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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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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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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 new [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 new [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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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: Mike B]
      #5598836 - 12/31/12 10:18 PM

Quote:

Simply induce additional suggestion to its natural inclination.




Exactly.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5598841 - 12/31/12 10:24 PM

Norme,

Yeah, watch those metaphors! They can get you into trouble.

The baffle covers almost the entire bottom of the OTA, only allowing enough space to adjust the collimation knobs. I like the fact that the baffle and fan are not directly attached to the OTA or mirror cell, but merely suspended by scrunchies or rubber bands. This must reduce the chance of vibration being transmitted to the optical train. I don't think air tight is good if it allows vibrations to disturb the image.

Several years ago I had the fan in both these Dobs fitted into black foam core circular baffles which fit snuggly into the mirror cell. My concern, though, was that the fan was too close to the back of the primary and that vibrations would enter the system. It was a much tighter seal, though.

I have no idea where to obtain or how to set up rubber (?) as a tight baffle for the fan. That should be better if it truly does not induce vibration. Hopefully Pete can explain the nuts and bolts of this in more detail.

I am not really an ATM and am barely a DIYer. When someone says, "Go ahead and just cut an 80mm hole in the tube," I have no idea how to do it. Really.

Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Mike B]
      #5598844 - 12/31/12 10:26 PM

My fans are mounted on rubber 1/8" thick. The end cap of same rubber seals the airflow so the 5" fan is very efficient and as a result does great at. 6 volts down from the rated 12v. Both side and rear fans can be shoved or pulled with some play and flex. That was deliberate. Neoprene sounds like a dynamite idea - thicker than thinner though as it'll have more dampening effects. I'd bet Dons vibration would cancel out if it were suspended with rubber sheet 1/8" thick. Mike - I don't get the spinning voted thing but I've got large ports above the mirror to aid in dumping heat.

Next thermal project: making a carbon fiber tube, adding a removable extension to the front to keep my breath from blowing over the opening and adding a guiding handle well away from the same front end. The hand kills the seeing!!!! The heat coming off it on a winters night is unreal.

Thanks for the input guys!!

Pete


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Mike B]
      #5598845 - 12/31/12 10:26 PM

My fans are mounted on rubber 1/8" thick. The end cap of same rubber seals the airflow so the 5" fan is very efficient and as a result does great at. 6 volts down from the rated 12v. Both side and rear fans can be shoved or pulled with some play and flex. That was deliberate. Neoprene sounds like a dynamite idea - thicker than thinner though as it'll have more dampening effects. I'd bet Dons vibration would cancel out if it were suspended with rubber sheet 1/8" thick. Mike - I don't get the spinning voted thing but I've got large ports above the mirror to aid in dumping heat.

Next thermal project: making a carbon fiber tube, adding a removable extension to the front to keep my breath from blowing over the opening and adding a guiding handle well away from the same front end. The hand kills the seeing!!!! The heat coming off it on a winters night is unreal.

Thanks for the input guys!!

Pete


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Mike B
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5598855 - 12/31/12 10:36 PM

Quote:

Next thermal project:



Woah, slow down there! Take the rest of the nite off... don't start any more projects this year...


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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5598857 - 12/31/12 10:37 PM

Quote:

I don't think air tight is good if it allows vibrations to disturb the image.




Mike, Agreed on the vibration. One idea is to use a piece if sheet rubber if you can find some. Fly to Hong Kong, they have venders with anything you could ever think of. Rubber bands are the same idea. Maybe they work well enough, dunno. Anyway, if you can isolate or dampen the vibration to nil, no vibration should make it's way to the tube.

My idea was using a rubber sheet as an air tight seal across the back end of the tube. But, with a fan or two in the middle of it, I was concerned it might budge out with the fans blowing full force. (YIKE) Smaller is better, it seems, mounted on a more rigid thingy.

Edited by Asbytec (12/31/12 10:39 PM)


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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5598923 - 12/31/12 11:46 PM

I am working on a similar baffle for my dob. So far I have a prototype made out of card board using the stock 80mm fan. I just bought a low noise 120mm fan to replace it with http://www.silenx.com/quiet.fans.asp?sku=efx-12-12. I am making the baffle out of a 3/16 sheet of dark grey PVC and mounting the fan to the baffle. I am going to use Dynamat http://www.dynamat.com/products_car_audio_speaker_kit.html to line the inside of the baffle arouond the fan to eliminate any and all vibration. Lastly I am going to seal the baffle with some weather strip that I have left from another project. I am going to run a strip of this http://www.homedepot.com/Tools-Hardware-Hardware-Weather-Stripping-Foam-Tapes... the outside of the collimation screws. So far last night was the first clear skies Ive had for weeks and just having the baffle and the stock fan seemed to help reduce the time it took to get good views from prior nights.

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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5598982 - 01/01/13 12:44 AM

Yes, that sounds nice. Forgot to mention the "silent" fans that are ultra smooth running. Thanks for bringing it up.

Are you going with a cooling fan configuration or a boundary layer scrubbing set up?


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5599002 - 01/01/13 01:06 AM

Well I am trying to direct the flow from the rear fan around the front of the mirror. I am probably going to make a ring similar to what is discussed in this thread http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3460080/Main... further move the air across the face of the mirror. I am trying to resist the urge to cut a hole in the side of my scope. From all that I have read the side mounted fans are not really needed on 10" and less mirrors although it has been done.

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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5599026 - 01/01/13 01:21 AM

Interesting post.

I have a large computer fan mounted to a baffle I can leave on the back of my reflector, or remove when I need to let the scope warm up after being in the extreme cold. The fan runs on 12 volts, (8 "AA" batteries hooked together in a plastic holder), but I find battery life way too short.

What other small battery sources can I use to prolong battery life?

PS: The fan has a three speed switch for low, medium and high speeds.

Cheers,


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5599039 - 01/01/13 01:41 AM

At home I use an Universal AC/DC adapter similar to this http://www.amazon.com/PowerLine-Universal-Compatibility-0900-91-Bi-lingual/dp... gives me the option ove 3-4 diferent speeds. For the road I am going to use 2 C cell 4 cell holders and wire them together for 12v. I am also going to use a rheostat to give me a variable speed control.

Edited by rlmxracer (01/01/13 02:01 AM)


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5599048 - 01/01/13 01:50 AM

I already have three speeds. I do a lot of observing away from home as well and need a 12 volt battery source that will make it run longer. When I first take the scope out, the speed stays on full, and when observing, I leave the fan speed on low.

I am just looking for another small 12 volt battery source and wanted to know what else I could use.

Quote:

For the road I am going to use 2 C cell 4 cell holders and wire them together for 12v.




Would these type of cells make it run longer?

I would use an adapter plugged into an extension cord in my backyard, but I am concerned that: 1) I may trip over the cord in the dark and 2) The cord might get tangled when slewing the scope to another area of the sky.


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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5599092 - 01/01/13 03:39 AM

That's an interesting approach, using a lip to draw the warmer air off the edge of the mirror and exhaust it out the back of the tube. Seems it should work if you can find the right volume of air to flow. The concept seems to create a rather strong low pressure area near the edge of the primary. Again, this seems very analogous to the cracked car window.

To preserve the low pressure at the sides of the mirror, it seems being air tight along the bottom of the tube is still desirable. Otherwise, surrounding air behind the mirror can easily flood the fan and be replaced by air outside the bottom of the tube. You really need that air flow coming down the tube, not from behind the primary.


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5599103 - 01/01/13 04:06 AM

As I understand the thread I posted above he had the fan blowing on the primary and up the tube. I will try both directions it's easy enough. Happy new year

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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5599110 - 01/01/13 04:44 AM

Happy New Year

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5599135 - 01/01/13 05:41 AM

Quote:

From all that I have read the side mounted fans are not really needed on 10" and less mirrors although it has been done.




Then they were mounted wrong and more than likely without proper exhaust ports. Even a three inch primary will benefit if it has a significant boundary layer. It's not an issue at all of question to me. The boundary layer is there even with the sealed 5" fan blowing on its back. Turn on the side mounted fan and it's literally blown clear off and right out the tube off the other side of the primary.

My first attempt at this failed due to improper positioning of the fan across the mirror. In that case it did litte to nothing. When that was addressed the boundary layer was cleared off. I'd suspect some folks have had poor results due to the side fan not intercepting the side of the mirror but blowing to high above it. The bottom tip of the fans flow was just barely touching the primary and it was pointless. I lowered to the measure that it was partially obstructing the flow and that's when success was had.

I'd refer you to Alan Adlers article on his successful side mounted fan in Skya andTelescope. He reaches the failures you may have read about then perseveres and corrects them.

Pete

Edited by azure1961p (01/01/13 05:46 AM)


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5599367 - 01/01/13 10:56 AM

Quote:

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.




It is because the two fans are large and operate at high speed. I could plug the fans into a regulated port in my power controller instead of the "straight-through" 12V port, and then I could simply turn the speed down until I couldn't see vibration. rubber or neoprene or sorbothane mountings would help, but these fans are powerful enough the vibration can be felt in you fingers touching the top of the UTA, transmitted through the Moonlite connectors, poles, and a lot of wood.
I can turn them on periodically if i think a boundary layer has reappeared, but I don't think it really does on this thin mirror.

The rear fan is smaller, slower, and doesn't move as much air, and so its vibration is a lot less visible. I can't see any evidence of vibration at 456X.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5599387 - 01/01/13 11:13 AM

Norme,

Quote:

Agreed on the vibration. One idea is to use a piece if sheet rubber if you can find some. Fly to Hong Kong, they have venders with anything you could ever think of.




Yeah, right ... seriously? For me a long trip is driving 53 miles to my dark site.

I have no idea where to obtain a sheet of rubber or neoprene locally. I guess that's what the internet is for?

Quote:

Rubber bands are the same idea. Maybe they work well enough, dunno. Anyway, if you can isolate or dampen the vibration to nil, no vibration should make it's way to the tube.




Actually, I'm using rubber bands now. They work well to isolate the optical train from vibration. But suspending the fan+baffle from rubber bands does not produce an air tight fit, of course.

Quote:

My idea was using a rubber sheet as an air tight seal across the back end of the tube. But, with a fan or two in the middle of it, I was concerned it might budge out with the fans blowing full force. (YIKE) Smaller is better, it seems, mounted on a more rigid thingy.




I have a pre-wired set of three fans that I bought months ago during a sale from Orion. They were meant for the Orion 14" Dob, but I was thinking about mounting them on a baffle below my 10" Dob's primary. If they are distributed evenly and the rubber sheet or foam core is attached by Velcro or some such to the edge of the OTA, it should be OK.

But then once again you introduce the possibility of vibration being transmitted to the optical system. A cure that is worse than the disease is less than useful. I don't want a fan system that is only good for cooling the system initially but needs to be turned off when I'm observing. Ideally the fans should be left on during the entire observing session.

There is also the problem of access to the collimation screws if a baffle is tightly sealed to the bottom of the OTA. And if you cut holes for the screws, then you allow possibility for air leakage or induction of vibration. But IME & IMO, close collimation is just as important - maybe more so? - than thermal issues. If your system is not very closely collimated - especially if you're observing planets - you might as well leave it in the house.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5599410 - 01/01/13 11:34 AM

Quote:

As I understand the thread I posted above he had the fan blowing on the primary and up the tube. I will try both directions it's easy enough. Happy new year




Yes, I've read the same thread. The air is supposed to be directed from the fan below the primary, up along the sides of the mirror, and across its surface. This direction should help cool the sides of the primary as well as break up the boundary layer.

I suppose this is actually what is happening in my 8" Dob to produce the vortex. The vortex might help disturb the boundary layer, but a laminar flow from the side is probably better. Maybe both simultaneously would be ideal?

For a Newt/Dob, pulling air out the bottom of the OTA never seemed like a good idea to me. That would make more sense in an SCT or Mak. But for a Newt/Dob, we should put that ambient air to good use by pulling it up into the OTA. The air should either vent out the top of the OTA - why not put that big opening to good use? - and/or out vent holes in the side of the OTA.

Recently, I did construct an annular baffle from foam core and pushed it up above the primary's surface. The row of holes that I drilled are situated between the surface of the primary and the bottom of the baffle. (Actually the holes are partially below the edge of the primary. Judging from Pete's report, this might be a good thing.) The top of the baffle is surmounted by a ring of ProtoStar that protrudes a little over the edge of the baffle, making a sharp-edged lip. (An added value to the annular baffle, is that if there is a turned-edge on the primary, the ProtoStar ring can be made a bit bigger to cover that defect. The aperture will be slightly reduced but that will be compensated by an improvement in the image.)

This is in my 8" Dob. If it works well, I'll do the same thing in my 10". But I haven't had a chance to take out the 8" and do a good test on the vent holes / annular baffle combo. We'll see.

Mike


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5599634 - 01/01/13 01:32 PM

Mike, I have been experimenting with fans, annular rings etc. and bringing fresh air down the tube can also be controlled in the sense that a laminar flow is set up and you are disturbing the air between the face of the primary and the secondary mirror. In my limited experiments with what I am calling a Comprehensive Boundary Layer Mitigation System, preliminary results are quite favorable for increasing the resoliving ability of both an equilibrated mirror as well and an un-equilibrated mirror. I think the key may be the whole issue of laminar flow and how one goes about trying to create that? Our experiments with a 20" f/3 have so far been pretty successful. Bob

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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5599749 - 01/01/13 02:39 PM

Bob,

How do you go about bringing fresh air down the tube? Wouldn't a fan at the top of the OTA get in the way of the optical train, as well as be a potential hazard to the primary if it isn't very tightly secured? And if the fan is secured tightly enough so that it won't fall, it might be a source of vibration.

For a 20" f/3 Dob, there should be plenty of access to ambient air, unless a shroud is around the UTA and mirror box. I wouldn't think you'd need to bring air "down the tube."

There is a very good chance I don't fully grasp what you're talking about here, so any further explanation - maybe pics? - would be appreciated! All my Dobs are solid tube, but maybe some of your ideas could be applied to them as well. And I do plan on acquiring about a 14" to 16" truss scope eventually.


Mike


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Dean Norris
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5599785 - 01/01/13 03:02 PM

Quote:



Next thermal project: making a carbon fiber tube, adding a removable extension to the front to keep my breath from blowing over the opening and adding a guiding handle well away from the same front end. The hand kills the seeing!!!! The heat coming off it on a winters night is unreal.

Thanks for the input guys!!

Pete




Wouldn't a dew shield on the end of the tube take care of that issue. I made a dew shield out of foam mat that I spray painted black. I built this to keep the dew off the secondary but I think it would also block thermals at this end of the tube.

Dean


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beatlejuice
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5599835 - 01/01/13 03:32 PM

Quote:

Recently, I did construct an annular baffle from foam core and pushed it up above the primary's surface. The row of holes that I drilled are situated between the surface of the primary and the bottom of the baffle.





Is the bottom of the foam baffle above the mirrors surface?
If so, what are the holes for?
I am trying to visualize this for my own scope, as I was orignally thinking of a solid ring baffle secured by my mirror clip screws with some kind of slightly flexible rubber molding glued to the inside of the tube such that it meets the top of the outside of the baffle to keep air loss up the sides of the tube to a minimum thus maximizing the flow across the mirror from my rear mounted fan.
Thanks

Eric


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: beatlejuice]
      #5599949 - 01/01/13 04:53 PM

I am going to replace my larger fan and make an anular ring later this week. I have had a rainey and cloudy three weeks so I am itching to do some testing.

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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5599953 - 01/01/13 04:56 PM Attachment (21 downloads)

Quote:

Bob,

How do you go about bringing fresh air down the tube? Wouldn't a fan at the top of the OTA get in the way of the optical train, as well as be a potential hazard to the primary if it isn't very tightly secured? And if the fan is secured tightly enough so that it won't fall, it might be a source of vibration.

For a 20" f/3 Dob, there should be plenty of access to ambient air, unless a shroud is around the UTA and mirror box. I wouldn't think you'd need to bring air "down the tube."

