky 6 volt battery on two fans too. It's the boxy looking thing about 2.5" wide and 4" tall with coiled springs sticking out the top. And brother they last. I've had one since the half doz obs since thanksgiving and its still humming fine. My sessions are often three hour run times.

A nice gentle power source for low noise vibration - looks badass too. Lol.

Pete


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Sarkikos
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Re: Thermal Issues and Fans Successes [Re: azure1961p]
      #5617230 - 01/11/13 12:50 PM

Pete,

Quote:

I reversed my rear fan so it too was sucking instead of blowing air up the tube. For me with its closed tube it was a disaster. Every single exhaled breath no matter how calm would rise up and fe sucked down the tube killing the images terribly. It was awful. Too the secondary fewer up n thirty minutes. What little heat is a ails me after the fan blows the air up the tube never allows secondary dewing. Residual heat at this low per centage now is actually benefitting my system - and to no ill effects. Boundary is running too.




I'm also skeptical about sucking the air down intead of blowing it up the OTA and out the top. Warm air rises ... why shouldn't we help it do what it wants to do?

As you say, there are the problems of heat from the observer's breath and body, and dewing on the secondary. In my area, I need to cool the primary, but I also need to make sure that dew doesn't form on the secondary and other optics. Dew control through warmth is a big part of my observing sessions.

Probably there is a difference in which method is optimal depending on whether the scope is open tube or truss, size and thickness of the primary, length of the OTA (or shroud for a truss scope), clearance between primary and sides of the OTA, observing area, etc, etc.

Mike


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Sarkikos
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Reged: 12/18/07

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Re: Thermal Issues and Fans Successes [Re: azure1961p]
      #5617248 - 01/11/13 01:01 PM

Quote:

Mike try the blocky 6 volt battery on two fans too. It's the boxy looking thing about 2.5" wide and 4" tall with coiled springs sticking out the top. And brother they last. I've had one since the half doz obs since thanksgiving and its still humming fine. My sessions are often three hour run times.

A nice gentle power source for low noise vibration - looks badass too. Lol.




Yes, now I know what you mean. Sounds like a good idea. I suppose the big 6v batteries should cut the rpm by half versus the 12v -just guesstimating here - but should last much longer than the little 6v batteries. Maybe two of them: one for the boundary layer fan, and one for the two fans under the primary. I could put them in my sports-pack counterweight at the lower end of the OTA, or in the basket hanging from the front of my Dob mount.

This is brainstorming, of course. I've not yet fully decided that I'm going to have a boundary layer fan! But between bad weather and the Moon, it looks like I'll have at least three weeks to work on the problem. I like to have my 10" Dob ready to go to the dark site around New Moon. Clouds, rain and fog this New Moon, though.

Mike


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