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Equipment Discussions >> Reflectors

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azure1961p
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Thermal Issues and Fans Successes
      #5594403 - 12/29/12 01:19 PM

Last night I completed the project of cooling the 8" primary mirror to my reflector. The following is how I did it...

Side mounted an 80mm 12v DC fan running off a 12v car jump battery pack via cig lighter plug with a potentiometer to control RPMs. The plug has an on/off switch. I may redo the potentiometer as Id like lower Rpm capability but got now its adequate. It's 2500 rpm at max which is good to initially cool the mirror but half that speed does fine. Neither setting causes vibration on any visible level. The 80mm fan is mounted onto a black rubber rectangle which in turn has the black rubber bolted to the tube. I used a lot of bolts as it would stress the soft rubber less to have the bond distributed among many points rather than four. Too it affords a better seal.
Lastly exhaust ports on the other side of the tube to allow quicker venting off of the heat. This is important.

That's the face of the mirror, here's whe other side ...

Using my rubber end cap for the bottom of the tube I cut a 125mm hole thru it for the like sized 12V DC fan. Unlike the 80mm this one has its own unattenuated energy source in a typical cube 6v battery. This voltage runs it at a perfect gentle but effective speed. The air rushing out alongside the mirror to exit up the tube does not impede the boundary layer flow as was my fear.

Both fans running offers no vibrational issues, just a soft whirring sound.

Design thoughts:

Alan Adler who was instrumental thru his article on my work here uses only a boundary layer fan exclusively and finds no need for a rear fan. He uses a thin flex mirror of 8" and in that is probably the reason a boundary layer fan by itself on my thick 8" primary is not effective. The heat from the primary slowly but surely pollutes the boundary layer. It's a simple matter then just to run both fans. Cooling from the front AND back is essential Id guess on any mirror that's non flex.

The results:

Jupiters moons in 5/10 Pickering were the test subjects. Being small and therefore particularly susceptible to the vagaries of plumes in the light path provided a perfect test object. Without any fans the moons would flare in all directions soft doubled images that extended then retracted as the lensing heat morphed 1.0" Europa. The central moon was still seen but soft edged and extended in numerous moving directions. The out of focus moon was a blurry boiling disk with visible poisson spot. The edges of the defocused moon were wildly hairy and diffuse while the interior had what look like black morphing bubbles in slow motion boiling off - forming then dissipating to be replaced nearby with other heat bubbles. They looked for all the world like mini central obstructions growing in and out of existence. That always gave me pause for concern.

Turning on the rear fan reduced the that gross diffusion and ambiguity of the out of focus moon. Too, the black bobs morphing in and out of existence were still there but reduced in size. Focus was cleaner and the moon had noticeably less flairing. It was a good step forward but it wasn't complete.

Turning on the side fan too literally washed off the blobs. Focus now had a clear 50% reduction in lensing off Europa and all moons actually and the brightness of the flaring that remained was far less too. My Pickering seeing literally climbed a notch or two simply by running two fans. The difference particularly on details approaching 3" or less was where benefit seemed most obvious.

Conclusion: no reflector should even be sold without fans back and side. When you see the improvement - at the arc seconds level - you'll never want to go back. I can't say enough things in support of this simple DIY project. If you are particularly interested in double stars and lunar and planetary you simply need these fans working.

Caveat Observer: my first attempt at a boundary fan did nothing . I had the bottom of the 80mm airflow just touching the surface of the mirror. I didn't want to block the airflow by having the airflow blowing on the primaries side. This was wrong. The air flowed wonderfully over the mirror and never touched the boundary layer - not one bit. By lowering the fan however so 35% was obstructed by the edge of the mirror insured the entire layer was scrubbed clean off. I tried tilting fan do it blew down into the mirror when it was in its former position above the glass but this offered no help as the angle needed would have had the low profile DC fan jutting out from the side of the tube so far the hole in the tube was now grossly exposed impeding airflow in fact. I thought about vented louvers but that reduces airflow efficiency. It's too simple just to move the fan so its diameter is obstructed 35% by the mirrors edge.

And what it won't do: even with both fans markedly steadying the views and the intensity of it as well, heat plumes still are detectable. Minor but still evident as the tube is not a vacuum and it'll never truly reach outside air temps. This is a minor but real point. Also having removed the offending thermals caused by the mirror now very apparent his the effect of my hands heat blurring details as I hand guide my scope. When pointed near zenith seeing Jupiter my breath kept introducing transient pluming in the light path as it blew across the scopes aperture even tho it was well in front of it.

These will bet next thermal problems to fix. Shouldn't be hard.

If anyone's considering this approach and you enjoy tighter stars and arc second sharpness I can't encourage this enough and its cheap.

