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Equipment Discussions >> Reflectors

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Mark Peterman
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Re: Thermal Issues and Fans Successes new [Re: demiles]
      #5631790 - 01/19/13 03:44 PM

Quote:

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




Well said. No argument here.


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azure1961p
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Re: Thermal Issues and Fans Successes *DELETED* new [Re: Bob S.]
      #5632152 - 01/19/13 08:05 PM

Post deleted by azure1961p

Edited by azure1961p (01/19/13 08:08 PM)


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johnnyha
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5632161 - 01/19/13 08:11 PM



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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5632417 - 01/19/13 11:14 PM

You know, I appreciate Steve's videos. He did everything right, IMO, by sealing the back of the tube. Doing so allowed some high pressure to build up behind the primary and, instead of leaking out the back, that higher pressure air moved around the front of the primary. It seemed to carry away some smoke (simulated boundary layer.) But, it was not aggressive enough for me, that smoke lingered in the optical path.

The volume of air flow changes with the amount of space it is flowing through. Coming directly off the back of the fan, a given volume of air is fairly compressed into the diameter of the fan. Then it spreads out in all directions around the back of a 10" mirror. This reduces both the velocity and the volume per unit area. The result is a much weaker flow of air from the annular baffle. In fact, it would seem to drive the boundary layer toward the center of the primary, then maybe up the tube as pressure built up on the primary's surface. The result would be a little mixing, but still some eddies of cooler and warmer (density) in the optical path.

Sucking would have the same affect in reverse, but tend to pull the air out of the optical path. However, the low pressure behind the primary could draw air from the huge volume above the primary. So, the effect is still weak, maybe even weaker than blowing air from behind.

A more aggressive approach is to attack the boundary layer directly. Bob showed one such approach earlier with a "spider" mounted fan blowing down directly onto the primary's business end...it's surface. Another is to blow across the primary's surface. But, where does the air go? It still /should be/ vented. So, you can pull it out the back or the side.

Now, there are problems with this design, too. Blowing air across the surface sets up a laminar flow that DOES NOT hug the mirror's surface. The laminar flow departs the mirror's surface and leaves a turbulent area below it. This is exactly what happens when an aircraft wing is near stall (the angle of attack has to be correct to keep the air hugging the surface of the wing or the mirror.)

But, where does this air flow go? Well, there's a good and sensible argument that some of it might be blown out the exhaust holes opposite the side blowing fan. Truth is, some of that air will bounce off the inside of the tube and recirculate into the optical path. But, the nice thing about this set up is the high pressure is applied directly to the mirror's surface and not immediately diminished as it was blowing and expanding along the entire surface of the primary's back. That's pretty aggressive provided you can prevent the laminar flow from departing the mirror's surface.

You could suck the air from the mirror's surface in much the same way - exhausting out the side, except that the fan will, again, be drawing from a large and weak low pressure area inside the tube. It will not be as aggressive as blowing directly across the surface. If you think about it, you cool yourself with a fan by blowing it at you, not by standing behind the fan allowing it to suck air past you. That doesn't work.

Apply some physics and come up with a design that suits you.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5632564 - 01/20/13 02:08 AM

Norme,

Quote:

Blowing air across the surface sets up a laminar flow that DOES NOT hug the mirror's surface. The laminar flow departs the mirror's surface and leaves a turbulent area below it. This is exactly what happens when an aircraft wing is near stall (the angle of attack has to be correct to keep the air hugging the surface of the wing or the mirror.)




Then what angle would the side fan need to have in relation to the surface of the primary in order to generate a consistent laminar flow across the entire surface?

There are differences between an aircraft wing and a primary mirror. An aircraft wing is an airfoil. The primary's surface is a parabola, but might be considered planar in the macroworld of air flow. What impact would these differences have in terms of breaking up the boundary layer?

Also, would more than one fan or maybe a larger fan to cover more of the mirror's surface be more effective than a single smaller fan in transforming the boundary layer into a laminar flow?

Mike


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Gray
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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5632570 - 01/20/13 02:18 AM

Lol, here we go.

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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5632589 - 01/20/13 02:49 AM

Quote:

Quote:

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




You got to be kidding me Bob. Turn down the heat so long as Im a fan of yours? At this point Im trying to figure out where to vent off the hot air in your post. Your stance on sucking in this case is entirely appropriate.