There is a very good chance I don't fully grasp what you're talking about here, so any further explanation - maybe pics? - would be appreciated! All my Dobs are solid tube, but maybe some of your ideas could be applied to them as well. And I do plan on acquiring about a 14" to 16" truss scope eventually.


Mike




Mike, Front fan is in the shadow of the 5" secondary and it held in place by .025 thick safety wire. The fan is a Sunon MagLev fan which is essentially vibration free. The rear fan is fully enclosed in the rear of the mirror box and is a MagLev fan suspended on Sorbothan and is essentially completely isolated from the scope.

Edited by Bob S. (01/01/13 07:29 PM)


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orion61

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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5599997 - 01/01/13 05:20 PM

I once had a 6" F10 Newtonian tube for Planetary, (wish I still had it) it had the tiniest secondary I've ever seen,
something like 2/3rds of an inch or so! Homebuilt 12th wave mirror.
but it suffered from tube currents in that LOOONG tube.
I simply bought a Radio Shack DC fan,hooked it up with rubber bands and Velcro taped the 9v battery to the tube with an on/off switch.
It worked superbly, no currents vibrations and the best Planetary views I have ever had. Easily as good as a Christian refractor had at the time!


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Mike B
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Re: Thermal Issues and Fans Successes new [Re: orion61]
      #5600021 - 01/01/13 05:38 PM Attachment (16 downloads)

Quote:

It worked superbly, no currents vibrations and the best Planetary views I have ever had.




Same happens with this classic 1960's Optical Craftsmen 6" F8, fitted with a single rear-blower PC fan, suspended with rubber (innertube) straps + velcro. VERY sharp planetary views result, and commonly! Will also take silly-high mags, as well.


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5600089 - 01/01/13 06:24 PM Attachment (30 downloads)

Quote:

Actually, I'm using rubber bands now. They work well to isolate the optical train from vibration. But suspending the fan+baffle from rubber bands does not produce an air tight fit, of course.




Post a pic of your fan / baffle setup Mike! Here is mine. it is a 12 Volt computer fan fixed to a light flat black, (on the inside it is flat black), thick cardboard baffle / seal. I get zero vibrations and have three speeds on the fan. I set it cranked at full speed for cool-down and leave it on low when I observe or until the 8 "AA" batteries expire. I need a new 12 Volt source so the fan can stay on longer, but I don't know what other alternatives there are for a small 12 Volt battery!


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5600090 - 01/01/13 06:24 PM Attachment (22 downloads)

Pic #2 showing the fan mounted to the thick cardboard baffle which I can leave on or remove. Note the small, beige three-speed switch at the top left.

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Re: Thermal Issues and Fans Successes new [Re: Dean Norris]
      #5600096 - 01/01/13 06:26 PM

Dean,

Quote:

Quote:



Next thermal project: making a carbon fiber tube, adding a removable extension to the front to keep my breath from blowing over the opening and adding a guiding handle well away from the same front end. The hand kills the seeing!!!! The heat coming off it on a winters night is unreal.

Thanks for the input guys!!

Pete




Wouldn't a dew shield on the end of the tube take care of that issue. I made a dew shield out of foam mat that I spray painted black. I built this to keep the dew off the secondary but I think it would also block thermals at this end of the tube.

Dean




I agree. Once in a while the secondaries in my 8" and 10" Dobs would begin to fog. But this ended immediately when I put dew shields on the scopes. They cut glare and keep the dew off the optics. FWIW, I never notice any effects from my body heat or breath in the optical train of my Dobs. It's a nonissue.

IME & IMO, the best dew/light shields are homemade from black foam sheeting fully-flocked on the inside with vertical strips of ProtoStar. PVC and metal dew shields are inferior by comparison.

Mike


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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600122 - 01/01/13 06:43 PM

Quote:

How do you go about bringing fresh air down the tube? Wouldn't a fan at the top of the OTA get in the way of the optical train...




Bob's solution is one way, in fact probably very effective. In the case I was describing, one would seal the bottom of the tube and exhaust air out the back of the tube. This arrangement sets up a low pressure area behind the mirror and draws air into it, hopefully carrying the boundary layer with it. The lip along the edge of the tube just above the primary serves to act as a vacuum aimed across the mirror's surface instead of pulling air directly down the tube wall. The idea is, this descending air will pile onto the mirror's surface and get sucked off the edge of the mirror.

I think Pete tried this. The challenge seems to be creating a sufficient volume of air moving in the tube. That's a lot of CFM to move (in either direction) from the back. In one review, a guy had fans strong enough to feel a slight breeze from the open end of the OTA. If it sucks into and down the tube, you run the risk of pulling your breath down the tube. If it blows up the tube, you run the risk of drawing in contaminants from the ground below the primary. Bob's system just forces air onto the surface to the same effect. It seems pretty effective at mixing and moving warm air. If it is mixed well it probably does not need to be evacuated from the tube.

With nothing to do, I watched some videos on laminar flow. One interesting thing is how a side blowing fan sets that up. We've all seen images of bow waves forming in supersonic flight. Well, a side blowing fan that strikes the mirror's edge will do the same thing. A weak bow shock will form over the mirror and depart the mirror's surface. This sets up a laminar flow that starts at the surface then arches above the mirror. Below that laminar bow wave and above the mirror is a turbulent zone much like an aircraft wing near stall.

I bet someone could do their Dissertation on this complex subject.

To me, again, it's all about moving air through pressure and the path of least resistance. Bob's set up creates high pressure on the mirror's surface, so air is driven into the surrounding low pressure near the tube wall. Sucking air out of the back of the tube (low pressure behind the primary) does the same thing but probably needs to move more air volume inside the tube. Blowing air up the tube (high pressure behind the primary) does, too. That high pressure air will make it's way to the lower pressure area at the upper tube and eventually out the top. Driving air inside the tube has the added benefit of cleaning out the entire optical path, if you can move that much air (even gently.)

Hitting the mirrors surface seems to require less air movement and to be more efficient. But the air probably won't evacuate the tube without a path for it to follow (like an exhaust port.) But, that may be just fine.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: beatlejuice]
      #5600127 - 01/01/13 06:45 PM

Eric,

Quote:

Quote:

Recently, I did construct an annular baffle from foam core and pushed it up above the primary's surface. The row of holes that I drilled are situated between the surface of the primary and the bottom of the baffle.




Is the bottom of the foam baffle above the mirrors surface?
If so, what are the holes for?
I am trying to visualize this for my own scope, as I was orignally thinking of a solid ring baffle secured by my mirror clip screws with some kind of slightly flexible rubber molding glued to the inside of the tube such that it meets the top of the outside of the baffle to keep air loss up the sides of the tube to a minimum thus maximizing the flow across the mirror from my rear mounted fan.




I experimented with a thin cardboard baffle ring screwed down directly to the tops of the mirror clips. But I could never get a good seal where the ring contacts the OTA. IME, rings directly on the clips are better to cover edge errors on the primary rather than to direct air flow.

For my baffle ring, I cut out a circle of black foam core just the right diameter to have a snug fit inside the OTA. I used the thicker type, so it would be sturdy and have a snug fit against the OTA. I cut out the central area a little larger than diameter of the primary. Then I cut a ring of ProtoStar whose central opening is just large enough to expose the entire surface of the primary mirror. I glued this ProtoStar ring on the top of the foam core ring. Finally I pushed the baffle up into the bottom of the OTA.

I drilled the lateral series of holes in the OTA so they will be at the edge of the primary surface. The baffle is positioned so that it is above these holes. The holes are between the baffle and the surface of the primary.

The idea is that the baffle will help direct the flow of air around and to the vent holes, helped by the fan(s) below the primary and the fan at the opposite side of the OTA (when I eventually place one there).

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5600149 - 01/01/13 07:04 PM

Norme,

Quote:

Bob's solution is one way, in fact probably very effective. In the case I was describing, one would seal the bottom of the tube and exhaust air out the back of the tube. This arrangement sets up a low pressure area behind the mirror and draws air into it, hopefully carrying the boundary layer with it. The lip along the edge of the tube just above the primary serves to act as a vacuum aimed across the mirror's surface instead of pulling air directly down the tube wall. The idea is, this descending air will pile onto the mirror's surface and get sucked off the edge of the mirror.




I think that we should have a fan below the primary blowing ambient air onto the bottom of the mirror. The dual goals are to adjust the temperature of the mirror to ambient and to eliminate/diminish the boundary layer on the surface of the mirror. A fan blowing air onto the bottom of the mirror, then pushing the air around the sides of the mirror and finally over the surface of the mirror to vent out holes immediately above the mirror seems like an elegant, multi-function solution to me. If the bottom of the OTA is sealed except for the intake of the fan, so much the better ... as long as no vibrations are induced. Now if another fan is placed at a hole at the lower edge of the primary's surface, to blow in air over the surface of the primary and out the vent holes at the upper edge of the mirror, the synergistic effect should be excellent.

Mike


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600167 - 01/01/13 07:17 PM Attachment (31 downloads)

The rear sucking fan is regulated by a potentiometer that allows it to move air fairly slowly at low speeds and move a lot of air at high speeds. The rear of the mirror cell is enclosed and the air cascades over the annulus and around the edge of the primary and out the back. Here is a picture of the rear fan imbedded in the mirror cell with Sorbothane. The fan is a MagLev fan and there is no noticeable vibration at any speed. The front blowing fan seems to create what is likely some semblance of laminar flow as the images definetly are improved with both fans running simultaneously. I guess I would have to do smoke tests to actually see what is going on?

Edited by Bob S. (01/01/13 07:32 PM)


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5600244 - 01/01/13 08:03 PM

Bob,

Quote:

Front fan is in the shadow of the 5" secondary and it held in place by .025 thick safety wire. The fan is a Sunon MagLev fan which is essentially vibration free. The rear fan is fully enclosed in the rear of the mirror box and is a MagLev fan suspended on Sorbothan and is essentially completely isolated from the scope.




OK, so the way I understand this is that both front and rear fans are in the vicinity of the mirror box. The front fan is far enough above the surface of the primary that it will still be effective in pulling air down the "tube," but close enough to the primary that it can hide within the shadow of the secondary. The suspension wires for the front fan would be another source of diffraction for the optical train, besides the spider vanes or wires that secure the secondary.

A good solution for large Dobs, maybe not so good for smaller scopes - especially solid tubes - but something to think about.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: orion61]
      #5600251 - 01/01/13 08:08 PM

Hi Larry,

Quote:

I once had a 6" F10 Newtonian tube for Planetary, (wish I still had it) it had the tiniest secondary I've ever seen,
something like 2/3rds of an inch or so! Homebuilt 12th wave mirror.
but it suffered from tube currents in that LOOONG tube.
I simply bought a Radio Shack DC fan,hooked it up with rubber bands and Velcro taped the 9v battery to the tube with an on/off switch.
It worked superbly, no currents vibrations and the best Planetary views I have ever had. Easily as good as a Christian refractor had at the time!




Where exactly did you place the fan? Below the primary and blowing air onto its bottom? Or in a hole at the side of the OTA?

Mike


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600264 - 01/01/13 08:19 PM

Just a note about suspending a fan in front of the mirror:

The "umbral" shadow of the secondary is always somewhat smaller than the secondary because of off-axis light shining on the mirror from "around the edge" of the secondary (off axis light).

The longer the tube, the less size the "umbra" of the secondary shadow has for the same off-axis angle.
[in practice, long tubed scopes don't have as large an off-axis angle, so the umbral shadow of the secondary is still nearly the same size as the secondary itself]

You can work it out with simple trig by figuring out 1/2 of the maximum field size of the scope as the upper acute angle, the distance from secondary to primary surfaces as one side of the triangle and the width of the penumbra of the secondary shadow as the bottom of the triangle. Since you know one side, and the angle, you can figure the width of the penumbra
[remember, the penumbra is actually larger than the size of the secondary because of the shadow from the opposite side angle, but for purposes of this, we can figure the penumbra to be a thin ring inside the diameter of the secondary].

So long as the fan is smaller than the umbra of the secondary shadow, no possible diffraction can occur from it. And the wires can be really thin.....


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beatlejuice
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600269 - 01/01/13 08:21 PM

Quote:

The idea is that the baffle will help direct the flow of air around and to the vent holes, helped by the fan(s) below the primary and the fan at the opposite side of the OTA (when I eventually place one there).




So the implication is that allowing the air to leave the bottom of the scope is superior for eliminating the boundary layer than letting it drift out the front of the scope?

Eric


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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600280 - 01/01/13 08:29 PM

Quote:

A fan blowing air onto the bottom of the mirror, then pushing the air around the sides of the mirror and finally over the surface of the mirror to vent out holes immediately above the mirror seems like an elegant, multi-function solution to me.

Now if another fan is placed at a hole at the lower edge of the primary's surface, to blow in air over the surface of the primary and out the vent holes at the upper edge of the mirror, the synergistic effect should be excellent.

Mike




Mike, that set up could work provided you could move a sufficient volume of air around the mirror to disperse the layer. As Bob mentioned, some smoke tests might help. One guy tested this "lip" system on youtube, but his results seemed to barely move any air. The smoke just kind of lingered above the mirror. He might could have used more CFM.

One thing to take into account is volume. The volume of fast moving air directly coming off the fan must spread itself around the entire rear surface, move in all direction with enough CFM to actually be felt in the huge volume of space above the primary. If the air flow is too weak, you will not get disruption toward the center of the primary (thus a cross blowing fan would help.)

Absolutely, if you can get the dual benefit of cooling and layer scrubbing with a single system, that's the way to go. Blowing cold air directly on the primary is probably most efficient, but the mere act of removing the boundary layer (blanket) will allow for some degree of cooling, too. I like Bob's set up because it kills two birds with one stone AND evacuates all that mixed up air out the back. (Even though I am adverse to adding fans or additional spider vanes into the light path.)

Again, there are so many ways to do this. The trick is to find the most efficient use of batteries. Another good design seems to be 3 high volume fans blowing from the rear and out the front. It moves so much air you can feel a slight breeze out the top of the tube. It evacuates the entire volume of air inside the tube and replaces is with cooler ambient air while it cools the primary. And the low pressure force might be sufficient to draw off the boundary layer, too. You can observe crisp images through gale force winds provided it is laminar in nature and a constant density.


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Re: Thermal Issues and Fans Successes new [Re: beatlejuice]
      #5600292 - 01/01/13 08:36 PM

Eric,

My physical intuition - later to be affirmed or refuted by the facts - is that it might be best to vent the warm air out of the system as close as possible to the source of the heat, that is, the primary mirror. So I think that in a solid tube Dob, probably the best place to vent the warm air from the primary is immediately at the uppermost edge of the surface of the primary.

I go back to my thought about high-end SCTs and Maks. They have a row of vent holes directly around the edge of the correcting lens. Might be a good idea in all solid-tube scopes of all types if there were vent holes around the rim of the objective, correcting lens or primary mirror.

It's really easy to drill a series of holes along the high edge of the primary mirror in Dobs. Depending on results in my 8" Dob, I'll be doing that for my 10" and 5" Dobs next.


Mike


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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5600300 - 01/01/13 08:42 PM

Bob,

Quote:

The front blowing fan seems to create what is likely some semblance of laminar flow as the images definetly are improved with both fans running simultaneously. I guess I would have to do smoke tests to actually see what is going on?




There is a program that you can download to help visualize air flow in a telescope system once you enter in data points. I've never used it. Looks intimidating to me, above my current knowledge and abilities about such things.

Yep, smoke might be easier.


Mike


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Re: Thermal Issues and Fans Successes new [Re: Asbytec]
      #5600319 - 01/01/13 08:54 PM

Norme,

Quote:

Mike, that set up could work provided you could move a sufficient volume of air around the mirror to disperse the layer. As Bob mentioned, some smoke tests might help. One guy tested this "lip" system on youtube, but his results seemed to barely move any air. The smoke just kind of lingered above the mirror. He might could have used more CFM.