Pete

Edited by azure1961p (12/29/12 01:22 PM)


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MessiToM
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Reged: 12/21/09

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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5594570 - 12/29/12 02:47 PM

isnt this a non issue if you just let your mirror cool down and then keep your rear fan going all night to make sure the mirror follows the ambient air temp?

good work though.


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stevetaylor199
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Re: Thermal Issues and Fans Successes new [Re: MessiToM]
      #5594603 - 12/29/12 03:15 PM

I have read a lot about the benefits of fans on a reflector. Most of the talk about side fans, and those addressing the boundary layer, are focused on Dobsonian reflectors.

I'm intrigued with Pete's post because I believe he is referring to his Parks reflector, which I imagine is a Newtonian on an EQ mount. I am definitely interested in adding a side fan to my tube reflector(s).

My question is: what's the best way to add a side fan to a round tube? First I imagine that I'll need to cut round holes in the sides of the tube for intake and exhaust, but how do you guys build mounts on the tube for the fan(s)?


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Ed D
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Re: Thermal Issues and Fans Successes new [Re: stevetaylor199]
      #5594674 - 12/29/12 04:11 PM

Pete, interesting and informative write up of how you went about resolving thermal issues in your particular scope and observing environment. When I installed a fan in my Dob one of the first things I noticed was that what I previously thought were heat waves coming off neighborhood roofs were actually tube currents! Knowing what I know now I would never use a Newt without a fan, even in a tiny 3" tube.

Ed D


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Ed D]
      #5594856 - 12/29/12 06:31 PM



Hi Guys,

MessiTom: If only it were that easy and if primary mirrors were super thin this would be possible but as Gary Seronik has pointed out, the thick glass used even in todays thinner mirrors acts as a heat battery. It radiates heat long into the night, and Id suspect in many cases it never truly reaches ambient air temperature all the way through ever [outside of a stable lab environment control]. The upshot here is the mirror is never temperature equal all the way through. A rear blowing fan still leaves the otherside of the glass warmer than the fan facing side and hence a bubbling boundary layer of heat. Turn the fans off and the interior heat that was never gotten rid of radiates back to the surface. A side blowing boundary fan never cools this interior temp to the ambient air but it blows the boundary layer physically off the optic and cools the surface as well helping to keep the layer from reforming - so long as the side fan is running ofcourse. Switch it off and the layer returns literally in seconds.

Tom a rear fan is a great thing but theres no doubt removing the boundary layer is beneficial too. For deepsky, it can mean tighter stars and/or galactic cores which in term can potentially mean success in reaching your faint magnitude limit. After seeing Europa tighten up due to fan cooling Ive got every confidence this can boost my faint magnitude limit.

Steve,

LOL - it USED to be on a GEM not its on a homemade dob mount and Im happier with the dob motions if only because the mount doesnt displace everything around it like a GEM.
On mounting the fan...

Its a radio shack 12v DC computer type fan. I have it so its on the bottom of my tube if the reflector were level at the horizon. The holes are on the top of the tube - naturally because heat rises and so this helps it along. Alan Adler and Richard Berry were adamant about getting the heat out in the shortest route possible and so having these holes on the other side of the primary mirror [just above the reflecting edge surface] is important. Adler found with out them too much heat was storing in the tube when the only exit was out the tube end. But how to mount it...

Its easy really. Cut a hole a little bigger than the 80mm fan opening as this allows the sheet rubber [ sleeping mat foam works well too] to be suspended somewhat so as to flex a little. This play allows flexure and so absorbs vibrations. I bolted the fan through its four mounting holes right to the rubber. These are small nuts and bolts. Next the over sized square of rubber is bolted to the tube. I did this with numerous bolts so as not to stress any one rubber point too much, tearing it. The end product looks b--n. Black fan mounted on the black rubber square [mines rectangular actually]. Has a neat look and I wasn particularly pretty sbout it.

Ill say Steve I was concerned about excessive vibration but it never reared its head. A five inch whirring lazily and even the side fan at a hefty 2500 rpms - and nary a dither at 240x anyway. Even at 2000x the rear fan never showed its presence. Didnt do the silly masgnification with bioth though. Im betting its impossible to see. Fans are rubber mounted so there you go.

Send me your email in a message and I can send a pic of the fan mounted. Holes by the way amount to five squares spaced 3/4" apart with each square [rectangle actually] 3" long and 1.5" wide. Adler used holes but when Im finsihed finishing off this part of the project itll look nice too. I have chicken wire on the inside of the tube so as to protect the exposed mirror. At an ouitting with public Id cover the area with air conditioning foam to prevent any debris that could fall in... who knows what. By myself the chicken wire spray poainted black is fine. You dont see it since its interior. I screwed the wire through the tube to hols it in place.

