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Equipment Discussions >> Reflectors

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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
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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Reged: 12/18/07

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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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Reged: 01/17/09

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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
Starstruck
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Reged: 04/06/05

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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
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
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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rlmxracer
sage
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Reged: 11/09/11

Loc: Motocross Mecca , CA
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
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
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
sage
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Reged: 11/09/11

Loc: Motocross Mecca , CA
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
Markus
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Reged: 12/26/04

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
sage
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Reged: 11/09/11

Loc: Motocross Mecca , CA
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
Markus
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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
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
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
sage
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Reged: 11/09/11

Loc: Motocross Mecca , CA
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
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
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
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
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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Reged: 12/18/07

Loc: Suburban Maryland, USA
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 [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.
Carpal Tunnel
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Re: Thermal Issues and Fans Successes [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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