Pete




Pete, Thanks for starting this thread. It is an important issue that has been frequently overlooked. I would prefer to not engage in personal attacks and will opt out of your thread to reduce any perceived noise from me that you are experiencing. Much good luck on your quest to optomize boundary layer mitigation issues. Bob


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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5632623 - 01/20/13 04:20 AM

Mike, I wish I knew what a good angle would be. Maybe one that just skimmed or slightly above the surface. There was a link a while back in this thread that had a pretty extensive link to a video on fluid dynamics. I watched some of it. This laminar break is one thing they discussed.

I believe it had to do with the hard edge of the primary as opposed to an air foil. Tilt an air foil enough and it puts a hard edge to the oncoming wind and the laminar flow breaks from the surface. But, I am no expert in the field, just regurgitating what I took from that link and applying it to what I think I know.

One would think a broader air flow either by a larger fan or several small fans would more efficiently cover the entire surface.


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nevy
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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5632735 - 01/20/13 08:22 AM

I think all these ideas on this subject are good and I enjoyed reading them but its getting a bit complicated and it's started to turn into a bit of an argument , I haven't got the time to experiment with all these options , I have a largish truss scope and a medium solid tube . All I know is from experience with my lightbridge is that planetary views are much nicer with the standard fan blowing the back of the mirror , it also cools the mirror quicker than not having a fan , that's good enough for me so I will be putting a fan on my 12" in the same way as the lightbridge ( blowing on the back of the mirror to cool & to sharpen up the image when cooled) but good luck with whatever way you all find best on your scopes.

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5632822 - 01/20/13 09:43 AM

Quote:

I think all these ideas on this subject are good and I enjoyed reading them but its getting a bit complicated and it's started to turn into a bit of an argument , I haven't got the time to experiment with all these options , I have a largish truss scope and a medium solid tube . All I know is from experience with my lightbridge is that planetary views are much nicer with the standard fan blowing the back of the mirror , it also cools the mirror quicker than not having a fan , that's good enough for me so I will be putting a fan on my 12" in the same way as the lightbridge ( blowing on the back of the mirror to cool & to sharpen up the image when cooled) but good luck with whatever way you all find best on your scopes.




The end game of it all which you've pointed out well is, it's simply a matter of what works best for you. There's theory a plenty and variables higher and yon but ultimately, you look through the eyepiece, slight defocus a star and observe . Like the star test itself its ruthlessly obvious wether or not your thermal management is successful or not and often the solutions to opt for are incrediy simple - like reversing fan flow. In the end what finally works for you is independent of countering theory, hypothetical models and so much testimony. The negative airflow or reverse sucki g flow for my closed system essentially made it a vacuum cleaner intake for dew and warm breath. In the face of that it would seem some folks with THEIR design have found success with it still. One being a lifetime ALPO mars observer with a 16" reflector. My disaster is his solution.

The one thing that can be stated with blanket authority is that optical tube assemblies vary enough in thermal properties for any number of reason s that one shoe does not fit all - no matter how well it fits one. I'm still mystified negative airflow in the tube can be beneficial for anybody but such are the details in the variables .

Pete


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Mark Peterman
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5632926 - 01/20/13 10:37 AM

Maybe I dreamed this but did I read somewhere that defocusing inside-focus shows the thermal stability of your mirror and defocusing outside-focus shows you atmospheric stability?

If so, like star testing, is a high magnification required or does it matter?

Should have clear sky tonight with my new fan and would like to test this.


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5633390 - 01/20/13 03:01 PM

I haven ever tried to Gerry that but it sounds plausible. Interesting notion. I don't think high power is needed in the same way for collimation for examples A point source like a star works better than a planet . I kno that the out of focus star will show thermals outsid the scope too like heat from my hand or breath acting across the aperture.

Pete


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Mark Peterman
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5636593 - 01/22/13 10:27 AM

I have a 12" Orion reflector. The mirror is 1.5" thick. The inside of the tube is flocked and the OD of the metal tube is 14".