Hi Ed,

I think its a crime I went as long as I did without it and as mentioned in the OP its not that I still didnt see a lot of great stuff, things I proud of even, but this makes the liklihood all the better of repeat performances . Im glad you saw the light - heh - or heard the fan as it were, too.

Ill post a pic of the intake side since its completed

Pete


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5594869 - 12/29/12 06:36 PM Attachment (183 downloads)

ok heres a pic

Edited by azure1961p (12/29/12 06:49 PM)


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MessiToM
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5594884 - 12/29/12 06:42 PM

Quote:



Hi Guys,

MessiTom: If only it were that easy and if primary mirrors were super thin this would be possible but as Gary Seronik has pointed out, the thick glass used even in todays thinner mirrors acts as a heat battery. It radiates heat long into the night, and Id suspect in many cases it never truly reaches ambient air temperature all the way through ever [outside of a stable lab environment control]. The upshot here is the mirror is never temperature equal all the way through. A rear blowing fan still leaves the otherside of the glass warmer than the fan facing side and hence a bubbling boundary layer of heat. Turn the fans off and the interior heat that was never gotten rid of radiates back to the surface. A side blowing boundary fan never cools this interior temp to the ambient air but it blows the boundary layer physically off the optic and cools the surface as well helping to keep the layer from reforming - so long as the side fan is running ofcourse. Switch it off and the layer returns literally in seconds.

Tom a rear fan is a great thing but theres no doubt removing the boundary layer is beneficial too. For deepsky, it can mean tighter stars and/or galactic cores which in term can potentially mean success in reaching your faint magnitude limit. After seeing Europa tighten up due to fan cooling Ive got every confidence this can boost my faint magnitude limit




OK, Ive had the wrong idea ALL THIS TIME about this boundary layer issue Ive heard about. thank you for clearing this up, no pun intended lol


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AutoPilot
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Reged: 12/14/12

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Re: Thermal Issues and Fans Successes new [Re: MessiToM]
      #5595140 - 12/29/12 09:01 PM

I am still very new to all of this, but I have done my share of reading (no actual hands on experience). I found this thread to be one of the best I've found on CN so far. http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/360923/page/... It gets into fans on page three as well as several other theories. I'm sure many people here have already read it front to back.

Congrats on your success.

Mike


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demiles
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Re: Thermal Issues and Fans Successes new [Re: AutoPilot]
      #5596040 - 12/30/12 12:15 PM

My own attempts at various fan mounting locations has taught me one thing. What works today under specific conditions may not work well next week when all the variables change. I use a front and rear fan configuration as well but what really helps the most is moving the scope to get it as close to ambient temp prior to viewing. If your delta T is too high no fan configuration will clean up the view and most likely make it worse.

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: demiles]
      #5597097 - 12/30/12 11:00 PM

Well nature is irregular like that but on the whole it's proven beneficial. I can't say its made anything worse but as u mention if huge temps are dropping fast enough it can cause issues . Still wouldn't turn the things off though.

Pete


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stevetaylor199
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5597161 - 12/30/12 11:43 PM

Thanks Pete. I'd love to see more pics and learn what materials to use.

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: stevetaylor199]
      #5597581 - 12/31/12 09:29 AM

Let me see if I can do that today after work.

Pete


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5598176 - 12/31/12 03:18 PM

I have two large side fans blowing across the mirror and a reasonably large one blowing on the rear.
I can see vibration at high power when using the side fans (though not at low power).
I run all three fans from sunset to darkness (about 90 minutes at my latitude), then run the rear fan all night. When I walk away from the scope for 5-10 minutes to take a break or look through someone else's scope, I turn the side fans back on.

My mirror is only 1.25" thick (a 10:1 width/thickness ratio), so it starts out the evening at ambient temperature, even if the temperature was falling.

I've checked at 2am and the mirror temperature isn't more than 2 degrees F above the ambient.

It's interesting how much better the seeing seems to have become at my observing site.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5598279 - 12/31/12 04:08 PM

Pete,

Good job on this. Congratulations on beating down much of the thermal problems in your Dob. There's a lot for Dob owners to think about and emulate in their own scopes.

I've suspended fans below the primaries of both my 8" and 10" Dobs. I made a circular baffle from black foam core and fitted it around the fan before attaching it, to help contain the air onto the back of the primary. I attached the fan to the scope with scrunchies to avoid transmitting vibrations to the optics. I've never seen any vibration in the scopes due to my fans.

There have been instances when a battery ran out of power or a cord became detached and so I could not use the fan. The difference in image quality was obvious. I've thought about setting up a more air-tight fan system for the primaries, but am concerned about inducing vibration into the optical train.

