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Equipment Discussions >> Reflectors

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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5622207 - 01/14/13 07:29 AM

What we need here are some pictures showing what is going on. There is too much confusion with just words.

Pictures really show how things are.

I was going to add a boundary layer fan, but I'm just going to wait and see what the fan on the back of my mirror does first. I used to just leave the fan on full speed to cool it down for an hour, then I would shut it off. I'm going to try with the fan on full speed for cool down, then I will run it at the slowest speed for observing to see how things are.

POST SOME PICTURES PLEASE !!!!

Cheers,


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5622211 - 01/14/13 07:38 AM Attachment (9 downloads)

Quote:

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.




Johnny, The CBLMS consists of a a a front fan blowing air onto the front surface of the mirror suspended in the shadow of the secondary and a second fan that is fully enclosed in the rear of the mirror box sucking air from around primary that is surrounded by an annulus and exhausts the air out the back of the telescope. Both the front blowing fan and the rear sucking fan are connected to potentiometers that allow for speed control of the fans. The picture below shows the Sunon MagLev fan suspended completely in Sorbothane and exhausts the air out the back of the mirror box. Bob

Edited by Bob S. (01/14/13 07:41 AM)


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5622217 - 01/14/13 07:47 AM Attachment (10 downloads)

The front Sunon MagLev fan is suspended on safety wire above the primary and gently blows the boundary layer off the primary while also additionally providing even cooling of the primary. The air that is blown off of the front surface of the primary is sucked out the back with the rear sucking fan in the picture in the previous frame. You can see the annulus in place that surrounds the primary mirror and creates a bit of a venturi effect for the air surrounding the primary and sucks the air out the back of the telescope. I have been hearing anecdotal concerns about sucking in exhalation air and have not found this to be any kind of a significant/noticeable problem in my temperate Florida viewing conditions. Bob

Edited by Bob S. (01/14/13 07:57 AM)


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5622225 - 01/14/13 07:53 AM

I kno one guy whose got it sucking air down the tube and likes it. Maybe my secondary and focuser are to close to the open front end? At anyrate the general consensus is to blow up but there are fans - no pun intended- of the other method.

Pete


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5622234 - 01/14/13 08:03 AM

This was a recent response from Mike Lockwood to my description of the CBLMS on the thread started in this Forum on my 20" Lockwood/JP Astrocraft.

"In comment to Bob's most recent post, what is important here is that now the conditions are present/satisfied that allow very fine differences in performance to be discriminated.

With 1) good optics, 2) good mirror support (edge and back), 3) good collimation, and 4) good equilibration made possible by the fans and thin mirror, the scope is performing up to the potential of a quality 20" visual instrument.

The conventional technique of blowing air on the back of mirror will help for a while, but it causes an uneven temperature distribution in the mirror and eventually when the mirror does cool it starts pulling the warm air off the ground back up into the mirror box. The cooling system in the 20" f/3 avoids that, and cools all sides of the mirror much more evenly, something that I have recommended for years to those that have asked me. Even cooling is vital for the ultimate performance on most nights. While one may get lucky with conditions on a few nights a year, with a cooling system like this there are going to be a lot more of those amazing nights."

--------------------
Mike Lockwood - Owner, Lockwood Custom Optics
20" F/3 MX Starmaster, 14.5" F/2.55 self-built Newt., nine self-built scopes, 4.25" to 30"
http://www.Loptics.com/
"


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5622254 - 01/14/13 08:32 AM

I wonder if there is any difference in effectiveness for scrubbing the boundary layer depending on whether a fan pulls air directly onto the surface of the primary from above or across the surface from the side? It seems the former would result in air striking the center of the primary's surface and flowing radially out toward the sides, while the latter produces a laminar flow from one side of the surface to the other. But wouldn't air striking directly at the primary's surface result in more turbulence? Wouldn't the side fan produce a more stable image?

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5622272 - 01/14/13 08:42 AM

I think any discussion of fans installed directly above the primary is a case of apples and oranges for solid tube Dobs. That's not going to happen - or it would be difficult to make happen, in any case.

Has anyone installed such a fan in a solid tube? Where did you put it exactly? If directly over the primary, did you cut out some big holes in the OTA so you can perform maintainence on the fan setup? Or is the fan mounted to the upper sides of the primary cell, and you have to remove the cell in order to work on the fan? I think either of those options would be more trouble than it's worth.