One thing to take into account is volume. The volume of fast moving air directly coming off the fan must spread itself around the entire rear surface, move in all direction with enough CFM to actually be felt in the huge volume of space above the primary. If the air flow is too weak, you will not get disruption toward the center of the primary (thus a cross blowing fan would help.)




That's why I suggested that ideally there should be a second fan at the side of the OTA to blow air across the surface of the primary. Then at least the fan below the primary would be cooling the bottom and sides of the mirror.

In my 8" Dob, though, the bottom fan by itself is apparently enough to create a vortex circulating around the surface of the primary. I still need to take the scope back out to see how adding vent holes has affected the air flow and the image.

Quote:

Another good design seems to be 3 high volume fans blowing from the rear and out the front. It moves so much air you can feel a slight breeze out the top of the tube. It evacuates the entire volume of air inside the tube and replaces is with cooler ambient air while it cools the primary. And the low pressure force might be sufficient to draw off the boundary layer, too. You can observe crisp images through gale force winds provided it is laminar in nature and a constant density.




Yep, I still have the three-fan setup I bought on sale from Orion. It was meant for their 14" Dob, but I want to try it on my 10". That ought to move some air.

Mike


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beatlejuice
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600340 - 01/01/13 09:09 PM

Quote:

It's really easy to drill a series of holes along the high edge of the primary mirror in Dobs. Depending on results in my 8" Dob, I'll be doing that for my 10" and 5" Dobs next.




Mike, I also like this idea,are you thinking something like 8-16 1/4" holes or larger?

Eric


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5600344 - 01/01/13 09:11 PM

Mark,

Quote:

Post a pic of your fan / baffle setup Mike! Here is mine. it is a 12 Volt computer fan fixed to a light flat black, (on the inside it is flat black), thick cardboard baffle / seal. I get zero vibrations and have three speeds on the fan. I set it cranked at full speed for cool-down and leave it on low when I observe or until the 8 "AA" batteries expire. I need a new 12 Volt source so the fan can stay on longer, but I don't know what other alternatives there are for a small 12 Volt battery!




I'll have to set aside some time to bring out the scope and take some pics. Here is the battery supply I use. It holds eight D-cells. The batteries last a long time. I only need to replace one or so within about a year. I try to take the 10" to my dark site once or twice each month, and I have the batteries running five or six hours each night. The battery pack has a strap to hang it from the side of a Dob mount. Apparently the battery pack is sold along with a fan now. When I bought mine a few years ago, all you received was the battery pack.

Orion Cooling Fan for Large Reflectors

I use a separate 12v battery for my dew buster strips. That battery is a PowerSonic PS-1270 F2. I keep it and the dew controller in a basket that hangs on the front of my Dob mount.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: beatlejuice]
      #5600358 - 01/01/13 09:22 PM

Eric,

Quote:

Quote:

It's really easy to drill a series of holes along the high edge of the primary mirror in Dobs. Depending on results in my 8" Dob, I'll be doing that for my 10" and 5" Dobs next.




Mike, I also like this idea,are you thinking something like 8-16 1/4" holes or larger?




As I said, I'm not a master of tools. I just used the largest drill bit I had at the time. I drilled seven 1/4" holes spaced evenly about 1" apart. They are lined up around the OTA at the level of the edge of the primary's surface, at the uppermost position to catch the rising warm air.

Mike


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5600569 - 01/02/13 12:00 AM

So, I just discovered that the bigger the cell, the more current it holds. Those big "D" cells would be a lot better than the 8 "AA" batteries I am using! I'm thinking that a Lithium battery that adds up to 12 volts or NiMH would be even better than Alkaline and last even longer.

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5600583 - 01/02/13 12:26 AM

Well I was out again in terrific typical CT garbage seeing that went from 5/10 to overcast with stratus clouds milking the contrast to pointless, but I got to evaluate my fans again...

It would seem to work like this: the turning on of the sealed 5" fan behind the primary cuts the Jovian moon flaring ( the BEST test subject ) in half. And it's not just the length of the flaring but the brightness or opacity. Now turning on the side fan along with it cleans that down to half or a third of that. The side mounted fan would seem to have negligible effects at 50x to 120x but 173x or higher and the benefit becomes more pronounced.

Tempted to say the rear fan cuts it in half and the side fan another half, but the latter at least tonight was a little more subtle: but all together real. Once the moons tightened up under both fans what was finally left was the garbage seeing over head that left the disks soft edged - Minsk flairing but soft edged. So it helps to be sure but as the old saying goes, you can't shine *BLEEP*. Such is the seeing.

It's little unreal and aggravating too that I haven't seen anything better than 5/10 seeing in five weeks.

Five weeks.

Pete

Edited by azure1961p (01/02/13 12:26 AM)


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5600596 - 01/02/13 12:44 AM

Thanks for the report Pete. I should get three nights of testing this weekend. I'll try a few differnt things and report back. It's looking like I'll end up with a hole in the side of my scope.

Edited by rlmxracer (01/02/13 12:44 AM)


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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5600597 - 01/02/13 12:47 AM

Quote:

So, I just discovered that the bigger the cell, the more current it holds. Those big "D" cells would be a lot better than the 8 "AA" batteries I am using! I'm thinking that a Lithium battery that adds up to 12 volts or NiMH would be even better than Alkaline and last even longer.



I didn't read the whole thread, so if these are covered already, forgive me

You could put a rechargeable 12V battery in the rocker. 7AH to 18AH, for example, doesn't take up much space. the 7AH size makes a good counterweight.

As for the fan speed control: I control the speed using a potentiometer within reach of the Ep. Since I built it, I see these units meant for computers where the entire circuit board is already built, and cost 5 bucks.


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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5600636 - 01/02/13 01:22 AM

Quote:

Thanks for the report Pete. I should get three nights of testing this weekend. I'll try a few differnt things and report back. It's looking like I'll end up with a hole in the side of my scope.




Just make sure a full 1" of the three inch fan is BELOW the mirrors reflected surface. Essentially so the bottom 1" of the fan is blowing the the side of the mirror. This forces the remaining two inches above the mirrors surface to powerfly blow the boundary layer off. If you have the tip of the 3" hole just above the mirrors surface it's a wasted exercise as the ai blows clean over the boundary layer doing nothing at all. You'd think it was close enough like the cyclonic wind off the fan would disrupt it anyway or at least partially. It did NOTHING. You need a third of the fan blowing on the side of the primary but believe me this works Wonderfully.

Good luck!!!!

Pete


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5600730 - 01/02/13 04:46 AM

Thanks for the tips. Did you ever try it without the holes on the opposite side of the tube having the rear fan also blowing up the tube?

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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5600816 - 01/02/13 07:54 AM

Quote:

So, I just discovered that the bigger the cell, the more current it holds. Those big "D" cells would be a lot better than the 8 "AA" batteries I am using! I'm thinking that a Lithium battery that adds up to 12 volts or NiMH would be even better than Alkaline and last even longer.




My experience with NiMH batteries is that they do not last very long. I have various red lights and other accessories which use AAA or AA NiMH batteries. I need to recharge them nearly everytime before I go to my dark site. The old-fashioned D alkaline batteries last much, much longer, at least in the 8-pack power supply for fans.

At one time I shopped around for rechargeable D batteries. IIRC, they are rather expensive and most chargers don't accommodate D batteries. But the old-style D's are cheap and last very long, a year or longer in my experience. So what's the point of getting NiMH's? I don't go "green" when it don't make sense.


Mike


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Re: Thermal Issues and Fans Successes new [Re: Tom and Beth]
      #5600829 - 01/02/13 08:09 AM

Quote:

Quote:

So, I just discovered that the bigger the cell, the more current it holds. Those big "D" cells would be a lot better than the 8 "AA" batteries I am using! I'm thinking that a Lithium battery that adds up to 12 volts or NiMH would be even better than Alkaline and last even longer.



I didn't read the whole thread, so if these are covered already, forgive me

You could put a rechargeable 12V battery in the rocker. 7AH to 18AH, for example, doesn't take up much space. the 7AH size makes a good counterweight.




I have a large capacity 12v battery that I use exclusively for dew control. Dew is huge in my area. The 12v battery needs to be recharged after every night at the dark site. I keep this battery along with the dew controller in a metal basket I hang from the front of the Dob mount. (No rocker box here.)

A separate power supply - the Orion 8-pack of D cells - powers the fan. I need to replace a D cell about every year or so. This power supply is in a little case that hangs from a side of the Dob mount.

This dual setup works great for me. I've never had either of these battery supplies fail me over the course of a five or six hour observing session.

Mike


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Re: Thermal Issues and Fans Successes new [Re: Tom and Beth]
      #5600831 - 01/02/13 08:11 AM

Quote:

As for the fan speed control: I control the speed using a potentiometer within reach of the Ep. Since I built it, I see these units meant for computers where the entire circuit board is already built, and cost 5 bucks.




I've never had any speed control for fans. Where can I order one of these circuit boards and how would I set it up?


Mike


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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5600834 - 01/02/13 08:18 AM

Pete,

Quote:

Just make sure a full 1" of the three inch fan is BELOW the mirrors reflected surface. Essentially so the bottom 1" of the fan is blowing the the side of the mirror. This forces the remaining two inches above the mirrors surface to powerfly blow the boundary layer off. If you have the tip of the 3" hole just above the mirrors surface it's a wasted exercise as the ai blows clean over the boundary layer doing nothing at all. You'd think it was close enough like the cyclonic wind off the fan would disrupt it anyway or at least partially. It did NOTHING. You need a third of the fan blowing on the side of the primary but believe me this works Wonderfully.




Placing a third of the fan below the edge of the mirror would also help cool the mirror as well as disrupt the boundary layer. I like it when improvements perform more than one function.

But here's a question from an experienced amateur astronomer but a newbie handyman craftsman: How did you cut the 3" hole in the side of the OTA? I don't understand why no one has asked this. Do most folks have a lot of prior experience cutting big holes in the side of metal tubes? I don't get this... I never had metal shop in high school.

Mike


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5601031 - 01/02/13 10:59 AM

Mike,
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.


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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5601052 - 01/02/13 11:05 AM

Quote:

Mike,
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.



Be sure to get one that cuts metal as most are designed for cutting wood


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5601115 - 01/02/13 11:44 AM

Don,

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.




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?

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5601117 - 01/02/13 11:44 AM

Thanks, Nevy, you answered my second question.


Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5601221 - 01/02/13 01:01 PM

Quote:

Thanks for the tips. Did you ever try it without the holes on the opposite side of the tube having the rear fan also blowing up the tube?




Alan Adler did and it was not as effective ad the heat hits the wall of the tube on the other side and basically goes all over the place. It needs a short direct path immediately out.

Pete


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5601230 - 01/02/13 01:05 PM

It would probably be better to have the vent holes at the top edge of the primary without the opposing fan, than to have the fan without the holes. At least the holes will allow some of the warm air from the primary to escape from the OTA without going all the way up the tube and further disturbing the image. Having all the warm air eventually vent out the top end of the OTA - rather than out a quicker route - is probably not a good thing if you can avoid it. In fact, I'm certain that it is bad.

Mike


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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5601246 - 01/02/13 01:16 PM

Has anyone else drilled vent holes at the primary without using an opposing fan? I really ought to take my 8" Dob out at the next opportunity to see the effects that it has on the image.

Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5601308 - 01/02/13 02:03 PM

I have for the time the fan was adjusted wrong it was turned off. The holes in and of themselves didn't seem to do anything though I'm sure some good was had.

Pete


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5601330 - 01/02/13 02:17 PM

I thought the vent holes would have some benefit, if nothing else than to more quickly acclimate the primary and possibly diminish tube thermals. They probably are not enough to substantially remove the boundary layer.

I keep thinking of those vent holes along the correcting lenses of SCTs and Maks. They must be doing some good.

Mike


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5601415 - 01/02/13 03:13 PM

I don't think it would be worthwhile to drill small holes as it would still take very long. Just run a fan at the back of the mirror, that should be fine. I've been doing that for years with good results.

Sure, the boundary layer is the culprit, I just run a computer fan on the back of my mirror for an hour or two and I am good to go. I know I won't be drilling holes all over my scope tube seeking perfection.

You'd be hard pressed unless you have a perfect sky to really notice anyways and you're in excellent skies all of the time. I know in my neck of the woods the skies are rarely perfect, or close to it. I'll stick with my fan on the back of my mirror, thanks.

Whoever is the first to do it here, let me know , LOL!


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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5601431 - 01/02/13 03:29 PM

Quote:

I don't think it would be worthwhile to drill small holes as it would still take very long. Just run a fan at the back of the mirror, that should be fine. I've been doing that for years with good results.

Sure, the boundary layer is the culprit, I just run a computer fan on the back of my mirror for an hour or two and I am good to go. I know I won't be drilling holes all over my scope tube seeking perfection.

You'd be hard pressed unless you have a perfect sky to really notice anyways and you're in excellent skies all of the time. I know in my neck of the woods the skies are rarely perfect, or close to it. I'll stick with my fan on the back of my mirror, thanks.

Whoever is the first to do it here, let me know , LOL!



From the articles I have read the big benifit of the side mounted fan is it clears up the views almost immeadiately upon turning the fan on even before the mirror has cooled.


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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5601533 - 01/02/13 04:23 PM

Quote:


From the articles I have read the big benifit of the side mounted fan is it clears up the views almost immeadiately upon turning the fan on even before the mirror has cooled.




I can attest to that. Scrubbing the boundary layer has an immediate effect. Leaving low vibration fans on at 1/2 speed or lower during an observing session, prevents the layer from forming at all while the mirror is still in cool-down.


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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5601554 - 01/02/13 04:33 PM

Quote:

Has anyone else drilled vent holes at the primary without using an opposing fan?



Several years ago I tried drilling a ring of vent holes even with the mirror surface, without using a fan. It did not seem to have any significant effect. This was a 10" mirror in a solid tube.

If you want to get rid of the boundary layer, I believe it is best to have the mirror totally out in the open, as in my post Naked Mirrors.


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Re: Thermal Issues and Fans Successes new [Re: rockethead26]
      #5601606 - 01/02/13 05:09 PM

Quote:

Quote:


From the articles I have read the big benifit of the side mounted fan is it clears up the views almost immeadiately upon turning the fan on even before the mirror has cooled.




I can attest to that. Scrubbing the boundary layer has an immediate effect. Leaving low vibration fans on at 1/2 speed or lower during an observing session, prevents the layer from forming at all while the mirror is still in cool-down.




This is what makes it a winner for me as I don't usually have an hour or two the let the mirror fully cool. I get home from work at 1am and if the skies are good I like to grab n go and not wait over an hour to get good high power views. I'm going to try a few things before I do the side mounted fan though.

Edited by rlmxracer (01/02/13 05:28 PM)


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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5601649 - 01/02/13 05:46 PM

Quote:

I don't think it would be worthwhile to drill small holes as it would still take very long. Just run a fan at the back of the mirror, that should be fine. I've been doing that for years with good results.

Sure, the boundary layer is the culprit, I just run a computer fan on the back of my mirror for an hour or two and I am good to go. I know I won't be drilling holes all over my scope tube seeking perfection.

You'd be hard pressed unless you have a perfect sky to really notice anyways and you're in excellent skies all of the time. I know in my neck of the woods the skies are rarely perfect, or close to it. I'll stick with my fan on the back of my mirror, thanks.

Whoever is the first to do it here, let me know , LOL!




Hi Markus,

Unfortunastely I disagree with that entire post - mutual respects here, but the results of a boundary fan while less than the rear fan is a very real and measurable improvement even in the rotten seeing/transparency lastnight that started at 5 then went way south. Infact because the thermals removed are within the scope it self, itd be measurable even in 1/10 seeing. Too its not a matter of drilling holes all over the tube, its a fairly resttricted lower end affair and frankly it looks better with the mods.