So far I haven't set up a boundary-layer fan for either scope. That would involve cutting out a large hole in the side of the tube. I'm not sure how to do that. Only recently I've become confident enough to drill holes in my scopes to attach finder mounts and such.

One thing I did to help vent tube currents and hopefully clean up the boundary layer in my 8" Dob is to drill a line of holes at the level of the primary's surface, along its upper edge. (That's an advantage to Dob's over GEMs when battling thermals: heat rises and for Dobs the upper side of the tube never changes.) I kept seeing a circular motion in defocused planet images in that scope. It was as if thermal currents were trying to escape from the tube but were caught circulating around the surface of the primary.

The holes allowed the thermals to escape and helped improve the image, even though I did not install a boundary-layer fan at the opposite side of the OTA. I think in practice these are similar to the venting holes near the upper edge of the corrector lens in some SCTs and Maks.

Mike


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Mike B
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5598384 - 12/31/12 05:08 PM

Quote:

I kept seeing a circular motion in defocused planet images in that scope.



I see this on my Dob, as well- it's induced by the rear fan's vortex.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Mike B]
      #5598697 - 12/31/12 08:18 PM

Are you sure? I believe I saw it whether I had the fan turned on or not. I've never seen the circular motion in my 10" f/4.8 Dob, only in the 8" f/6. I'll definitely have to double check this when I take the 8" out again.

Whatever the source, I thought what might have sustained the vortex, not allowing it to dissipate, was the longer and narrower OTA of the 8" relative to my 10" Dob. But if you have the same problem in a 15" f4.55, that can't be the case. Does the vortex occur whether or not you have shroud around the 15"? If the optics are open to the surrounding air, shouldn't a fan-induced vortex be disrupted quickly?

In any case, do you think the vortex appreciably worsens the image? Or possibly could it actually help disrupt the boundary layer? IME, the vortex was not a good thing.

If you have tried to get rid of the vortex and succeeded, how did you do it?

Mike


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Mike B
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5598804 - 12/31/12 09:52 PM

Tempest in a teapot, IMHO.

I have not done anything with the vortex, pro or con... just noticed it was.

Quote:

Or possibly could it actually help disrupt the boundary layer?



This was my suspicion, as well.

Remember, any "boundary layer" hap'nin' is the warming of air right at the glass-air interface... but this warmed air ain't gonna sit still- it's gonna rise, as warm(er) air does. If allowed to rise naturally, in a rolling, tumbling manner, it'll induce optical "noise" to the light passing thru it- BOTH incoming AND reflecting. But if there's movement to usher it out with some haste, it'll smooth the billowing effect out.

The air movement i'm seeing in this "vortex" should be plenty to induce some significant degree of ushering. Sure, a direct application of forced air might usher more persuasively. Yet too much might cause its own effects?


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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5598810 - 12/31/12 09:56 PM

Don, so your side fans are really for cooling due to vibration. Have you tried some vibration suppression material? I thought of mounting them on a neoprene sheet (with a hole in it, of course.) Let the neoprene absorb the vibration sparing the rigid tube of that task.

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Asbytec
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Re: Thermal Issues and Fans Successes [Re: Sarkikos]
      #5598819 - 12/31/12 10:03 PM

Mike, air tight is good. To me, it's all about pressure. An air tight rear end (so to speak) allows pressure to build (I am giggling at the metaphor.) This pressure has to go somewhere and that somewhere is up the tube. Well done.

Now, we've all seen cigarette smoke work it's way to an cracked car window. That's pressure in action, this time low pressure pulling the smoke out the window. I often thought about a side fan doing the same, exhausting the tube rather than blowing into it (causing the trapped air problems you mentioned.) So, it might work to pull the boundary layer from the top of the Dob tube.

On the vortex thing with air blowing up the tube, it's exactly like cigarette smoke being pulled out the open car window. When the car is traveling, there is lower relative pressure outside the car. Same with air blowing up the tube, it creates low pressure along the tube wall. This low pressure should draw the boundary layer toward the tube wall. But, the effect is probably weak, maybe just enough to cause a little swirling motion in the boundary layer without completely evacuating it. To do that, you need much lower pressure along the tube wall using much greater wind speed (bigger, faster fans with much greater ability to move air.)

I do not have a Dob anymore, so these are just thought experiments...something I would try. It's all about high and low pressure, those differentials are what move air from one place to another.


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Mike B
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Re: Thermal Issues and Fans Successes [Re: Asbytec]
      #5598828 - 12/31/12 10:11 PM

Quote:

So, it might work to pull the boundary layer from the top of the Dob tube.



Yes, this would be my suspicion. Simply induce additional suggestion to its natural inclination.


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