Mike


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5622276 - 01/14/13 08:46 AM

Quote:

I wonder if there is any difference in effectiveness for scrubbing the boundary layer depending on whether a fan pulls air directly onto the surface of the primary from above or across the surface from the side? It seems the former would result in air striking the center of the primary's surface and flowing radially out toward the sides, while the latter produces a laminar flow from one side of the surface to the other. But wouldn't air striking directly at the primary's surface result in more turbulence? Wouldn't the side fan produce a more stable image?

Mike




Mike, The side fans seem good in theory but have not proven to be particularly helpful in my environments. You have the problem of air stacking up on the opposite side and even with scopes that have had exhaust holes on the opposite side or even sucking fans on the opposite side, the hoped for laminar flow or benefits have not been particularly evident. I keep hearing these strategies proposed but extensive testing I have done and others have done has netted limited results with the fans blowing across the face of the primary. Joe Wambo's approach with the front fan blowing air down on the primary has appeared to be the most efficacious strategy that I have personally experienced in the many years of trying to find the right combo. Bob


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5622282 - 01/14/13 08:56 AM

Bob,

Quote:

The conventional technique of blowing air on the back of mirror will help for a while, but it causes an uneven temperature distribution in the mirror and eventually when the mirror does cool it starts pulling the warm air off the ground back up into the mirror box.




Maybe a concern about warm air coming up from the ground is more pertinent to warmer locales such as Florida and SoCal. I don't think I really have a big problem with that here in Maryland, especially if I set up a telescope on grass. The big release of heat is when the scope is on pavement, gravel or concrete. I virtually never put any telescope over those materials.

Also, when I go to my dark site, which is in a grassy field, I always put down a carpet on the ground and then set up the scope on that. That's a much easier fix than having to worry about which way to blow the fan to avoid heat from the ground.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5622292 - 01/14/13 09:06 AM

Bob,

Quote:

The side fans seem good in theory but have not proven to be particularly helpful in my environments. You have the problem of air stacking up on the opposite side and even with scopes that have had exhaust holes on the opposite side or even sucking fans on the opposite side, the hoped for laminar flow or benefits have not been particularly evident. I keep hearing these strategies proposed but extensive testing I have done and others have done has netted limited results with the fans blowing across the face of the primary. Joe Wambo's approach with the front fan blowing air down on the primary has appeared to be the most efficacious strategy that I have personally experienced in the many years of trying to find the right combo.




Have those side fans been on solid tube Dobs or truss Dobs? We can discuss front fans all day, but I don't see that as a viable alternative for solid tube scopes. Apples and oranges. If the side fans show some improvement in the image for solid tube scopes - and AFAIK they have, both here in the northeast and out west - they should be tweaked for further improvement rather than thrown out entirely. It's not as if centrally-suspended front fans are an alternative. I don't see front fans as even a possibility for solid tube scopes. Somebody prove me wrong.

Mike


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5622300 - 01/14/13 09:12 AM

See this thread HERE.


Quote:

Now the fan placement. This is the most controversial issue regarding planetary Newtonians. The way I determine which method works best is based on the views I see while comparing them to other methods. We've tried just about all of them and I will tell you how Pons and Grissom do theirs.

It has been suggested that elastic rubber bands will help filter out vibrations, however in my opinion, this also has much to do with the tube material and the speed of the fans themselves. During our tests, you can go without the elastic bands just as Pons and Grissom have done as illustrated in these pictures but there's a crucial element that makes a HUGE difference and I believe that this is where the main problems stem from. You should have a thermostat of some sort to regulate the speed. In some cases you can be pushing too much air or not enough.

You can also place a separate 8" fan behind a primary of 12.5" which is not attached to the scope at all, however you will need a thermostat in either situation. Remember, I'm talking about the tube first. There are differences in fan placement for truss's and tubes.

Many people have attempted different positions of fan placement either sucking down, forward or sideways but I will tell you what has proven to win in the field beyond the shred of a doubt. I've seen magnifications up to 600x using 12.5" scopes with Pons and Grissoms fan placements. You are better off getting Boxer fans which are brushless to minimize vibrations. In these designs there are three 4" fans placed directly behind the primary and attached to the tube with a solid wood board. The side fan is another option but I have some issues and I'll give numerous reasons why depending on the situation, but first I'll explain what's going on with the rear fans pushing forward, which the best way to go in our opinion.