The boundary fan might not be for everyone. If the observer is enjoying typicaly low power wide field deepsky on the order o 15x per inch or less, its not evgen worth it. At typical planetary magnifications of 25x per inch its another story. The enlargement is enough at that point that the gallilean moons for example showed reduced flare after a minute or so of turning the boundary fan back on. Normally it should be virtually instantaneous the moment the fan blows off the layer, but the heat accumulates the moment its off and needs to be removed - too the surface of the mirror chills back down to ambient -or wonderfully close to it.

You dont HAVE to have a boundary layer fan and if it were impossible to have Id begrudgingly get on without it, but its a simple thing to install afterall and again... medium and especially high power truly favor its use.

If you had one installed today Id say look at the moons of jupiter at 250x. Get used to how they appear without the fan scrubbing the boundary layer - then turn it on, keep the image of what youve been seeing on Io for example, give it a couple minutes and then look.

another nice thing - those AWFUL bubbling thermal blobs in the out of focus stars and such are history. The ones that form like dark shadowy amoebas and fade then reform anew. Gone.

Lol its a beautiful thing Marcus, but look you have a rear fan already so most of the benefit is realized. still...

Pete


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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5601660 - 01/02/13 05:53 PM

Quote:

Quote:

Quote:


From the articles I have read the big benifit of the side mounted fan is it clears up the views almost immeadiately upon turning the fan on even before the mirror has cooled.




I can attest to that. Scrubbing the boundary layer has an immediate effect. Leaving low vibration fans on at 1/2 speed or lower during an observing session, prevents the layer from forming at all while the mirror is still in cool-down.




This is what makes it a winner for me as I don't usually have an hour or two the let the mirror fully cool. I get home from work at 1am and if the skies are good I like to grab n go and not wait over an hour to get good high power views. I'm going to try a few things before I do the side mounted fan though.




It shouldnt require more than 60 minutes cool time and if i wanted to stir the pot id bet i could have my full thickness 8" mirror equalized in 40 minutes. Its a guess but its getting win on both sides now so. At anyrate, with the rear fan i NEVER needed over an hour for cool down and Im talking from 70F to 30F or less. Brother, if it took that long... I dont know. Ive heard the horror stories of maks needing 2 to 2.5 hours to chill. I havent go the time or the patience.

Id be extraudinarly surpised if you needed a penny over 60 minutes.

Pete


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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5601675 - 01/02/13 06:01 PM

Quote:

Quote:

Quote:

Quote:


From the articles I have read the big benifit of the side mounted fan is it clears up the views almost immeadiately upon turning the fan on even before the mirror has cooled.




I can attest to that. Scrubbing the boundary layer has an immediate effect. Leaving low vibration fans on at 1/2 speed or lower during an observing session, prevents the layer from forming at all while the mirror is still in cool-down.




This is what makes it a winner for me as I don't usually have an hour or two the let the mirror fully cool. I get home from work at 1am and if the skies are good I like to grab n go and not wait over an hour to get good high power views. I'm going to try a few things before I do the side mounted fan though.




It shouldnt require more than 60 minutes cool time and if i wanted to stir the pot id bet i could have my full thickness 8" mirror equalized in 40 minutes. Its a guess but its getting win on both sides now so. At anyrate, with the rear fan i NEVER needed over an hour for cool down and Im talking from 70F to 30F or less. Brother, if it took that long... I dont know. Ive heard the horror stories of maks needing 2 to 2.5 hours to chill. I havent go the time or the patience.

Id be extraudinarly surpised if you needed a penny over 60 minutes.

Pete



I have a similar temperature delta as you 72deg f to 40s f in the winter. So far with the stock 80mm fan and no baffle it seemed to never cool. My last outing I had my prototype rear baffle and it still took about an hour. I'll see what my new 120mm fan will do this weekend.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5601725 - 01/02/13 06:40 PM

Mark,

Quote:

I don't think it would be worthwhile to drill small holes as it would still take very long. Just run a fan at the back of the mirror, that should be fine. I've been doing that for years with good results.




Yes, a fan below the primary is the first thing to do to control thermal issues. But I'm looking to do everything possible to help the problem, as long as it doesn't have an exorbitant price or takes higher DIY skills than I possess.

Quote:

Sure, the boundary layer is the culprit, I just run a computer fan on the back of my mirror for an hour or two and I am good to go. I know I won't be drilling holes all over my scope tube seeking perfection.




It doesn't take very long to drill seven or so holes for venting. And it doesn't take a lot of skill. I did it! Now that 3" hole at the other side will be a bit more difficult. You don't need to drill holes all over the OTA. Just where they'll do some good.

Quote:

You'd be hard pressed unless you have a perfect sky to really notice anyways and you're in excellent skies all of the time. I know in my neck of the woods the skies are rarely perfect, or close to it. I'll stick with my fan on the back of my mirror, thanks.




I'm not in excellent skies. Most of the winter has poor seeing here ... those twinkling stars in a cold sky are not a good sign. Usually we only have really good seeing here in Maryland now and then in the summer and fall.

Quote:

Whoever is the first to do it here, let me know , LOL!




Well, Pete and many others have already done it.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Dick Jacobson]
      #5601735 - 01/02/13 06:49 PM

Dick,

Quote:

Quote:

Has anyone else drilled vent holes at the primary without using an opposing fan?



Several years ago I tried drilling a ring of vent holes even with the mirror surface, without using a fan. It did not seem to have any significant effect. This was a 10" mirror in a solid tube.

If you want to get rid of the boundary layer, I believe it is best to have the mirror totally out in the open, as in my post Naked Mirrors.




Well, I'm not doing anything like that to my 10" solid-tube Dob! As mentioned in the thread, this would also leave the mirror exposed to dewing. The dew in my area is substantial. I need to have dew strips at eyepiece, the finder eyepiece and my Telrad or they will dew quickly. I can't imagine having a completely exposed mirror in this neck of the woods. Not gonna' happen' here! I'd have to completely encircle the primary with a dew strip. What would that do to the thermal issues?

I think I'll continue to experiment with other solutions, including venting and scrubbing the boundary layer with a fan.


Mike


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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5601764 - 01/02/13 07:07 PM

Pete,

The boundary layer fan is something I definitely want to do, and I've wanted to do it for sometime, first in my 8", then in my 10" Dob. The only thing that's stopping me is my lack of experience and confidence rigging up electric devices and using power tools to cut big holes in metal tubes. The theory is easy. The execution ... maybe not so much.

First, where do you get the potentiometers to control the speed of the fans and how do you set them up? Second, where do you get the rubber to mount the fans in and exactly how did you go about working with it? Maybe these are obvious things to most, but to me ... again, not so much.

Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5601805 - 01/02/13 07:32 PM

Lol Mike,

Hell if I know! I just googled potentiameters and got a schematic image that looked kinda right, wired it by twisting copper. When it worked I went and soldered it and sealed with shrink tubing! Works tho by golly!

Same thing with the dew heater and led light for my finder - googled schematics and such online! Wiring a pot is simple but a little peculiar to me as I'm not an electrician at all. My crowning achievement to date is repairing/replacing blown capacitors on my flat screen.

Pete

Edited by azure1961p (01/02/13 07:33 PM)


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5601833 - 01/02/13 07:48 PM

Wiring is kind of my forte as I grew up racing electric R/C cars and the graduated to installing car audio. Try this link http://www.allaboutcircuits.com/vol_6/chpt_3/7.html
All the stuff you need can be found at your local Radio Shack. You can get one of their small project boxes to install the potentiometer into. Check out my photos of a remote focuser I wired up for my refractor. In the members galleries just put my screen name in the search box. Rob


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5601835 - 01/02/13 07:49 PM

I was thinking of going to a Radio Shack and seeing what they have there in the way of pots (that's what the wireheads call'em!) and free advice. Parallel, series, PIE ... I don't remember much from those electronics kits I put together back in the '70's. But I was able to listen to Russian on that short wave I built.

Soldering I've done. Sealing with shrink tubing? I've seen that stuff but never used it. Don't know how to use it.

I hate climbing the learning curve ...


Mike


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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5601853 - 01/02/13 07:58 PM

rlmxracer,

Thanks for the link and your advice. This boundary-layer fan is something I definitely want to do.

Mike


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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5601876 - 01/02/13 08:14 PM

Quote:

rlmxracer,

Thanks for the link and your advice. This boundary-layer fan is something I definitely want to do.

Mike



No problem feel free to pm me with any wiring ?s.


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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5601922 - 01/02/13 08:38 PM

I'm giving the thumbs down on RadioShack. They used to be good. A dedicated electronic component supplier would be far better. I found a great place locally but had I not it'd been an online purchase.

Pete


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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5601928 - 01/02/13 08:41 PM

Oh shrink tubing: black rubber tubi g that shrinks around wire insulating it when a heat gun blows hot air on it. A hair dryer will do in a pinch if your diligent. Oh... Advice at RadioShack???? Forget it. At least locally these guys didn't even kno what the hell a potentiometer was!!!! I'm serous. They sell cell phones and at screen cables, GPS and assorted junk these days.

Pete


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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5601944 - 01/02/13 08:51 PM

I'd have to concur RadioShack has been slipping for quite a while. I have ordered from Mouser electronics with good luck. They seem to have everything.

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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5602252 - 01/03/13 01:06 AM Attachment (18 downloads)

I recall reading an extensive article by Brian Greer and it had videos attached showing that the boundary layer will hug the face of the mirror ALL NIGHT....

http://www.fpi-protostar.com/bgreer/fanselect.htm

Notice in the article link I just posted, front fans are needed on mirrors that are THICKER and it also depends on your area as to how fast temperatures change. As I stated, my mirror is just over an inch thick, so I think the rear fan I have with the baffle is sufficient.

One thing I'd like to mention is that I am using speaker wire for the connections....would that have any effect on battery life? or does it not matter? I also need batteries that last longer and I think I will go for 8 "D" cell batteries instead of using the 8 "AA" batteries I was using.

Cheers,


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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602299 - 01/03/13 02:17 AM

Quote:

I recall reading an extensive article by Brian Greer and it had videos attached showing that the boundary layer will hug the face of the mirror ALL NIGHT....

http://www.fpi-protostar.com/bgreer/fanselect.htm

Notice in the article link I just posted, front fans are needed on mirrors that are THICKER and it also depends on your area as to how fast temperatures change. As I stated, my mirror is just over an inch thick, so I think the rear fan I have with the baffle is sufficient.

One thing I'd like to mention is that I am using speaker wire for the connections....would that have any effect on battery life? or does it not matter? I also need batteries that last longer and I think I will go for 8 "D" cell batteries instead of using the 8 "AA" batteries I was using.

Cheers,



The speaker wire is fine. These fans only draw .1 amp or so. The 120mm Silenex fan I just got only draws .09amp. Mouser electronics has several variations of 8 D cell battery holders.

Edited by rlmxracer (01/03/13 02:22 AM)


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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5602385 - 01/03/13 05:41 AM

I'm not saying you can't get a decent image, but over coming a 40 degree differential with continuing dropping temps in 40 minutes? I'm not sure that's even possible.

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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: demiles]
      #5602424 - 01/03/13 06:57 AM

Yea,

I just talked to a good friend on the phone and I'm going to switch to 8 D cell batteries and run my fan on those.....Should last a lot longer than 2 or 3 nights of observing, LOL!

If you guys wanna drill holes in your telescope tubes, or add boundary layer fans for a 1" mirror, all the power to you.....but for a 1" mirror it is not needed. I'm sticking with what I currently have now because I am getting excellent views the way things are.

Cheers,


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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602434 - 01/03/13 07:04 AM

Quote:

Yea,

I just talked to a good friend on the phone and I'm going to switch to 8 D cell batteries and run my fan on those.....Should last a lot longer than 2 or 3 nights of observing, LOL!

If you guys wanna drill holes in your telescope tubes for a 1" mirror, all the power to you.....but for a 1" mirror it is not needed.

Cheers,




Markus, I hate to challenge folks but how did you arrive at such a firm conclusion about 1" thick mirrors? A friend recently had Zambuto make him a 1" thick 10" f/6 mirror that he placed in an older Starmaster structure. He drilled 3 large holes in his mirror box just above the primary mirror with a fan blowing from behind the mirror and the views we had through it down at Chiefland, Florida a few months ago were some of the finest I have ever seen. I was amazed at the contrast and the rock steady views we were getting with his modified setup. Bob


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: demiles]
      #5602555 - 01/03/13 08:56 AM

Quote:

I'm not saying you can't get a decent image, but over coming a 40 degree differential with continuing dropping temps in 40 minutes? I'm not sure that's even possible.




Maybe with a big mirror its problematic but I routinely have drops in temp from inside and out down to ten degrees from 70f - after an hour cool down its good to go. Kinda simple really but my mirrors just 8" .

Pete


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5602557 - 01/03/13 08:56 AM

Quote:

Markus, I hate to challenge folks but how did you arrive at such a firm conclusion about 1" thick mirrors? A friend recently had Zambuto make him a 1" thick 10" f/6 mirror that he placed in an older Starmaster structure. He drilled 3 large holes in his mirror box just above the primary mirror with a fan blowing from behind the mirror and the views we had through it down at Chiefland, Florida a few months ago were some of the finest I have ever seen. I was amazed at the contrast and the rock steady views we were getting with his modified setup. Bob




http://www.fpi-protostar.com/bgreer/fanselect.htm

Good for you Bob, I think that's awesome. I have had rock steady views with no fan and just the back of the mirror exposed. Scroll 3/4 of the way down and you can read it for yourself, (Front-blowing fans). I guess I was referring to front boundary layer fan. Brian Greer says that a fan on the back of a mirror an inch thick, (or just over that as my mirror is 1 1/8"), is sufficient.

Cheers,


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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5602570 - 01/03/13 09:08 AM

I had a Parks 6 in. F8 and I would say it's almost impossible to over come that kind of differential in such a short time. Glass will only dissipate heat so fast, even with fans running.

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602573 - 01/03/13 09:13 AM

Well that's the difference between reading about someone else's experience and seeing for yourself. The boundary layer fan isn't needed if you're "scanning for comets" but dust storms on mars for example is another matter. The idea of running no fan at all even if the aperture is 4" is just terrible. Again if all your looking at are low power views of fuzzy comets u can get away with it.

It's not a debating point. Either you get rid of your thermals and see more detail or you don't and u see less. It's one or the other. If you use primarily low power you don't need it but your stars are still going to flare.

Pete

Edited by azure1961p (01/03/13 09:14 AM)


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Re: Thermal Issues and Fans Successes new [Re: demiles]
      #5602579 - 01/03/13 09:17 AM

Quote:

I had a Parks 6 in. F8 and I would say it's almost impossible to over come that kind of differential in such a short time. Glass will only dissipate heat so fast, even with fans running.




Ill post pics of my setup maybe I've overlooked something. The heat never truly leaves the tube 100% but what residual percent is left does a nifty a nifty job of keeping my secondary dew free.

Pete


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Bob S.
Carpal Tunnel
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602589 - 01/03/13 09:25 AM

Quote:

Quote:

Markus, I hate to challenge folks but how did you arrive at such a firm conclusion about 1" thick mirrors? A friend recently had Zambuto make him a 1" thick 10" f/6 mirror that he placed in an older Starmaster structure. He drilled 3 large holes in his mirror box just above the primary mirror with a fan blowing from behind the mirror and the views we had through it down at Chiefland, Florida a few months ago were some of the finest I have ever seen. I was amazed at the contrast and the rock steady views we were getting with his modified setup. Bob




http://www.fpi-protostar.com/bgreer/fanselect.htm

Good for you Bob, I think that's awesome. I have had rock steady views with no fan and just the back of the mirror exposed. Scroll 3/4 of the way down and you can read it for yourself, (Front-blowing fans). I guess I was referring to front boundary layer fan. Brian Greer says that a fan on the back of a mirror an inch thick, (or just over that as my mirror is 1 1/8"), is sufficient.