I previously attached the 80mm, 32 CFM, fan directly to the mirror cell as the cell is drilled and tapped for this purpose. After reading Bryan Greer's article on fans, I added a small 7" diameter baffle to it in order to make sure the fan was just not recirculating its own air. It was still mounted using the holes in the mirror cell.

Visually, I could never tell if the fan was making a difference in either of these configurations. Testing with the fan on and off, the view never changed. At the front of the tube, no air current could be felt.

This past weekend, I made a temporary baffle 14" in diameter. I placed weatherstrip around the edge and attached it to the OTA using 4 velcro straps, 90 degrees apart. The fan was mounted to the outside of this baffle, blowing towards the back of the primary. Now, when I place my hand in the OTA just past the secondary, I can feel the flow of air over my hand.

Sunday evening with the scope outside and the fan running for two hours before observing I believe I could tell a slight difference with the fan running. I'm not sure my mirror had ever been as close to ambient air temp as it was that night. Leaving the fan running and looking at Jupiter, there was good detail a majority of the time, however, the detail would come and go as the seeing seemed to get soft again. Turnning the fan off, the detail was soft a majority of the time and the better seeing short lived. Reversed from with the fan running.

I tried this throughout the next 2 hours of observing and just prior to stopping for the night, it seemed as if the mirror got to the point where it did not matter if the fan was on and off, the view was the same, the best detail I have had of Jupiter in recent memory.

I think the fan and sealed baffle made a difference in my case, at least until it got to the point where the mirror was very close to the ambient air temperature. It might have been the sole reason the mirror got to this point at all.

I want a more active 'scrubbing' of the boundry layer so my next test will be to install a baffle in front of the mirror. However, instead of a complete ring, I am going to try a semi-circle at the bottom of the mirror, and leave unobstructed, the top half of the mirror. It is my hope that the air coming from the baffle scrubbs the mirror from bottom to top, and the linear flow of air around the top portion of the mirror will wisk the air along the top edge of the tube and out the front. We'll see.

OLD FAN NEXT TO NEW:




ON SCOPE:


EDIT - I forgot to add that my wife came out about halfway through my viewing session. She likes to look at the planets and she had no idea about anything being done differently with the scope. When she looked at Jupiter with a 10mm Delos (175X with Paracorr), she commented without any prompting by me, that it was the most detail she had ever seen.


Edited by Mark Peterman (01/22/13 11:09 AM)


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Chucky
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Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5637044 - 01/22/13 02:32 PM

<< will be to install a baffle in front of the mirror. >>

Can anyone please show me a picture or two of this type of baffle? I'd be most interested.


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tnranger
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Reged: 01/14/12

Re: Thermal Issues and Fans Successes new [Re: Chucky]
      #5637435 - 01/22/13 05:44 PM

Excellent report, Mark!

What are you using for power, and a switch with your setup? I am trying to "recycle" a computer fan and would like to find a high/low/off switch to run the fan. Any suggestions?


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Mark Peterman
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Reged: 08/07/12

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Re: Thermal Issues and Fans Successes new [Re: tnranger]
      #5637464 - 01/22/13 06:01 PM

Chucky, if you look around page 7 of this thread, user rlmxracer posted a few pics of the ring/baffle/annulus that he made for his 10" scope.

tnranger, I am currently using 8 D cells to power the fan but I have a rechargeable LiPo pack on order to replace that setup. I have also ordered a potentiometer to vary the speed instead of using a high/low switch. You can see the switch I was using with my old baffle in the first pic. It's on/off with a red LED. You can find it at Radio Shack.

Everything will be re-done and neatly installed/painted after I finish testing with the baffle in front of the mirror.


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johnnyha
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Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5637522 - 01/22/13 06:41 PM

I've been following this thread with interest! After some tests I realized that my primary fan in back was causing vibration and softening the image just a hair. I dug out my old 12V RV box fan a few days ago and started using it again with very nice results, just blowing it up the back of my mirror box as Kriege recommends. The separate box fan scrubs the area around the mirror box as well. Sometimes it's nice to go back to the basics.

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nevy
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Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5637584 - 01/22/13 07:22 PM

Quote:

I have a 12" Orion reflector. The mirror is 1.5" thick. The inside of the tube is flocked and the OD of the metal tube is 14".