I remember waking up to test Grissom's fans and the differences were so amazing, I could see it right before my eyes. I pointed the 13.5" at Saturn without the rear fans on. I had asked one of my friends to turn the fans on. Within about five seconds, the image literally froze rock solid. I then told him to turn the fans off. Within seconds, the image started to soften.

We kept doing this over and over under the sub arc second skies of Mt. Wilson. At one point I stood in front of the tube while the fans were on and could actually feel the subtle breeze flowing against my face. These fans remain on at all times because these large scopes have nearly endless thermal drawbacks when compared to refractors.

I was so impressed with fans I became obsessed with them and tried different methods. Bryan Greer at Protostar has conducted numerous tests and is very well informed. I have had numerous conversations with him and I believe he's an amazing source of excellent information on these issues if you have questions.

Think about the tube for a moment and what's going on inside of it. Depending on how long it takes things to settle, some of these issues may play an issue, while some may not. But, a proper fan set up eliminates practically all the thermal gremlins no matter what time you observe. You can role these Newtonians out and get startling images right off the bat. The system as a whole does not have to depend so much on settling down. These fans work so efficiently, that there isn't any time for thermal gremlins to cause these problems.

There's heat exchange coming off the secondary, heat exchange at the top of the tube, there's a boundary layer in front and back of the primary, tube currents, etc. Think about light having to contend with all these issues before it even enters the eyepiece, it's no wonder people like their refractors better and I don't blame them. These combination of fans along with Grissoms curved spiders are unsurpassed in visual planetary performance. The boundary layer gets sucked off the face of the mirror and the diffraction spikes, the scintillation and thermal gremlins are completely flushed out of the picture like a vacuum. The remaining image is a steel ball that looks like a Voyager picture. You wouldn't even want to look through refractor in a side by side after you've seen through on of these scopes, it's truly amazing.

So what's up with the side fans? Side fans basically help to speed up the cooling process and break down the boundary layer to produce a more laminar effect. In Alan Alder's Sky&Tel review, he had a push/pull mirror system. This prevented him from being able to use a rear fan. The concept of the side fan was to help break the boundary layer and have it flow out the holes residing at the opposite side of the tube. This sounds cool, but it's not that simple. Both Pons and I tested this method in a tube and on a truss during observations. Even though some observers claim it helps during visual observations, our tests proved otherwise.

During a star test, you can rack the image out and see the boundary layer and watch how it behaves. But more importantly you can watch a planet with them on and off. Personally, I thought the images were not as impressive with them on. The air isn't just flowing out the other side. Those gremlins ricochet inside the tube. They don't just magically flow out the other side and the star test proves this. Nothing beats having no boundary layer at all. If you have a full thickness mirror, side fan can help speed up the process of cooling.

I know some observers will object to my opinion on this matter and that's OK. I'm not searching for the perfect image. I've already found it, so it's up to you. These pictures show Ed Grissom and Pons fan set up.




Another thing to think about is the diffraction you'd get from those wires suspending a fan over the mirror. Doesn't matter how thin the wires are, light will need to bend around it causing further diffraction on top of the spider that holds the secondary.

Note the two small fans on the side of the scope which proved to be not as good as just having the fan at the back BLOW air over the mirror. Suspending a fan over a mirror in a solid tube dob would be pointless IMHO.

I'm going to stick with the fan over the back of my mirror and use the THREE SPEEDS I have for cooling. I have set up my telescope right on my driveway which is concrete and have had incredible views of Jupiter. It all depends on many factors. Daniel is right about side fans being detrimental because the air will not magically blow out holes on the other side of the tube,(if you are using a solid tube scope), some of that air will just bounce off part of the scope walls and blow around causing more turbulence. However, they WOULD be good for a faster cool down before observing to scrub the boundary layer, and can be turned off when observing but leaving the fan at the back on.

I dunno if I want to cut any holes in the side of my solid tube dob because I feel that the back where the mirror is needs to be as solid as possible. I have a thin metal tube completely flocked front to back, and I don't want to weaken it by cutting more holes in it. YMMV.