Cheers,




Markus, As a point of reference, I have owned more non-fan Starmasters than anybody in the country/world and was not a big believer in fans. I had tried a fan strategy from a great guy out in California that was making elegant fan retrokits for Starmasters and unfortunately did not see any clear evidence from that approach to improved views at the time. However, when I saw how Joe Wambo's 32" f/3.7 scope performed in the Florida Keys, I was totally blown away. He attributes his incredible views to the additive benefits of a frontal surface blowing fan. We saw more detail on Mars than any of us experienced observers had ever seen at 950x. I then had a conversation with Jimmy Lowery of Texas with his 48" f/4 and he says that he never views without running his 10 MagLev fans in the enclosed mirror box and says it really cleans up his DSO work.

I know that we are comparing watermellons to grapes with the sizes you are mentioning but my preliminary tests on my 20" f/3 Lockwood/JP Astrocraft with front blowing and rear sucking fans seems to be bearing incredible fruit in terms of resolving abilities. I have read Brian's missive on mirrors that was recently put up and his early work with cooling and until recently, was not a convert. However, after seeing and hearing such glowing reports, I decided to create with John Pratte what we call a Comprehensive Boundary Layer Mitigation System (CBLMS). The complete system seems to be additive. However, have not had very many nights with the new system and will need many months of testing before I can sort out and try and control all the variables I am interested in.

I am delighted that this subject about boundary layers is resurfacing. It may prove to have more benefits than we may have realized? Cannot really say for sure at this point?

Bob

Edited by Bob S. (01/03/13 09:34 AM)


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Dick Jacobson
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5602598 - 01/03/13 09:32 AM

Quote:

Dick,

Quote:

Quote:

Has anyone else drilled vent holes at the primary without using an opposing fan?



Several years ago I tried drilling a ring of vent holes even with the mirror surface, without using a fan. It did not seem to have any significant effect. This was a 10" mirror in a solid tube.

If you want to get rid of the boundary layer, I believe it is best to have the mirror totally out in the open, as in my post Naked Mirrors.




Well, I'm not doing anything like that to my 10" solid-tube Dob! As mentioned in the thread, this would also leave the mirror exposed to dewing. The dew in my area is substantial. I need to have dew strips at eyepiece, the finder eyepiece and my Telrad or they will dew quickly. I can't imagine having a completely exposed mirror in this neck of the woods. Not gonna' happen' here! I'd have to completely encircle the primary with a dew strip. What would that do to the thermal issues?

I think I'll continue to experiment with other solutions, including venting and scrubbing the boundary layer with a fan.


Mike



I have not had any problems with dew on my "naked mirror" even though I have a lot of dew/frost problems on eyepieces and secondary mirrors. The difference is that the primary mirror is extremely thick (2-5/16" in my case) and it is nearly impossible for it to cool down below the dewpoint.

The mirror seems to give better images when it is out in the open than it ever did when it was in a tube. With the front fan off, I can still sometimes see a turbulent boundary layer when looking at a bright defocused star, so I think a boundary-layer fan is needed when your mirror is more than a few degrees warmer than the air.


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JasonBurry
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5602602 - 01/03/13 09:36 AM

I added a boundary layer fan and speed control, along with secondary dew heating to my 8" truss dob this fall. I've been observing with this instrument for the best part of 10 years.

The difference in performance was immediate and obvious.

My setup is simple. My mirror (8", F6, about 1.25" thick) is passively cooled on the back side, no fan there (yet). Just some mighty holes in the 3 point mirror cell to speed things a touch. I may add a small fan to improve slightly, but right now it's happily loosing 20C in about 1.5 hours.

My fan is for boundary layer control. It's an old computer "ISA slot cooler", a thin squirrelcage style blower. It's fed thru a POT from a 2000mah 8.4V NiMH pack mounted in the mirror cell. The pack also powers the dew heater and the red dot finder, giving about 12 hours endurance for the fan at full speed, or 6 hours with both fan and heat (though I only use the heat in "defog" mode).

I created a plastic louvre on the fan's outlet to distribute the flow across the mirror face. Observing the clearing of condensation from the mirror with the fan running after a session allowed me to tweak the louvre to optimize the airflow as best I could. The pot allows the fan to run down to double-digit RPM's, and I usually run it at about 1/3rd speed during observing. It's velcro mounted to the mirror box, and causes no observable vibration.







The fan installation really helped in seeing fine detail in Jupiter, festoons and detail galore, indeed views I'd thougth impossible with this scope. The Jovian moons have become miniscule worlds unto themselves. Fine galactic detail has improved. Now if only my skies would.... LOL.

J


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5602604 - 01/03/13 09:38 AM

Pete,

Quote:

Well that's the difference between reading about someone else's experience and seeing for yourself. The boundary layer fan isn't needed if you're "scanning for comets" but dust storms on mars for example is another matter. The idea of running no fan at all even if the aperture is 4" is just terrible. Again if all your looking at are low power views of fuzzy comets u can get away with it.

It's not a debating point. Either you get rid of your thermals and see more detail or you don't and u see less. It's one or the other. If you use primarily low power you don't need it but your stars are still going to flare.




I have a 6" f/5 Dob, 5" f/5 Dob, a 4.5" f/4.4 Ball Scope, and a 3" f/4 Hand-Held Newt. I haven't bothered to put fans in any of them yet. But I mostly use them for deep sky or grab-n-go. Sometimes the 5" goes with me to my dark site instead of the 10" when I bring the family and want more room in the car. It does fine without a fan, but then I'm not trying to tease out fine detail on bright planets at the dark site.

On the other hand, I do take out that 5" at my house for lunar observation. It might be a good idea to rig a fan up to the bottom of the primary.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602620 - 01/03/13 09:48 AM

Mark,

Quote:

I just talked to a good friend on the phone and I'm going to switch to 8 D cell batteries and run my fan on those.....Should last a lot longer than 2 or 3 nights of observing, LOL!




Yep, you should be able to go about a year or so without having to replace any of the D cells. Don't even bother getting rechargeable D's.

I have a hand-held battery charge checker that I check all my batteries with before I go to my dark site. It's a bother to take the eight D cells out and check them, because I know that they'll seldom show a low charge. I take extra D cells with me just in case, too, but I don't think I've ever had to replace any of the D's in the field.

Mike


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5602641 - 01/03/13 10:00 AM

Download this free calculator:
http://www.cruxis.com/scope/mirrorcooling.htm
You can enter the data for your mirror and fans and see how effective active cooling is.
With the fans in my scope, and my 1.25" thick 12.5" mirror, I can overcome a 40 degree differential in temperature in around 30 minutes (mirror is then within 1 degree C of ambient).
So, fast cooling of glass is possible, but only with high capacity fans doing the job.
If you play with the calculator, you'll see that normal radiative cooling isn't adequate for any mirror in a falling temperature environment.


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Scanning4Comets
Markus
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5602706 - 01/03/13 10:41 AM

Quote:

Well that's the difference between reading about someone else's experience and seeing for yourself. The boundary layer fan isn't needed if you're "scanning for comets" but dust storms on mars for example is another matter. The idea of running no fan at all even if the aperture is 4" is just terrible. Again if all your looking at are low power views of fuzzy comets u can get away with it.

It's not a debating point. Either you get rid of your thermals and see more detail or you don't and u see less. It's one or the other. If you use primarily low power you don't need it but your stars are still going to flare.

Pete




I have to respectfully disagree here Pete. I am talking from experience here. My last quote was just showing something about the boundary layer fan in an article.

I have been observing since I was 19 and I am 50 now. Back when I got my first 10" F/5.6 plate glass mirror telescope I had incredible views of Jupiter and other planets as well. I had no fans on the back, nothing. It's not a matter of "if you're just scanning for comets" here either. I have had my telescope up to 488x and the views were excellent, which is pretty rare in my neck of the woods. It IS a debating point and you haven't seen what I have seen in my telescope as you were not here when I saw what I did.

I'm not saying boundary layer fans or fans at the back of telescopes is useless at all because I have a large 12cm fan on the back of the mirror now. All I was saying was that I have seen incredibly sharp detail on the planets and had my scope up at relatively high powers and I do NOT have a boundary layer fan.

It appears that going with a boundary layer fan even with my 10" scope and smaller is the way to go from what I just read....so now I will be looking into making that happen on my scope seeing as testimonials outweigh not having a boundary layer fan.

This should prove interesting below...

http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/342304/page/...



Cheers,


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602755 - 01/03/13 11:11 AM

Quote:

Quote:

Well that's the difference between reading about someone else's experience and seeing for yourself. The boundary layer fan isn't needed if you're "scanning for comets" but dust storms on mars for example is another matter. The idea of running no fan at all even if the aperture is 4" is just terrible. Again if all your looking at are low power views of fuzzy comets u can get away with it.

It's not a debating point. Either you get rid of your thermals and see more detail or you don't and u see less. It's one or the other. If you use primarily low power you don't need it but your stars are still going to flare.

Pete




I have to respectfully disagree here Pete. I am talking from experience here. My last quote was just showing something about the boundary layer fan in an article.

I have been observing since I was 19 and I am 50 now. Back when I got my first 10" F/5.6 plate glass mirror telescope I had incredible views of Jupiter and other planets as well. I had no fans on the back, nothing. It's not a matter of "if you're just scanning for comets" here either. I have had my telescope up to 488x and the views were excellent, which is pretty rare in my neck of the woods. It IS a debating point and you haven't seen what I have seen in my telescope as you were not here when I saw what I did.

I'm not saying boundary layer fans or fans at the back of telescopes is useless at all because I have a large 12cm fan on the back of the mirror now. All I was saying was that I have seen incredibly sharp detail on the planets and had my scope up at relatively high powers and I do NOT have a boundary layer fan.

It appears that going with a boundary layer fan even with my 10" scope and smaller is the way to go from what I just read....so now I will be looking into making that happen on my scope seeing as testimonials outweigh not having a boundary layer fan.

Where would be the best place to put it on my solid tube dob? The bottom? or the sides?

Cheers,




+1 on what Markus said. I too have spent most of the past decade not using fans and in my steady Florida skies, have gotten incredible views from Newts from 6" to 28". However, there appears to be a small body of evidence to suggest that fans properly placed may have some beneficial results that might not be comparable to a non-fan situation?

I personally maintain some level of skepticism about fans and I have recently made a rather sizeable investment in a fan system on a new premium Newtonian telescope. Only time will tell me how effective my intended strategy is?
Bob


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602761 - 01/03/13 11:17 AM

A fellow out here in CA decided to optimize his full-tubed reflector for planetary observing.
He did the usual things:
--lots of clearance between mirror and tube so tube currents would be out of the light path
--tube extended a foot or so above the focuser
--flocking
--large rear fan
etc.

He showed me, actually proved to me, that all you need is a rear fan to make the images great. And that the mirror doesn't have to be at ambient temperature to give good views as long as the heat is blown away smoothly.

We were looking at Jupiter (altitude about 50-60 degrees at the time) and the image was mush. Soft and scintillating. The out-of-focus image was filled with what looked like worms and plumes and feathers of heat.

He turned the fan on and within a couple seconds, while I watched, the image got sharp and contrasty and tons of small details all over the planet became visible.

He then turned off the fan and within a few seconds the image was mush again. When the fan was turned on again, the image resumed its incredible sharpness in only a few seconds.

The back of the tube was not closed off. There were no boundary layer fans.

The fan created a positive pressure at the back that blew air up the tube, blowing the heat out of the tube in a fairly laminar manner (due to the clearance between the mirror and tube wall. It kept any body heat from drifting up in front of the tube. You could stand at the front of the tube and feel a breeze on your face.

The difference between fan and no fan was simply stunning. It convinced me a rear fan CAN pull the boundary layer off the face of the mirror with a simple pressure differential between the edge of the tube and the edge of the mirror.

A couple years later, I saw this same test performed with a 6" f/8 and the effects were the same.

It's not just heat removal that fans provide--it's also the ability to dispose of the boundary layer in front of the mirror. And while side fans blowing on the mirror certainly do all this a lot faster, they weren't necessary on his 12.5" scope.

Commercial scopes are rarely built with as large a clearance between mirror and tube, so the effects of air flow *may* be more visible in the image. Difficult to say. But I see a positive effect from just a rear fan on my truss-tubed 12.5", so I let it run all night. Side fans can cool a mirror quickly, but, other than an occasional blast, after the mirror is cooled are probably not necessary. But that rear fan certainly is.

IME, that is.


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5602780 - 01/03/13 11:26 AM

Quote:

Mark,

Quote:

I just talked to a good friend on the phone and I'm going to switch to 8 D cell batteries and run my fan on those.....Should last a lot longer than 2 or 3 nights of observing, LOL!




Yep, you should be able to go about a year or so without having to replace any of the D cells. Don't even bother getting rechargeable D's.

I have a hand-held battery charge checker that I check all my batteries with before I go to my dark site. It's a bother to take the eight D cells out and check them, because I know that they'll seldom show a low charge. I take extra D cells with me just in case, too, but I don't think I've ever had to replace any of the D's in the field.

Mike




How long the D cells will last really depends on how often they are used and at what draw.

Typical alkaline D-cells are good for at least 12,000maH. You can get somewhat better if you spend more. Seems rechargeables are much poorer and provide around 5000mah. (Apparently some rechargeable D-cells are actually AA's in a D-cell package.)

If your fan runs at 200ma, then you can get about 60 hours use from a fresh D-cell battery. You get proportionally longer if you can reduce the current by slowing down the fan. You get proportionally less if you run multiple fans. Around here 60 hours observing could take years. It wouldn't be a month in better sites.

If you can slow your fan for most observing, and run it at 100ma then you should expect at least 120hrs of use from good fresh D-cells. Double that if you can get away with only 50ma draw. So yeah, they can last for years.

dan


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5602819 - 01/03/13 11:48 AM

Markus and so I've had tremendous views without too, sub arc second binaries beyond Dawes with unequal magnitude components , detail on Ganymede. It's all there without fans but its even better with. I'm 51 now started in 74 - used my first full time fan a year ago and a boundary later fan a week ago - ill never go back. Ill post some picks of before and after images with different configurations.

Pete

Edited by azure1961p (01/03/13 11:49 AM)


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Scanning4Comets
Markus
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5602863 - 01/03/13 12:04 PM

Detail on Ganymede.

Bionic eyes?


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rguasto
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5602864 - 01/03/13 12:05 PM

There was (and is ) a huge difference in my 8" F8 reflector with a rear fan. The rear of the tube is fully enclosed, with <1/2" space around the cell. The fan is 80mm with a 60mm step-down adapter which is the size of the central rear "vent". A huge improvement was immediately appreciated during star testing. Tube currents are minimal if any at all times. Cool down is about 20 minutes. The fan runs constantly. I could never get the quality of images I do now without the fan.

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demiles
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5602938 - 01/03/13 12:44 PM

Don, I have that program on my laptop, but the issue with it is its based on one dimension. Thickness. While testing with my Parks 6 in F8 and then with a Discovery10 F6 with a 2 in. thick mirror I couldn't come close to the cool down times that program gave me. I had a fan that I would hang up on the top end of the tube and blow into it for cool down prior to viewing as well as one blowing on the back. The single best thing i've found to do is to move the scope to get close to ambient prior to viewing. For instance I'm going out tonight, so at 5:30AM this morning I put my mirror box outside in my truck, temps here are going to top out at about 32 degrees so by 5:00 PM its gonna be pretty close to 32. Don't get me wrong I run a back and front too and they help but they themselves have never been able to over come high temperature differentials at the eyepiece.If I were to make a rough guestimate I'd say getting within 10 degrees of ambient has worked well for me.