I previously attached the 80mm, 32 CFM, fan directly to the mirror cell as the cell is drilled and tapped for this purpose. After reading Bryan Greer's article on fans, I added a small 7" diameter baffle to it in order to make sure the fan was just not recirculating its own air. It was still mounted using the holes in the mirror cell.

Visually, I could never tell if the fan was making a difference in either of these configurations. Testing with the fan on and off, the view never changed. At the front of the tube, no air current could be felt.

This past weekend, I made a temporary baffle 14" in diameter. I placed weatherstrip around the edge and attached it to the OTA using 4 velcro straps, 90 degrees apart. The fan was mounted to the outside of this baffle, blowing towards the back of the primary. Now, when I place my hand in the OTA just past the secondary, I can feel the flow of air over my hand.

Sunday evening with the scope outside and the fan running for two hours before observing I believe I could tell a slight difference with the fan running. I'm not sure my mirror had ever been as close to ambient air temp as it was that night. Leaving the fan running and looking at Jupiter, there was good detail a majority of the time, however, the detail would come and go as the seeing seemed to get soft again. Turnning the fan off, the detail was soft a majority of the time and the better seeing short lived. Reversed from with the fan running.

I tried this throughout the next 2 hours of observing and just prior to stopping for the night, it seemed as if the mirror got to the point where it did not matter if the fan was on and off, the view was the same, the best detail I have had of Jupiter in recent memory.

I think the fan and sealed baffle made a difference in my case, at least until it got to the point where the mirror was very close to the ambient air temperature. It might have been the sole reason the mirror got to this point at all.

I want a more active 'scrubbing' of the boundry layer so my next test will be to install a baffle in front of the mirror. However, instead of a complete ring, I am going to try a semi-circle at the bottom of the mirror, and leave unobstructed, the top half of the mirror. It is my hope that the air coming from the baffle scrubbs the mirror from bottom to top, and the linear flow of air around the top portion of the mirror will wisk the air along the top edge of the tube and out the front.

EDIT - I forgot to add that my wife came out about halfway through my viewing session. She likes to look at the planets and she had no idea about anything being done differently with the scope. When she looked at Jupiter with a 10mm Delos (175X with Paracorr), she commented without any prompting by me, that it was the most detail she had ever seen.




I'm surprised you couldn't feel any air coming out of the front of the scope because I can feel it from my 16" lightbridge with just the standard fan that came with the scope , no baffle of any kind , if I look into the empty focused when the fan is running I can feel the breez hitting my eye also when the cloth dust cap is on the top of the scope it's blown to a dome shape and also the shroud is puffed out aswell. And if I collimation my scope when the fan is running , I can feel the breez blowing at my face, it definitely makes a difference to the views while the fan is running. I'm wondering what power source you are using and if the fan is running at the full speed , I run mine off a 12 volt car battery or a plug in 12 volt mains adapter.

Edited by nevy (01/22/13 07:28 PM)


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Sarkikos
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Re: Thermal Issues and Fans Successes [Re: nevy]
      #5637685 - 01/22/13 08:34 PM

I can also feel a breeze on my face or hands as I look into the sky end of my solid-tube 8" or 10" Dob when the fan is blowing onto the bottom of the primary. It is not a strong wind, but a noticeable breeze.

Mike


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azure1961p
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Re: Thermal Issues and Fans Successes [Re: nevy]
      #5637722 - 01/22/13 08:50 PM

Mark,

Nice write up with good pictures. I'm glad that you sealed the bottom end as its the better way to go. I went the other way and it worked fine actually but at poor economy to the actual air movement potential of the fans rating.
When I sealed off the bottom end as you have the improvement was clear and more airflow could be felt out the open opposite end. Please do keep in mind a couple things - my tube is 10" OD for an 8" mirror with a 5" fan behind the mirror. Lol, I can't dry my hair with the airflow out the open end but it is probably a little higher velocity air due to the narrow airflow compared to your significantly larger OTA. What you might want to consider for your large mirror is more fans. I haven't got the numbers needed to an exact value per scope size but it probably wouldn't be overkill if you had two 100mm fans or three 80mm fans. I'd opt for two 100mm fans. One lone 80mm fan seems a little lean. There's a point a mirror cooling will reach where no greater fan power speeds cooling than a lesser, but in this case you would seem to need more air movement. The 5" for my 8" is a little large but its also run at 6v rather than the zany 12volts but I have no vibration issues either.