Heat coming off the grass? I've never had a problem with that myself.

Cheers,


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5622324 - 01/14/13 09:32 AM

Mark,

No, you might not, because grass is probably the best surface that you can set your scope on. More heat will come up from concrete or asphalt pavement during the night than from grass. I think that asphalt pavement would probably be the worst, though.

These little measures are additive. If I have the choice, I will always set up on grass. Placing a carpet over the grass just makes it better. Try putting down a carpet to see if that will help.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5622347 - 01/14/13 09:49 AM

Mark,

Quote:

I dunno if I want to cut any holes in the side of my solid tube dob because I feel that the back where the mirror is needs to be as solid as possible. I have a thin metal tube completely flocked front to back, and I don't want to weaken it by cutting more holes in it. YMMV.




There are holes and then there are HOLES. A line of a few 1/4" holes in the OTA along the upper edge of the primary should not diminish the sturdiness of the OTA. Now, if you were to completely encircle the OTA with a line of these holes, there may be a problem. But that isn't necessary, since heat rises.

If I see any improvement at all in thermal flow and image clarity in my 8" Dob after drilling the vent holes, I'll probably start doing it for all my Dobs. It's not a big deal to do. If the holes don't improve these problems - but how could they not? - they can be easily covered by a strip of Velcro. It makes sense that the vent holes would decrease inital cooldown time, at the least. If they don't improve the image or thermals after that, they can be covered.

Mike


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5622640 - 01/14/13 01:00 PM

Since my 12.5" Teeter has 3 fans--one rear (80mm) and 2 side fans (100mm each), and my mirror is thin (32mm, 1.25"), cooling is not the issue. I can bring the mirror down to within 1 degree F of ambient temperature in <30 minutes with all 3 fans running.

I've experimented with having fans running while looking at a high-resolution target (in this case, Jupiter and the center stars of M15).
I wanted to see if having all 3 fans running was better than just the rear fan, and whether any of the fans imparted visible vibration in the image at, say, 280-305X.

The rear fan is unnoticeable, vibration-wise, even at 456X. The side fans do impart visible vibration above 200X, but not below.

Once I knew what magnification I could use with fans running, I measured the tracking of the temperature drop with only the rear fan running. I discovered that as long as the temperature was quickly dropping, it was inadequate to keep the mirror at ambient. When the two side fans were also turned on, the mirror quickly returned to ambient temperature. So I made it a habit that, when I take a break from viewing to drink some coffee or eat something, I turn the side fans on and then turn them off again when returning to the scope. That keeps the mirror cooled and disperses the boundary layers.

The question was which combination of fans or no fans resulted in the steadiest image. I definitely noticed that if I turned all the fans off, within a few minutes (5-10), the image quality and steadiness definitely diminished, as if the seeing got poorer. Turning the rear fan back on resulted in a return of better star images, but it took 5-10 minutes for the seeing to get "better". Turning the side fans on did not have an immediate effect of sharpening the image, but also took only a few minutes to return the star images to sharp.

Use of a laser thermometer told me what was happening. When I turned the fans off, the mirror essentially stopped cooling and "froze" at whatever temperature the mirror was. While the temperature continued to drop, the mirror gradually grew warmer, relative to the air, and thermal issues returned. Turning the fans on did not immediately correct the temperature differential, but took about five to ten minutes to return the mirror to the ambient temperature. This told me that it is necessary, even on a thin mirror, to have fans continuously running as long as the temperature is dropping.

But why didn't turning on the side fans immediately blow away the boundary layer and restore the image quality? I could only conclude that
the turbulence caused by the fans, even though there are substantial exit vents on the other side of the box from the fans, was just as deleterious to the image quality as the boundary layer itself. The most effective use of the side fans was as cooling devices to help speed the return of the mirror temperature to ambient rather than as devices to simply rid the mirror of a boundary layer.

However, the fans run at full speed at 12V, and I do not yet have a power control connected that allows me to slow the fans down to a slower speed.
It's ironic, since the fans get their power through a Kendrick Control box that would allow me to slow the fan down with the simple push of a button. I merely need to plug the cables into another port on the box.
At the moment, the control cables are a little too short to do that, so it will entail my getting a short extension for the RCA cable so I can do that.