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Quest
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Re: Thermal Issues and Fans Successes new [Re: demiles]
      #5602971 - 01/03/13 01:05 PM

Love this thread! With all the talk about fans blowing air this way and that, I see little, if anything, mentioned about filtration. I don't yet have a fan on my 8" closed tube but I'm concerned about drawing in dirty air - especially during the summer when it doesn't rain often and dirt is easily kicked up by just walking around. If it was an open truss design, I would think the air would adequately dissipate but is there any concern about dust and other particulates accumulating inside a closed tube telescope? I'm a little concerned to install a fan on the back of my telescope without a filter on the intake side and a decent baffle. Perhaps my concerns are unwarranted?

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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: dan_h]
      #5603011 - 01/03/13 01:31 PM

Dan,

Quote:

If your fan runs at 200ma, then you can get about 60 hours use from a fresh D-cell battery. You get proportionally longer if you can reduce the current by slowing down the fan. You get proportionally less if you run multiple fans. Around here 60 hours observing could take years. It wouldn't be a month in better sites.

If you can slow your fan for most observing, and run it at 100ma then you should expect at least 120hrs of use from good fresh D-cells. Double that if you can get away with only 50ma draw. So yeah, they can last for years.




At this time the eight D-cells only run one fan beneath the primary. I run the fan at full speed for the entire time I'm at the dark site; I don't have a potentiometer to vary the speed. At a conservative estimate, I go to the dark site an average of once a month and run the fan for about five hours each time. This would be about 60 hours a year for each battery. As I said, the batteries seem to last at least a year, so this is about right.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5603019 - 01/03/13 01:35 PM

I'm considering taking that three-fan harness I bought from Orion and adapting it to the 10" Dob. I might rig up one fan as the boundary-layer fan, and attach the other two to a baffle beneath the primary.

As long as there is no vibration transmitted to the optics, I'll run them at full speed all night. Maybe I'll have to replace the D cells every four months or so. Big deal. I'll spend much more than that in gas.


Mike


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NHRob
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5603043 - 01/03/13 01:46 PM

The value in using cooling fans is highly dependent on your local temps and region where you observe.
Up here in NH, I often have a delta-T of > 50deg-F, moving the scope from inside to outside. It takes forever for the mirror to approach outside ambient. Without a fan ... forget it. That's not even taking into account the drop in ambient temp during the night.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5603059 - 01/03/13 01:55 PM

Quote:

Detail on Ganymede.

Bionic eyes?




The problem isn't so much catching some albedo features on Ganymede ... it's having to bump up the power and follow that little bugger without tracking!


Mike


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Jeff B
Carpal Tunnel
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5603278 - 01/03/13 03:58 PM

Anybody considered coring out the center of the mirror, attaching a fan on the back and sucking the boundary layer off out through the middle?

Jeff


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Jeff B]
      #5603534 - 01/03/13 07:03 PM

Sure. I'll get me one of them hole borers and knock that out during lunch tomorrow.


Mike


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nevy
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5603599 - 01/03/13 07:46 PM

Do you think if I put a 12 inch fan on the bottom of my 12 inch dob it would cool down quick?:-)

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5603606 - 01/03/13 07:51 PM

Too big!

Vibration hassles. Try several 80mm fans instead.

Pete


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Quest]
      #5603627 - 01/03/13 07:59 PM

Quest,

Quote:

Love this thread! With all the talk about fans blowing air this way and that, I see little, if anything, mentioned about filtration. I don't yet have a fan on my 8" closed tube but I'm concerned about drawing in dirty air - especially during the summer when it doesn't rain often and dirt is easily kicked up by just walking around. If it was an open truss design, I would think the air would adequately dissipate but is there any concern about dust and other particulates accumulating inside a closed tube telescope? I'm a little concerned to install a fan on the back of my telescope without a filter on the intake side and a decent baffle. Perhaps my concerns are unwarranted?




A filter might be a good idea. I was thinking of putting one on my fan. But dust is not a big deal here in Maryland. It's so dewy, any dust settles quickly as mud before it can be sucked into the fan!

Seriously, a good way to keep your area clean is to bring an indoor/outdoor carpet or tarp to put your mount on. I think a nice, black carpet is best to retain your dark adaptation. The bad thing about tarps is that they can be slippery.

I just picked up a couple black carpets with rubber backing I can put together to make about a 6'x8' area under my Dob mount. Cheap, too.

Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5603636 - 01/03/13 08:06 PM

Quote:

Quote:

Quote:

However, there appears to be a small body of evidence to suggest that fans properly placed may have some beneficial results that might not be comparable to a non-fan situation?

Bob




A small body of evidence? How big does it need to be?

Pete

Edited by azure1961p (01/03/13 09:01 PM)


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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5603699 - 01/03/13 09:07 PM

Filtration I think is overkill. You do want a light mesh foam air conditioner type filter to keep from sending Beatles and such from bring shot at the glass but beyond that I don't think it's needed. Chicken wire underneath the ac filter material adds stiffness. You may find a boundary fan actually does a terrific job of blowing dust off that normally would settle without wind interruption. The other thing to consider is children. You don't want little fingers getting wacked. I'm a solo observer but Id rethink the issue of fingers if I were at a public affair.

Pete


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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: Dick Jacobson]
      #5603706 - 01/03/13 09:19 PM

Quote:

so I think a boundary-layer fan is needed when your mirror is more than a few degrees warmer than the air.




Quote:

And that the mirror doesn't have to be at ambient temperature to give good views as long as the heat is blown away smoothly.

Side fans can cool a mirror quickly, but, other than an occasional blast, after the mirror is cooled are probably not necessary. But that rear fan certainly is.




That seems reasonable. However you get rid of that layer the views should sharpen (side, back, top fan. Whatever.) If the layer is gone and stays gone, no need to blow something that is not there. But it should only take a few degrees change in ambient to create a new boundary layer.

However, being stable thermally seems important, too. Some folks report various amounts of correction error as the mirror is settling. They are thermally stable, and that seems to mean they - pretty much - retain their shape while resisting changes in temperature. Right?

In Daniel Mounsey's review above, this seems consistent with Mike and my own belief you help warm air do what it wants to do: rise. Pulling it out seems a good solution since it adds low pressure (to which air wants to move) and exhausts the air. He did mention, about his smoke test, the entire tube was evacuated in 5 seconds. That /should/ result in better seeing - instantly.

And, after viewing the laminar flow problem recently, I can see how the air blow across the primary departs from the mirrors surface. It leaves a turbulent layer below (on the primary's surface, much like an aircraft wing at stall) That turbulent air can remain trapped near the primary. There is likely some improvement in the view, but it might be less efficient in terms of total evacuation.

Detail on Ganymede (and elongation "effect" of Io) are certainly doable in modest apertures...provided collimated in good seeing and, of course, cooled.

Edited by Asbytec (01/03/13 09:33 PM)


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5603714 - 01/03/13 09:26 PM

Quote:

Quote:

Quote:

Quote:

However, there appears to be a small body of evidence to suggest that fans properly placed may have some beneficial results that might not be comparable to a non-fan situation?

Bob




A small body of evidence? How big does it need to be?

Pete




I don't know the answer to that one Pete. If you do, by all means share your wisdom/experience. Results with various cooling strategies have been somewhat equivocal by people in different locales and with different Newts. If they were a sure bet, wouldn't you think that everybody would be using fans and be reporting perfect views? Well, the data is not much clearer than mud at this point, IMO. Maybe we will be able to understand the most important variables? I sure hope so and have set out on a journey of discovery recently to see if a combination of cooling factors results in generally overall improved performance. I frankly cannot predict the outcomes. If I could, I wouldn't be wasting my time here. I would just go buy a Powerball ticket and be done with it


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Tom and Beth
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5603841 - 01/03/13 10:53 PM

Quote:

Quote:

As for the fan speed control: I control the speed using a potentiometer within reach of the Ep. Since I built it, I see these units meant for computers where the entire circuit board is already built, and cost 5 bucks.




I've never had any speed control for fans. Where can I order one of these circuit boards and how would I set it up?


Mike




http://www.newegg.com/Product/Product.aspx?Item=N82E16835118217

This one controls 2 fans.


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: demiles]
      #5603919 - 01/04/13 12:00 AM

Quote:

Don, I have that program on my laptop, but the issue with it is its based on one dimension. Thickness. While testing with my Parks 6 in F/8 and then with a Discovery 10 F/6 with a 2 in. thick mirror I couldn't come close to the cool down times that program gave me. I had a fan that I would hang up on the top end of the tube and blow into it for cool down prior to viewing as well as one blowing on the back. The single best thing I've found to do is to move the scope to get close to ambient prior to viewing. For instance I'm going out tonight, so at 5:30 AM this morning I put my mirror box outside in my truck, temps here are going to top out at about 32 degrees so by 5:00 PM its gonna be pretty close to 32. Don't get me wrong I run a back and front too and they help but they themselves have never been able to over come high temperature differentials at the eyepiece.If I were to make a rough guestimate I'd say getting within 10 degrees of ambient has worked well for me.



The issue isn't the thickness of your mirror, but the program's assumption of linear cool down of the night air when you enter beginning and ending temperatures.
Where I observe, the temperature can drop 30 degrees in the first 2 hours, followed by 1 degree per hour.
The mirror is hopelessly warmer than the ambient temperature until the temperature stops falling rapidly. Though the calculator says it should take only 20 minutes to bring my thin mirror to ambient, it often takes more than an hour to as much as two hours. Fans can only do so much.


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nevy
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Re: Thermal Issues and Fans Successes new [Re: Quest]
      #5603920 - 01/04/13 12:01 AM

Quote:

Love this thread! With all the talk about fans blowing air this way and that, I see little, if anything, mentioned about filtration. I don't yet have a fan on my 8" closed tube but I'm concerned about drawing in dirty air - especially during the summer when it doesn't rain often and dirt is easily kicked up by just walking around. If it was an open truss design, I would think the air would adequately dissipate but is there any concern about dust and other particulates accumulating inside a closed tube telescope? I'm a little concerned to install a fan on the back of my telescope without a filter on the intake side and a decent baffle. Perhaps my concerns are unwarranted?




Won't a filter on the fan decrease the airflow and make the fan motor work harder and also make it slightly noisier?

Edited by nevy (01/04/13 12:03 AM)


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FineArt
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Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5604027 - 01/04/13 02:58 AM

This is the fan.

http://video.canadiantire.ca/liveclicker/fb/?ref=aHR0cDovL3d3dy5jYW5hZGlhbnRp...

Edited by FineArt (01/04/13 03:00 AM)


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Tom and Beth]
      #5604163 - 01/04/13 07:09 AM

Quote:

http://www.newegg.com/Product/Product.aspx?Item=N82E16835118217

This one controls 2 fans.




Nice. Not very expensive either. They could be attached with Velcro to the OTA or Dob mount. I'd rather go with something like this than try rigging up my own pots.

The only concern for me would be finding adapters for the power supply and fans, or cutting and splicing to the leads I already have. Some folks seem to be able to do this in their sleep. I don't have much prior experience, so I have to figure it out step by step.

Mike


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yowser
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5604263 - 01/04/13 08:47 AM

Quote:

...It's not just heat removal that fans provide--it's also the ability to dispose of the boundary layer in front of the mirror. And while side fans blowing on the mirror certainly do all this a lot faster, they weren't necessary on his 12.5" scope.
--Jim

Commercial scopes are rarely built with as large a clearance between mirror and tube, so the effects of air flow *may* be more visible in the image. ...




I'd be interested to know just how much clearance is acceptable between the mirror and tube in relation to air currents. The reason I ask is that I added a rear fan to an 8" f/4 reflector and also flocked the tube with Protostar flockboard, but also placed 1/4" thick ribs between the tube and flockboard to essentially provide a "thermal window pane" affect. But by doing this, it reduced the clearance between the mirror and tube.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5604292 - 01/04/13 08:59 AM

I wonder if I could use my Dew-Not Controller to also vary the speed on fans? This controller has two channels, with two outputs per channel. It is connected to a 12v PowerSonic PS-1270 F2 battery. Now I plug three dew strips into the controller, leaving one output free. If I power a fan - or several fans - on the remaining output, it would share one channel with a dew strip. Not too convenient if I want to vary the fans but not that dew strip.

Dew-Not Dual Channel Controller

Maybe the best gizmo for my projected three-fan setup would be another controller to connect to my eight D-cell power supply. I might be able to get by with one channel one output, since the three fans are on a pre-wired harness. But it'd probably be better to have a separate channel for the boundary-layer fan.

But $119 for a second Dew-Not Controller is more than I want to spend.

Any thoughts?

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5604353 - 01/04/13 09:19 AM

This version is a little less expensive:

AstroZap Dual Channel Controller $98

Mike


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JasonBurry
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5604478 - 01/04/13 10:27 AM

I simply controlled the speed of my fan with a potentiometer inline with the fan. Can't remember the resistance value of the POT, but it was in the 25K range, IIRC. A lower value would be better, but it's what I could scrounge that day. Ideally, the pot would have a watt rating similar or greater than the fan's, but mine doesn't. It has been reliable so far.

J


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Project Galileo
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Re: Thermal Issues and Fans Successes new [Re: JasonBurry]
      #5604610 - 01/04/13 11:38 AM

I sealed up the rear of my LB16...



...added a Mauro Da Lio baffle...



...installed one of these super cool fans as a sucking fan with silicon gaskets and neoprene vibration barriers that doesn't show vibration at any speed or magnification...



...and control fan speed with one of these.



In this picture you can see how hard this fan sucks at 73 cfm. The sides of the shroud are being sucked in against the truss poles with the top cover in place. In the huge space of the LB16's tube it easily turns over the air very rapidly and creates a wonderful laminar flow and very clean image. Just like others have reported it all works almost magically. I don't believe I need a side fan. This single set up works great for the size and thickness of mirror I have. I am now a believer in active cooling and boundary layer removal.



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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: Project Galileo]
      #5604858 - 01/04/13 01:53 PM

Clean looking setup P Galileo. I just made the mistake of looking for the quietest 120mm fan I could find. Well I found out how they got it so quiet... it ony flows 40cfm which is not even close oh well. One good thing I found while removing the stock fan I had to remove my primary. I checked the center spot and it was off center by almost 2mm.
Now I just need a fan that flows more air.


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Bob S.
Carpal Tunnel
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5604935 - 01/04/13 02:38 PM

Very nice installation of that sucking fan on the 16" LB Doc! I think one of the potential advantages to our sucking systems is that we are using a pretty homogeneous body of air taking it in from the top. Someone mentioned to me that mixing different kinds of air for frontal boundary layer fans and rear fans is a potentially less optimal strategy than pulling in the air source for the entire primary mirror from one air mass. Not much data on this but it intuitively makes sense. Bob

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Tom and Beth
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5605114 - 01/04/13 04:09 PM

Quote:

Quote:

http://www.newegg.com/Product/Product.aspx?Item=N82E16835118217

This one controls 2 fans.




Nice. Not very expensive either. They could be attached with Velcro to the OTA or Dob mount. I'd rather go with something like this than try rigging up my own pots.

The only concern for me would be finding adapters for the power supply and fans, or cutting and splicing to the leads I already have. Some folks seem to be able to do this in their sleep. I don't have much prior experience, so I have to figure it out step by step.

Mike




If you're using computer fans, they often have a connector, yes? This speed control (and there's literally dozens of them to choose from) will attach using that connector. All that's left then is wiring to the battery (or power supply) and that's usually a positive and negative lead.

Some of these fan controllers are better than others, and one can find them on Ebay, Amazon, Newegg and the like. One could even buy a 5 way unit and also power little lights, for a tripod, or map reader...all it takes is removing the circuit card from a computer bay adapter and drilling holes in your scope.

I owe all this to somebody here on CN who posted a pic of one of these.


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5605368 - 01/04/13 06:51 PM

Nevy,

By some percentage sure but in all practical purposes no. These filters are highly cavetized - you can see through them. Perhaps that's the difference, if you can't see thru it its too dense. I wouldn't bother with a rear filter but again at public outtings chicken wire across the back would safe guard from kids unknowingly -- "!!!!"