The annulus is a neat idea. I'm totally sold on my boundary fan mounted on the side of the tube but for folks wanting to preserve the clean lines of the classic reflector this seems like the way to go. I've seen the video with smoke and such and I can't say I've done similar but the absence of boundary layer on either side of focus and at focus tells me for my system anyway its as good as its going to get. Plus for my tastes the fan on the side with exhaust ports has this business look about it even if it isn't a classical look.

I'm leaning with the idea more air movement would suit your large scope but here's a few things I noticed in judging wether it works or not...

1. Find a point source. Sirius is terrific for that. Jupiter is a nice object to be sure but its complexity tends to mask the results a bit. A bright lone star is ruthlessly obvious as to wether your set up is working or not.

2. First look on either side of focus. Even at 75x throw the star out of focus till it looks like billiard ball at arms length. The beauty here is that every single trace if temperature differentiation is glaringly obvious as morphing shadows boiling away in plumes and bubbles. A major caveat here is making sure your hand thermals if you are hand guiding have dissipated. They will linger and masquerade as interior heat signatures but given ten seconds they vanish- then just try not to exhale heavily. What's left is in your scope. The out of focus star tells all and about as honest as a test for quality of figure but not nearly so complex.

3. After you've ascertained the quality of the out of focus star, proof is in the focused image. Without fan cooling the out of focus star is a horrible blur of heat boils with even the perimeter of the out if focus disc ambiguously hairy with lost focus flaring out of control. Applying the rear fan should do two things: 1. The edge of the out of focus disc sharpens considerably - losing much "hair". 2. The shadowy boils or plumes forming and dissipating within the interior of the out of focus star shrink. Not ghe suze of the out of focus star but the suze of those nasty shadowy plumes. When I had no boundary fan they shrank but never disappeared. The difference was significant but subtle too. Focusing had more snap. The in focus star would hold tighter and less random flaring was clearly evident. Nothing erases a poor sky but here's an additive that was reduced and the focus on all things had less halo and morphing rays.

Another great object to focus on to ascertain wether things are working are the Jovian moons. Particularly with your size scope they should resolve as discs. The hallmark (so I learned after dealing with it ) of scope thermals damaging effects is these finite discs flaring. Poor seeing will do it but the thermals in a scope really have at it. The discs double up, triple up extending like random hands on a clock sometimes seemingly over one arc sec beyond the moon limb. The flaring here is bright too close to the intensity if the moon proper. Turning on the fan pulls these flarings down I've found by half. Too they aren't as opaque anymore either, the brilliance is faded some. All that gross distortion of an uncooled mirror had its own hand in focusing these objects hither and yon every which way. Again, poor seeing can do it in spades but reducing it here by even just a rear blowing fan was having unmistakeable benefits. The boundary layer was still there but the most improvement of either rear or side was the rear. That's the lions share of improvement. Contrary to Alan Adler with his thin flex mirror my 8" needs more than a side blower and if I had to choose I think I'd go with the rear. The boundary fan is real but its a refinement on the rear fans improvement for me anyway.

Had I not had singular test objects like Sirius or tiny moons the results would've been more convoluted. Less flaring sharpens everything but the simpler (and ruthless) the target the plainer the pluses or minuses of wether a given set up is working.

A closing caveat: when judging between no fan to working fan give the moving air time to make the difference. It was seen to shrink unmistakeable over minutes but nothing instant. I found it easier to observe the cooled mirror image first then shut things down and let it go to hell rather than let an uncooled mirror improve with a fan turned on. This can take a while but the other method seems to work faster. The cooled mirror with rear fan turned off slowly has interior heat come to the surface and then all hell breaks loose. Typically testing on Sirius or Ganymede is best around 200x or more. The low power defocus is fine to examine for heat but the infocus benefits are easier seen at medium to high magnifications of the in focus object.

I like your work and the attention to detailing with weather stripping and such.

Nice job!

Pete
PS: so as not to cause undo waves here: my closed tube is 10" wide and 72" long and the same solutions for me may not be best for a truss design.

Edited by azure1961p (01/22/13 09:02 PM)


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