It's quite possible that, slowed to a mere gentle push, the scrubbing of the boundary layer would not result in a build up of turbulence in the image and return the image quality immediately, or even allow me to run the fans continuously. I need to perform a further experiment.

Having a mirror at ambient temperature has meant I see good seeing conditions a lot more often than I used to. Of course, the seeing conditions that have improved are inside the scope, not in the atmosphere.


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5622679 - 01/14/13 01:23 PM

Quote:

Since my 12.5" Teeter has 3 fans--one rear (80mm) and 2 side fans (100mm each), and my mirror is thin (32mm, 1.25"), cooling is not the issue. I can bring the mirror down to within 1 degree F of ambient temperature in <30 minutes with all 3 fans running.

I've experimented with having fans running while looking at a high-resolution target (in this case, Jupiter and the center stars of M15).
I wanted to see if having all 3 fans running was better than just the rear fan, and whether any of the fans imparted visible vibration in the image at, say, 280-305X.

The rear fan is unnoticeable, vibration-wise, even at 456X. The side fans do impart visible vibration above 200X, but not below.

Once I knew what magnification I could use with fans running, I measured the tracking of the temperature drop with only the rear fan running. I discovered that as long as the temperature was quickly dropping, it was inadequate to keep the mirror at ambient. When the two side fans were also turned on, the mirror quickly returned to ambient temperature. So I made it a habit that, when I take a break from viewing to drink some coffee or eat something, I turn the side fans on and then turn them off again when returning to the scope. That keeps the mirror cooled and disperses the boundary layers.

The question was which combination of fans or no fans resulted in the steadiest image. I definitely noticed that if I turned all the fans off, within a few minutes (5-10), the image quality and steadiness definitely diminished, as if the seeing got poorer. Turning the rear fan back on resulted in a return of better star images, but it took 5-10 minutes for the seeing to get "better". Turning the side fans on did not have an immediate effect of sharpening the image, but also took only a few minutes to return the star images to sharp.

Use of a laser thermometer told me what was happening. When I turned the fans off, the mirror essentially stopped cooling and "froze" at whatever temperature the mirror was. While the temperature continued to drop, the mirror gradually grew warmer, relative to the air, and thermal issues returned. Turning the fans on did not immediately correct the temperature differential, but took about five to ten minutes to return the mirror to the ambient temperature. This told me that it is necessary, even on a thin mirror, to have fans continuously running as long as the temperature is dropping.

But why didn't turning on the side fans immediately blow away the boundary layer and restore the image quality? I could only conclude that
the turbulence caused by the fans, even though there are substantial exit vents on the other side of the box from the fans, was just as deleterious to the image quality as the boundary layer itself. The most effective use of the side fans was as cooling devices to help speed the return of the mirror temperature to ambient rather than as devices to simply rid the mirror of a boundary layer.

However, the fans run at full speed at 12V, and I do not yet have a power control connected that allows me to slow the fans down to a slower speed.
It's ironic, since the fans get their power through a Kendrick Control box that would allow me to slow the fan down with the simple push of a button. I merely need to plug the cables into another port on the box.
At the moment, the control cables are a little too short to do that, so it will entail my getting a short extension for the RCA cable so I can do that.

It's quite possible that, slowed to a mere gentle push, the scrubbing of the boundary layer would not result in a build up of turbulence in the image and return the image quality immediately, or even allow me to run the fans continuously. I need to perform a further experiment.

Having a mirror at ambient temperature has meant I see good seeing conditions a lot more often than I used to. Of course, the seeing conditions that have improved are inside the scope, not in the atmosphere.




Don, Your experience with side fans very closely mimicks what I have found on numerous attempts to to have them be additive. I think that your belief that the fans are creating more turbulence than they are mitigating above the boundary layer is probably accurate. I hope that there is some way that you can also vibration isolate your side fans such that they do not induce vibration that is noticeable. I suspect that they are always inducing some vibration that would be best eliminated if possible. One of the things that I have found with the complete mirror cooling system is that the closer we can keep the primary to ambient temperature, the less we are having to deal with boundary layer degradation. Of course, even with my mirror at ambient temperature, my CBLMS is providing significantly additive benefits to the overall performance of the scope. Thanks for sharing. Rob Teeter introduced that fan strategy many years ago when it was kind of the rage and I always felt like there was more work to be done to make those maximally effective. At the time of the big interestest following Adler's article, there were lots of us trying different things and most of us only getting only mediocre payback for our efforts. As I had mentioned earlier and I think you keenly appreciate, fans and boundary layer issues are certainly not a slam dunk. If they were, we would all have been using them with great rewards over this past decade. The truth is that it has been a bit of slog to figure out what works well and what notions really are based more on anecdotal reports than more studious evaluation of various systems implementations. Bob