In your own backyard I think it's a personal call - right now my boundary dies t have the ac filter. I might use simple window screening cut out and mounted over the intake. The rear fan - ill never bother covering lest it's an outreach deal and I haven't done one in a good ten years. I miss it actually.

Pete


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Scanning4Comets
Markus
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5605466 - 01/04/13 07:50 PM Attachment (7 downloads)

Quote:

If you're using computer fans, they often have a connector, yes? This speed control (and there's literally dozens of them to choose from) will attach using that connector. All that's left then is wiring to the battery (or power supply) and that's usually a positive and negative lead.

Some of these fan controllers are better than others, and one can find them on Ebay, Amazon, Newegg and the like. One could even buy a 5 way unit and also power little lights, for a tripod, or map reader...all it takes is removing the circuit card from a computer bay adapter and drilling holes in your scope.

I owe all this to somebody here on CN who posted a pic of one of these.




I cannibalized a comp fan with a three way switch on it and have it on the back of my mirror. I used to just have it on elastics over the open mirror, but now have it enclosed.

Note the micro switch for changing between low, medium and high speeds as circled.


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nevy
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5605495 - 01/04/13 08:12 PM

Quote:

Nevy,

By some percentage sure but in all practical purposes no. These filters are highly cavetized - you can see through them. Perhaps that's the difference, if you can't see thru it its too dense. I wouldn't bother with a rear filter but again at public outtings chicken wire across the back would safe guard from kids unknowingly -- "!!!!"

In your own backyard I think it's a personal call - right now my boundary dies t have the ac filter. I might use simple window screening cut out and mounted over the intake. The rear fan - ill never bother covering lest it's an outreach deal and I haven't done one in a good ten years. I miss it actually.

Pete



Ok , thanks for clarifieing that , I wouldn't use one myself as I find the mirror accumulates a bit of dust from normall use after a few sessions anyway even without the fan running , but I have got one of those metal finger guards on the fan , I know from experience that if a finger was to accidentally touch the fan it won't cut my finger tips off as it would just stop the fan from turning but I would be more worried about bending or distorting one of the blades and causing it to vibrate.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Project Galileo]
      #5606629 - 01/05/13 01:26 PM

Project Galileo,

Quote:

...and control fan speed with one of these.






A picture may be worth a thousand words, but sometimes you need more than just seven words to explain a picture.

What is this thing, what type of connectors does it have on both ends, how do you use it, where do you get it?

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5606645 - 01/05/13 01:38 PM

I see this thread has come to the usual controversy between fans that suck and and fans that blow.


Mike


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nevy
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5606663 - 01/05/13 01:49 PM

Quote:

Project Galileo,

Quote:

...and control fan speed with one of these.






A picture may be worth a thousand words, but sometimes you need more than just seven words to explain a picture.

What is this thing, what type of connectors does it have on both ends, how do you use it, where do you get it?

Mike



That looks like it could be very usefull as I have two scopes with fans so it could be used on either scope to mess with the fan speeds , it would save the trouble of fitting pots to both scopes , come on spill the beans where do you get such a device :-).


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Project Galileo]
      #5606666 - 01/05/13 01:50 PM

Project Galileo,

Quote:

...added a Mauro Da Lio baffle...




Here is an interesting quote from that thread:

Vortex

Quote:

In a tube, a rear fan also creates a vortex which scrubs and lifts much of the boundry layer away too.

At least it does in my 10" f/5.




So that vortex I saw in the defocused image through my 8" f/6 Dob might be a good thing? Perhaps it needs to be modified or enhanced somehow to improve its effectiveness.

Mike


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5606671 - 01/05/13 01:53 PM

nevy,

Quote:

That looks like it could be very usefull as I have two scopes with fans so it could be used on either scope to mess with the fan speeds , it would save the trouble of fitting pots to both scopes , come on spill the beans where do you get such a device :-).




Exactly. The less after-market cutting, splicing and wiring I have to do the better!


Mike


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MessiToM
scholastic sledgehammer
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Reged: 12/21/09

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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5606705 - 01/05/13 02:10 PM

This thread has got me cruising for fans. Check out this 110cfm fan at "low" rpm even
http://www.frozencpu.com/products/13819/fan-901/Akasa_140mm_x_25mm_VIPER_PWM_...

$9 4 fan controller!
http://www.ebay.com/itm/4x-Port-FAN-SPEED-CONTROLLER-3-5-Bay-Black-Aluminum-P...

Edited by MessiToM (01/05/13 02:15 PM)


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MessiToM
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Re: Thermal Issues and Fans Successes new [Re: MessiToM]
      #5606729 - 01/05/13 02:29 PM

O guys..........now I have a new project..

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MessiToM
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5606743 - 01/05/13 02:39 PM

Quote:

nevy,

Quote:

That looks like it could be very usefull as I have two scopes with fans so it could be used on either scope to mess with the fan speeds , it would save the trouble of fitting pots to both scopes , come on spill the beans where do you get such a device :-).




Exactly. The less after-market cutting, splicing and wiring I have to do the better!


Mike




I use something similar for dew busting. Its just a 12volt cigarette adapter with variable voltage outputs. Mine was from radioshack


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: MessiToM]
      #5606765 - 01/05/13 02:52 PM

I'll have to start looking for those. There are a lot of neat off-the-shelf gizmos in the 12-volt Universe!

Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5607630 - 01/06/13 12:38 AM

Quote:

Detail on Ganymede.

Bionic eyes?




Not at all. At least 8/10 seeing and preferably about 400x though you can see it with less. It's an uncompromising kind of feature that demands a steady image. I've tried with 7/10 which is still good seeing but its too soft at the magnification needed to show some albedo here.

Pete


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Scanning4Comets
Markus
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5607719 - 01/06/13 02:50 AM

I'll give it a try, LOL! Maybe I'll get lucky and see the footprints on the moon too.

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Asbytec
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5607752 - 01/06/13 03:57 AM

Quote:

Maybe I'll get lucky and see the footprints on the moon too.




If you do, report back. I wanna try it.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5607943 - 01/06/13 09:27 AM

Pete,

Quote:

Quote:

Detail on Ganymede.

Bionic eyes?




Not at all. At least 8/10 seeing and preferably about 400x though you can see it with less. It's an uncompromising kind of feature that demands a steady image. I've tried with 7/10 which is still good seeing but its too soft at the magnification needed to show some albedo here.




I've caught albedo features on Ganymede during excellent seeing but have never seriously tried to tease them out, much less attempt a drawing. That probably requires higher magnification than I like to use on a scope that does not track.

But I have gone up to 600x for Mars when it was at about 6 arcsec just to see what could be seen, and had good results. Didn't attempt a drawing, though. Ganymede is appreciably smaller at about 1.5 arcsec. So Ganymede's apparent diameter is about 4 times smaller than Mars when I was viewing that planet. With steady seeing, though, I should be able to tease out gross albedo markings.

I always resolve the Galilean Moons as disks. That is no problem for a decent 10" Dob.

Mike


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Scanning4Comets
Markus
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5608242 - 01/06/13 12:21 PM

Of course I can see the moons as discs, as I did during Oct 25th....but to see DETAIL on one of the moons? Gimme a break.

All the power to you.....Something isn't adding up here.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5608382 - 01/06/13 01:29 PM

Seeing gross albedo markings isn't exactly seeing a lot of detail. You can probably see a lot more "detail" with your naked eyes looking at the Moon than moderate-sized amateur scopes can see on Ganymede in excellent seeing. I don't see this as incredible, but just a big bother to do if you don't have tracking on your scope.

Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5608813 - 01/06/13 04:57 PM

Markus,

It's not unheard of for a good medium aperture scope to pull this off. Here's a link where one (of several) did a drawing of Ganymede details on a recent night of excellent seeing. http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5606142/page...

Too, one observer seemed go do even a little better with a 6" Maksutov.

The key is excellent seeing. I've never pulled it off with 7/10 but 9/10 seeing yes. Also Io if compared to Europa shows a subtle ovular shape, a diffraction effect caused by the brighter equatorial zone versus the dimmer polar regions.

Ganymede s surface features for me need unusually good seeing had here in Connecticut only in the summer. Io tho just 1.2" wide to Ganymedes 1.7" curiously shows the effects of its equatorial zone in just 6/10 seeing.

As far as albedo details on the other moons not counting Io it would seem to be substantially more difficult due to the smaller angular size of the features and inherent lower contrast.

It all adds up - but it takes unusually good observing conditions. And again that's without fans running. I'd imagine the probability now would seem to go up a picketing point or two.

Pete

Edited by azure1961p (01/06/13 05:00 PM)


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5611849 - 01/08/13 12:20 PM

Just wondering something here,

Would it be ok to use a 9 volt battery on my 12 Volt fan? I tried it and the fan spins....but I am wondering if the battery will get too hot and malfunction over time?


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5611861 - 01/08/13 12:25 PM

Quote:

Just wondering something here,

Would it be ok to use a 9 volt battery on my 12 Volt fan? I tried it and the fan spins....but I am wondering if the battery will get too hot and malfunction over time?



The lower voltage reduces the fan speed and the battery won't last more than one night (spoken from experience). The battery won't overheat--it just won't last very long.
The standard "D" cell lasts a lot longer, and 8 of them in series should last several full nights.
However, I use a 12V 8 amp-hour deep-cycle rechargeable for 3 fans, and it needs recharging after one night, so just be aware of the limitations of batteries when running fans. Some 12V fans are small and run fairly slow and consume little power. Others push a lot of air and consume a lot of power.


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5611875 - 01/08/13 12:36 PM

Thanks for the info Don! I'm going to pick up an 8 "D" cell battery holder and 8 D cells for the fan.

Cheers,


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De Lorme
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5614778 - 01/10/13 12:45 AM

Hi Don, It's De Lorme, If you don't remember I have a old 17.5" Odessey. Do you think 3 39cfm would be enough? I had
planned on putting them in the rear. But after reading that a side one really makes a differance I'm going to put one
on the side to. Do you know of anybody that has a picture
how it should exactly look. You Know every picture tells a
Story!LOL. I had not even considered this option.
I really appreciate you Veterans here on CN pointing me in the right direction. Thanks again! De Lorme


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: De Lorme]
      #5614800 - 01/10/13 01:09 AM

Quote:

Hi Don, It's De Lorme, If you don't remember I have a old 17.5" Odyssey. Do you think 3 39cfm would be enough? I had
planned on putting them in the rear. But after reading that a side one really makes a difference I'm going to put one
on the side to. Do you know of anybody that has a picture
how it should exactly look. You Know every picture tells a
Story! LOL. I had not even considered this option.
I really appreciate you Veterans here on CN pointing me in the right direction. Thanks again! De Lorme



I think 3 fans like that would be fine. One behind the mirror because otherwise heat doesn't dissipate from the rear very well, and two blowing across the mirror.
http://www.fpi-protostar.com/bgreer/fanselect.htm
and
http://www.deepskywatch.com/Articles/rear-fan-newtonian.html
and
here
and
http://www.skyandtelescope.com/howto/basics/3304176.html
and
http://www.atmlist.net/pipermail/atm/2005-August/009756.html
and
http://www.teeterstelescopes.com/images/125casella5.jpg

I'm sure, after that, you should be able to figure it out.
The rear fan should blow directly on glass. I presume you've either replaced or extensively modified your mirror cell by now so that wind on the mirror can actually cool it.


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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5614840 - 01/10/13 02:25 AM

Hi Don, Weather must be bad there to. LOL. Would you place the fans on the side of the tub facing each other or on the same side next to each other? I'm inclined{opinion, educated
guess} that two next to each other in the back and one on the side is better. The reason for one on the side is less air turbulance. Just enough to get the air up the tub.
If I where to put two on the same sides should I put them
where all the air goes right across, or move then down a little so that only 80% goes across and 20% goes acroos the back? Thanks for the advice. BTW,The mirror is washed. I got the base{painted} done with the lazy attached,and setting circles on it.
Tub is sitting on the kithen table{RoseMary is such a great wife} painted inside and out with a color called very black by Valspare. I bought a use heleical focuser that matched with the base plate. Got the digital angle setter today from Sears. Almost done! Once I get the fans I'll be able to drill holes and be finished. Eric told me about the Astro-Fix Locator. Have you or anybody you know used it?
This is so exciting. I really like my CR6" on the CGEM but
I cannot wait to look through this! De Lorme


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Re: Thermal Issues and Fans Successes new [Re: De Lorme]
      #5614939 - 01/10/13 06:33 AM Attachment (15 downloads)

I have been reading this thread with great interest and thinking back to all of my previous experiments with fans blowing across the front face of telescope mirrors. On a Teleport 14.5" with the "air knife", a 20" Starmaster with Floyd Blue's corner fans and rear blowing fans, Daniel Mounsey's "air knife" on a 12.5" Starmaster, I had very limited success in effectively mitigating boundary layer issues in my steady North Florida skies. There is also the issue of fan vibration that is often not mentioned but some report is a reliable nemesis of our boundary layer solutions.

In my thread on my latest attempt with what we have termed a Comprehensive Boundary Layer Mitigation System (CBLMS), it appears that borrowing successes from of Joe Wambo's front downward blowing fan in his 32" and Jimmy Lowery's 10 enclosed rear MagLev fans in an enclosed rear mirrorbox of his 48" are the only way that I have been able to achieve reliable success in mitigating the boundary layer issues here in North Florida. It involves the use of Magnetic Levitation fans controlled by potentiometers, comprehensive damping with sorbothane, an annulus strategically placed above the primary to create a venturi effect for sucking the air away from the front boundary and out the back of the scope.

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.

I am hesitant to throw a wet blanket on many of these well-intentioned attempts but my take on it after having experimented for about a decade is that the use of fans is pretty darn tricky in terms of not creating potentially more turbulence or just not having a particulary effective fan system? I also think that location, thickness of the primary and the size of the scope have a lot to do with how effective fans are. Gary Myer's of StellarCat fame had equipped a 30" Newtonian he had with all kinds of different fan systems out in Arizona and was telling me years ago that most of his experiements had not born much fruit. John Pratte of JP Astrocraft that did the design and engineering on my scope also advised that he has had limited success with rear fans blowing on the primary with vent holes above his 25" primary.

I was out with the CBLMS last night and in fairly steady seeing was getting sharp views of Jupiter at 373x and 474x intermittently. Unfortunately, the first iteration of this CBLMS probably cost about $2k to implement because we were doing first time engineering from scratch. I suspect that existing scopes could be modified using the techniques we incorporated for substantially less than this in subsequent iterations. For those that have not followed my thread on this forum, here is a picture of the front fan portion of the system. In the next post, I will show a picture of the rear sucking system of the CBLMS.

Edited by Bob S. (01/10/13 07:29 AM)


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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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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
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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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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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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.
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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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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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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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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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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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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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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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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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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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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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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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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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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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Re: Thermal Issues and Fans Successes new [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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Re: Thermal Issues and Fans Successes new [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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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5617300 - 01/11/13 01:37 PM

Yes mike I recommend a seperate 6v for each fan, its still tiny enough it's not cumbersome. They sit inside the rocker box. Well they used to. The 12v battery that's adjustable voltage sits on the grass. I tried one 6v and it was too taxing to run both and for lesser time too.

Pete


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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5617487 - 01/11/13 03:46 PM

FYI, not all fans are not created equal. Some require as little as 4vdc to run others may need much more.

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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: demiles]
      #5617685 - 01/11/13 06:03 PM

Best to test before taking everything 50 miles to a dark site!

Mike


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5618030 - 01/11/13 09:14 PM Attachment (16 downloads)

I finally finished my larger fan and baffle install. It really moves air now. I have it sucking out of the bottom. The weather has turned bitter cold (for So Cal lol) it going well into the 30s by midnight so I am going to time how long it takes to get good views. The mirror will start at 70 degrees so thats a big drop to equalibrium. attached a few pics.

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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5618032 - 01/11/13 09:15 PM Attachment (17 downloads)

The Di Leo (I think)ring.

Edited by rlmxracer (01/11/13 09:26 PM)


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5618051 - 01/11/13 09:28 PM Attachment (22 downloads)

Ring installed after painting it flat black. I used 1/4" birch plywood for all the pieces.

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5618684 - 01/12/13 09:41 AM

Well it's terrific but if you find like with my experiences your hot breath gets sucked down the tube and the secondary dews over its a simple fix by removing the fan, flipping it over the other way and reattaching. All else is intact.

Nice clean job'

Pete


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5618720 - 01/12/13 10:07 AM

rlmxracer,

Did you construct the fan baffle from 1/4" plywood, also? How did you attach it to the OTA? I'd be concerned that a material like plywood would induce vibration into the optical system. I think I'd go with rubber or maybe foam core with Velcro attachments.

Mike


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5619072 - 01/12/13 01:25 PM

Quote:

rlmxracer,

Did you construct the fan baffle from 1/4" plywood, also? How did you attach it to the OTA? I'd be concerned that a material like plywood would induce vibration into the optical system. I think I'd go with rubber or maybe foam core with Velcro attachments.

Mike



I had the first night out with the new set up. The fan I used is a very low noise unit rated at 19db. The standard 80mm fan was 28db. I ran a bead of black silicone around the outer edge of the mirror cell where the wood rests on it. I have it held on by the "locking" screws which are also isolated with rubber gromets. Last night the seeing was just avg but I did use 208x on Jupiter with no notable vibration. One night is not enough to fully acertain how well this set up works but it seemed to cut my time to get good views by more that half. Before I did this I made a cardboard version of the ring and baffle for my stock 80mm fan. It cooled faster than no baffle but still took over an hour for the views to get better. That was with a temp delta of 72f indoors to 50f outdoors. Last night it was cold for So cal down into low 40s. After 20min of placing the scope outside with the 120mm fan running I had very clean views of Jupiter at 208x so I think its a step in the right direction.

I live in the inland empire of So cal a very dry place 99% of the time. I have only had a dew problem once in the last year and that was after 3 straight days of rain.


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5619356 - 01/12/13 04:08 PM

Glad the 120mm worked . That's what works for my 8" though if you go the boundary layer fan, 80mm is perfect - again for my 8". I never did the cool down versus time test. I still give it a full hour. It'd be great if I could cut this down like you have.

Pete


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5619534 - 01/12/13 05:50 PM Attachment (14 downloads)

Picked up the D cell holders today and wired everything up. Tested it and the fan runs and I'm good to go. I just have to re-solder the bottom left terminal because it is coming off.

Cheers,


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5619581 - 01/12/13 06:16 PM

Looks serious!!!

Pete


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nevy
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5619924 - 01/12/13 10:21 PM

Does anyone know if the Duracell procell ( the black & red ones that's sold in bulk on eBay) are better or worse than the copper ( gold) normal Duracell batteries ( D size) for running fans in the eight batterie holder power pack.

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Mark Peterman
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Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5620033 - 01/12/13 11:47 PM

So what is the verdict???

Use a fan to blow or suck on the rear of the mirror?

How many CFM is needed (blowing or sucking) to effectively scrub the front of the mirror? (I assume this is a function of the size of the primary.)

With an enclosed rear cell (blowing or sucking) is a fan scrubbing the front of the mirror still necessary?

If mounting a fan to scrub the front of the mirror, is it best to mount the fan under the primary blowing up or on the side of the primary, blowing across?

This is all just blowing my mind.


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5620289 - 01/13/13 08:34 AM Attachment (13 downloads)

Just finished getting the battery pack all fixed up. I re-soldered the one wire about to come off and I found an old plastic magazine rack that had been lying around for a while and used a hack saw to cut it to size and sanded the edges so the 8 D cell battery holder fits in it.

The whole contraption fits inside of the dob stand. Now I can run my fan for a much longer time blowing air onto my 10" / 1" thick primary.

Cheers,


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5620420 - 01/13/13 10:08 AM Attachment (14 downloads)

Here is the 3 speed fan attached to thick cardboard which is held to the back of my mirror cell with elastic bands. I run the fan at full speed for cool down and cut the speed back to the slowest speed when observing. Looks crude right now, but it works. I might make a new backing with different plugs because I am using clips right now like the kind you have for a 9 volt battery.

I found a small CP fan yesterday and I considering cutting a hole in the side of my telescope tube to mount it there and have air blow across the face of the 10" primary.

I'll see how it goes first with just the fan on the back first before I get to cutting a hole in the side of the tube.

Cheers,


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Datapanic
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5620576 - 01/13/13 11:27 AM Attachment (10 downloads)

Here's what I came up with for the Horsetrail Cave. The fan is mounted on 1/8" plexiglass which is sandwiched between two foam rings. A DC power jack and on/off switch complete the setup. The foam dampens any vibrations and the whole thing can be flipped around to change direction of air flow. I could wire the switch to reverse voltage on the fan, but the fan blades are designed to go in one direction only.

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Datapanic]
      #5620880 - 01/13/13 02:08 PM

Just remember Markus that 30% of the 80mm fan, or 100mm for a ten inch needs to be below the mirrors reflecting surface so the full wash of the blades is blowing on the mirror. If you have the fan just above the mirrors siurface it literally does no good at all. It ll look like you r partly cooling the side of the glass and you are but the benefit is the stram of air is flooding the boundary layer and brushing it off.

Pete


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5621093 - 01/13/13 03:49 PM

So if I made a small hole for a boundary layer fan, you're saying I would need it at the bottom?

Cheers,


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5621239 - 01/13/13 05:04 PM

Pete,

Quote:

Just remember Markus that 30% of the 80mm fan, or 100mm for a ten inch needs to be below the mirrors reflecting surface so the full wash of the blades is blowing on the mirror.




Are you saying that an 80mm fan should be used for an 8" mirror to scrub the boundary layer, but a 100m fan should be used for a 10"? Where did you get this idea? Is it really necessary to use a larger fan for the 10"?

Mike


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tnranger
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5621306 - 01/13/13 05:39 PM

Not trying to highjack this thread, but I'm trying to understand it all.

I have a XT10i and 2 scavenged computer fans. One fan's mounting holes line up exactly with the fan holes on the mirror mount. The other fan is slightly bigger (~8.5-9cm). I do not want to cut holes in the OTA for side fans.

Will a single fan at the bottom of the mirror improve cooling enough to noticeably improve seeing? Is the consensus view that the bigger fan mounted/suspended further away will do a better job of cooling than one mounted directly to the mirror mount? Should the fan blow or draw?
Thanks for any help.


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: tnranger]
      #5621319 - 01/13/13 05:45 PM

Quote:


1. Will a single fan at the bottom of the mirror improve cooling enough to noticeably improve seeing?
2. Is the consensus view that the bigger fan mounted/suspended further away will do a better job of cooling than one mounted directly to the mirror mount?
3. Should the fan blow or draw?
Thanks for any help.



1. Yes, it will.
2. Probably, but it depends on the conditions of use, too.
3. blow. This is the most effective way to cool the mirror, force hot air out of the tube, prevent heat from the observer from being sucked in, and improve the nature of the tube currents in the tube.


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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: tnranger]
      #5621337 - 01/13/13 05:58 PM

I felt the mounting the fan on the mirror mount was too close to the mirror an would impeade the air flow reguardless of fan direction.

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tnranger
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5621362 - 01/13/13 06:06 PM

That makes sense. A ceiling fan moves more air if it's away from the ceiling instead of flush mounted.

Thanks guys!


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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: tnranger]
      #5621497 - 01/13/13 07:34 PM

This thread seems to be chock full of mass confusion.

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nevy
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5621543 - 01/13/13 08:02 PM

The best way is just to suck it and see , as the saying goes.

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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5621553 - 01/13/13 08:09 PM

Quote:

Quote:


1. Will a single fan at the bottom of the mirror improve cooling enough to noticeably improve seeing?
2. Is the consensus view that the bigger fan mounted/suspended further away will do a better job of cooling than one mounted directly to the mirror mount?
3. Should the fan blow or draw?
Thanks for any help.



1. Yes, it will.
2. Probably, but it depends on the conditions of use, too.
3. blow. This is the most effective way to cool the mirror, force hot air out of the tube, prevent heat from the observer from being sucked in, and improve the nature of the tube currents in the tube.




Don, Blowing does not seem to be as effective as sucking at least for larger mirrors. I am not sure where all of this talk of sucking in body heat has come from. My 20" scope with an AstroSystems shroud does not seem to suffer at all from sucking in body heat. It also uses a blowing fan on the front face of the mirror about 9" above the mirror which is quite effective when used with the rear enclosed sucking fan and annulus that surrounds my 20" mirror. Bob


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621576 - 01/13/13 08:20 PM

Bob,

I wouldn't be surprised if there's a substantial difference in what will work best to control thermals for solid tube 8" and 10" Dobs as compared to larger truss scopes ... especially much larger 20" scopes. I can't imagine that the optimal solution would be the same for both optical systems.

Mike


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5621583 - 01/13/13 08:24 PM

Quote:

Bob,

I wouldn't be surprised if there's a substantial difference in what will work best to control thermals for solid tube 8" and 10" Dobs as compared to larger truss scopes ... especially much larger 20" scopes. I can't imagine that the optimal solution would be the same for both optical systems.

Mike




Mike, I do not see why not? The issue is to uniformily cool the primary mirror on all sides, reduce hot air masses rising off the ground, bring in cool air to the primary and mitigate thermals from surrounding scope materials. I am not sure what would be different in any scope? Bob


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demiles
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621607 - 01/13/13 08:33 PM Attachment (12 downloads)

I use the a 80mm front and rear on my 15 in. And used a 70mm on my 10.

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Starman1
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621609 - 01/13/13 08:34 PM

I had it affirmatively demonstrated to me that:
1) you don't need to seal the back end of a tube to get the mirror to blow up the tube
2) blowing up the tube was sufficient to establish a laminar flow of air out of the tube and disperse the boundary layer. It wasn't the most effective cooling regimen, but it was adequate for a 12.5" full thickness mirror in a tube.
3) the experiment was fan on/fan off. The difference was immediate, and profound.
I do not believe that sucking air toward the mirror will be as effective at cooling as blowing on the gloss from up close.


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621611 - 01/13/13 08:35 PM

Bob,

Differences between large truss Dob and 8" or 10" solid tube Dobs for thermal issues:

- Shroud or open truss vs solid tube: allows easier access to install and maintain a front-blowing fan directly above the primary (I don't see this as practical for a solid-tube scope)

- Larger mirror: requires more time to thermally stabilize and more difficult to maintain thermal stabilization over change in delta during the night

- Longer distance from primary to sky end for larger scope: might require different solution in which way to exhaust the warm air from the optical system

There are probably other differences but these popped immediately into my head.

Mike


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demiles
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5621645 - 01/13/13 08:47 PM

Mike if you look at the specs on an 80mm fan it could range from 8cfm up to 60cfm plus. And my opinion not just any fan will do, fans from Noctua, Noiseblocker are of higher quality than most. I have bought many fans and those two are the best for having minimal vibration. BobS I've tried those Sunon maglev fans and you can do much better, the one I have had lots of vibration.
Lots of fans

Edited by demiles (01/13/13 09:16 PM)


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5621709 - 01/13/13 09:19 PM

Quote:

I had it affirmatively demonstrated to me that:
1) you don't need to seal the back end of a tube to get the mirror to blow up the tube
2) blowing up the tube was sufficient to establish a laminar flow of air out of the tube and disperse the boundary layer. It wasn't the most efeective cooling regimen, but it was adequate for a 12.5" full thickness mirror in a tube.
3) the experiment was fan on/fan off. The difference was immediate, and profound.
I do not believe that sucking air toward the mirror will be as effective at cooling as blowing on the gloss from up close.




Don, If you go over to the thread I started on the 20" f/3 Lockwood/Starmaster, you will find that indeed, the blowing face, sucking rear fan has been extremely effective. I termed it a CBLMS which is an acronym for Comprehensive Boundary Layer Mitigation System. I have tried blowing fans from the rear in an open system with very limited success in many different scopes (both small and large closed/open tubes). I have tried side fans just above the primary's surface coupled with rear blowing fans with limited success. The problem I have with these strategies is that they seem to create some turbulence with limited efficacy for my North Florida/South Carolina viewing conditions. When a fan blows from the back, I am not sure how the air reliably disperses the boundary layer that is just above the mirror's front surface? When I saw Joe Wambo's 32" scope with a fan placed in the secondary shadow gently blowing air onto the front surface, I witnessed extremely sharp images. I then talked with Jimmy Lowery who advised that 10 sucking fans in an enclosed structure were producing amazingly good images in his 48" scope. Having combined those two strategies together, I have found that the CBLMS that also incorporates an annulus surrounding the primary with about 1/2" larger diameter is providing exceptionally good boundary layer mitigation with views that are consistently better with both blowing/sucking fan strategy operating in tandem. The combined strategy also seems to be uniformily cooling the entire primary which is turning out to be a big deal. If you only cool one side, you can tend to get unequal cooling of the primary which can lead to suboptimal primary mirror substrate performance and subsequent impact on the mirror's figure. Bob

Edited by Bob S. (01/13/13 09:42 PM)


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tnranger
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621822 - 01/13/13 10:21 PM

Don,

I assume you meant "you don't need to seal the back end of the tube to get the FAN to blow up the tube"?


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Datapanic
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5621831 - 01/13/13 10:26 PM

That's why I made my design reversible - blowing for cooling the mirror down and as needed, sucking to bring cooler air at the end of the 8-foot tube on down inside to equalize the temps all around. Most of the time, I've found that once things have near equilibrium, best results for imaging have been from sucking the air in from behind the mirror. When imaging, I'm nowhere near the eyepiece end of the tube so there's no problem with body heat.

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5621947 - 01/13/13 11:55 PM

Well I'm guessing a 100mm for the ten inch tho the 80mm is perfect for the 8". If 80s too small just cut a bigger hole and modify. Just remember the 80mm or 3" hole needs to have an inch of that hole intercepting the SIDE of the mirror. It looks wrong to see the fan overlap but that's what's need to put the fan wash across the mirrors face. Sure some will hit the side of the mirror and really do little good but the rest of the flow is doing fine.

In the sky and tel article by Adler the magazine published a very misleading illustration. It shows the fan hole cut ABOVE the front of the mirror and the airflow inexplicably cascading down upon it . Folks it does no such thing - ignore this bad illustration. I even tilted the fan to vector the air downward which looked utterly stupid from the outside of the tube and it did no good still. To reach the correct angle would have meant constructing air duct tubing to seal the flow of an angled fan. The whole thing was a mess.

Overlap an inch of the fan coverage on the side of the mirror and it becomes a neat tidy effective affair.

Pete


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5621953 - 01/14/13 12:03 AM

Quote:


3. blow. This is the most effective way to cool the mirror, force hot air out of the tube, prevent heat from the observer from being sucked in, and improve the nature of the tube currents in the tube.




I had a night of 7/10 drop to periods of 3/10 that came in these horrendous bouts of soft seeing followed by the secondary dewing. All because I tried having the air sucked down the tube. Exhalation doesn't belong inside the tube!!!!

Pete


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johnnyha
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5622017 - 01/14/13 01:09 AM

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.

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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5622067 - 01/14/13 02:37 AM

Johnny, it seems different people are having success both ways. Several have tried both and seem to have good results eather way. I currently have a baffled 120mm fan behind the primary sucking air out the bottom. There are pics a page or two ago. I have only used it two nights but I've had great results. I honestly think its mostly because the mirror is cooled much faster than the standard 80mm fan. If you follow the air flow the cool air flows down the tube hitting the front of the mirror then wraps completely around the mirror. I have cut my time to get good high power views by 2/3 with this set up. It could be because the air flow is disrupting the boundry layer but i think the wrap around flow cools the mirror very efficiantly. Next week I plan to reverse the fan and see how that works. Rob.

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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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