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5622701 - 01/14/13 01:34 PM

So we shouldn't be expecting an authoritative how-to book on thermal issues in Dobs any time soon? Even an historical survey and discussion with examples would be better than what we have now - nearly nothing except for mostly contradictory opinions.


Mike


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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5622717 - 01/14/13 01:45 PM

Don,

Quote:

It's quite possible that, slowed to a mere gentle push, the scrubbing of the boundary layer would not result in a build up of turbulence in the image and return the image quality immediately, or even allow me to run the fans continuously. I need to perform a further experiment.




This might be the crux of the matter. The boundary layer fan needs to be run gently in order to prevent both vibration and turbulence. This agrees with the OP, since Pete was running his boundary layer fan at only 6v. The bottom fan only needs to be running slowly enough - or be isolated well enough - to prevent vibration.

Mike


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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5622718 - 01/14/13 01:46 PM

Mike,
There is probably more than one effective way to do it, and there can be variations in the execution of one idea that might affect efficacy of the idea.

And what works for smaller mirrors may not work at all for larger mirrors.

For instance, the holes in my mirror box opposite the fans have "grilles" over them. They serve no function and impede the passage of air, so why are they there? Esthestics, or keeping little kids from touching the mirror.
Otherwise, an open hole would be better if the goal is to allow the fans to blow across the mirror and vent out the other side.

And the demands of cooling could be quite different than the demands of scrubbing the boundary layer. It's why I'm beginning to believe fans need to have a variable speed control.

The best we can hope for is a compendium of ideas to solve the problems and allow different manufacturers to solve the problems in their own ways and to allow the tinkerer to modify in whichever way is most convenient to solve the problems of thermal issues in his own scope.

I could tell you about the scary "swiss cheese" SCT I saw--not only open tube, but the tube was reduced to only a very open lattice work. He completely solved the thermal issues, but injected a healthy dose of fragility.

What's fun about these forums is reading all the different ways people have solved various problems in contemporary scopes.

I'm waiting for refractor owners to rediscover "Hargreaves Struts" .


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Sarkikos
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Re: Thermal Issues and Fans Successes [Re: Starman1]
      #5622762 - 01/14/13 02:10 PM

Don,

Quote:

And what works for smaller mirrors may not work at all for larger mirrors.




Yes, exactly. That is why I keep trying to steer the thread back to more moderate-sized, solid-tube Dobs - which is, after all, what the OP was concerned with. What works for a 20" truss scope will not necessarily - probably won't - work for an 8" or 10" solid-tube. But speaking about all these different examples does carry the discussion along and might stimulate ideas for solutions for Dobs of various types and scales.

Quote:

And the demands of cooling could be quite different than the demands of scrubbing the boundary layer. It's why I'm beginning to believe fans need to have a variable speed control.




Yes and yes.

Quote:

I could tell you about the scary "swiss cheese" SCT I saw--not only open tube, but the tube was reduced to only a very open lattice work. He completely solved the thermal issues, but injected a healthy dose of fragility.




Here ya go:

The Holey Telescope

And I'm reluctant to put just one moderate-sized hole in the side of my OTA!


Mike


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Scanning4Comets
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Reged: 12/26/04

Loc: Ontario, Canada
Re: Thermal Issues and Fans Successes [Re: Starman1]
      #5622785 - 01/14/13 02:27 PM

Mike,

Quote:

There are holes and then there are HOLES. A line of a few 1/4" holes in the OTA along the upper edge of the primary should not diminish the sturdiness of the OTA. Now, if you were to completely encircle the OTA with a line of these holes, there may be a problem. But that isn't necessary, since heat rises.




1/4" holes won't do squat....the holes will have to be bigger for air to get out or it will just bounce around in the tube.


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