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Boundary layer fan position relative to primary

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#1 SchrödingersCat

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Posted 28 April 2013 - 08:37 AM

Forgive me if this has been covered before. I could not find this specific info. I intend to employ boundary layer fans blowing across the side of my dob's primary mirror. What should the positional relationship of the fans centerline relative to mirrors surface be? Should the CL be inline with the mirrors face or should the edge of the fans swept area just graze the mirrors surface? Ideally I would like the side mounted fans to not only handle the boundary layer but cool the mirror as well.

#2 catboat

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Posted 28 April 2013 - 11:18 AM

There's a lot of variance in people's experience. The following thread is the one I find most promising for a large dob (and the one I plan to implement). The fans are placed above the mirror and exhaust rather than blow on the surface. (I think the fans are positioned so they overlap the mirror's edge -- if you're sighting "down" through the fans-- but I'm not finding that info in the text this time around).

link

#3 MessiToM

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Posted 28 April 2013 - 11:25 AM

I've been experimenting with this type
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#4 don clement

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Posted 28 April 2013 - 11:46 AM

How about a ring around the mirror that uses Dyson's air multiplier technology see: http://www.howstuffw...adeless-fan.htm

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#5 Chuck Hards

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Posted 29 April 2013 - 02:56 PM

As long as you break-up the boundary layer with faster-moving air, the fan geometry isn't critical. Pulling it directly out of the optical path can't hurt, as it can help speed cooling, but once the warmer boundary air is mixed, it ceases to have such harmful effects on imagery.
I've seen great results on smaller,closed-tube scopes that don't even blow on the mirror face, but on the back. Because the back of the tube is closed, there is effectively a column of fresh air moving up the entire tube length and pulling the boundary layer with it. The effect in the eyepiece is dramatic, like flipping a switch. The image sharpens up within seconds. It's obviously not related to mirror cooling because of the rapid result seen.
It probably won't work with a primary in an open arrangement such as a truss. Then you have to blow across the face of the mirror.

#6 SchrödingersCat

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Posted 29 April 2013 - 04:25 PM

Catboat, thanks for that link. I had not read that thread before.
Don, that is a great idea but I am afraid it's more involved than I can commit to right now.
I was contemplating the approach taken by Rob Teeter, but before I cut into the mirror box I think I will experiment with closing off the back of my scope like what has been explored in recent threads and pioneered most famously by Mauro Da Lio.

#7 Chuck Hards

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Posted 29 April 2013 - 07:35 PM

Check out my 6" f/8 Newt in the March '99 S&T. It has a closed tube with a fan on the back and disburses boundary layer air instantly. And it's a fairly small fan. My editor, Gary Seronik, captioned the photo as if it were for mirror cooling, but the image snaps into clarity instantly as soon as the switch is turned-on, clearly demonstrating that it's removing the boundary layer.

I have a 10" version of the cell under construction, done in plate aluminum. Will post photos on my next day-off, if interested.

#8 SchrödingersCat

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Posted 29 April 2013 - 08:12 PM

Chuck, please do post.

#9 don clement

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Posted 30 April 2013 - 01:17 PM

My goal is to remove the fans from contact with the scope to prevent vibration. The air multiplier would allow doing just that. I was also thinking of an easier to build compressed air distribution system (nozzles and tubing) with a small air reservoir and compressor located off scope. The compressed air nozzles would be located around the mirror to homogenize the boundary layer and no need for ducting or a closed tube.

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#10 SchrödingersCat

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Posted 30 April 2013 - 02:11 PM

I just came across a reversible case fan that could offer cooling by blowing on the back of the mirror and boundary layer disruption by evacuating the mirror box. I have a circular baffle forward of the primary and the back is sealed off.

http://www.frozencpu...120mm_x_25mm...

#11 SchrödingersCat

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Posted 30 April 2013 - 05:57 PM

Did a little digging on the Akust. It's a bit noisier and pushes less air than a Noctua. Maybe too inefficient at 19-67 cfm for a 18ish mirror box and likely too noisy at 22-34 db. I already have a Noctua 140 that would be quite ideal at 89-115 cfm and 14-19db if I could devise a reversible mounting config. Any ideas?

#12 Chuck Hards

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Posted 30 April 2013 - 09:13 PM

My goal is to remove the fans from contact with the scope to prevent vibration. The air multiplier would allow doing just that. I was also thinking of an easier to build compressed air distribution system (nozzles and tubing) with a small air reservoir and compressor located off scope. The compressed air nozzles would be located around the mirror to homogenize the boundary layer and no need for ducting or a closed tube.


Most people just use too large a fan. It's surprising how little air needs to be moved to break up the boundary layer. Also all fans aren't created equal. Some are noticeably out-of-balance and that, coupled with too large a unit, is usually what contributes to vibration issues.
I've found that it's not really necessary to have a homogeneous air density all along the optical path, from my experience.
The compressed air idea sounds intriguing, Don, but seems like a lot more trouble in practice than a well-placed, properly-sized fan or two. That said, I'm keen to see the results if you build it.

#13 just lookin

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Posted 30 April 2013 - 10:14 PM

The boundary fan on my 18" Obsession may or may not give you some ideas. It starts with a PVC pipe with a slightly larger diameter than the truss pole. It is secure with a nylon bolt. The 92mm Noctua fan is bolted to a piece of flexible plastic. Silicone disks were made to dampen vibration. To further dampen vibration, a bungee cord pulls the fan down just enough so it's out of the mirror's light path. Eight AA batteries can power the fan all night.

Looking at tight doubles at high magnification shows no vibration. I also have a larger Noctua fan behind the mirror strung from rubber bands at each corner.

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#14 just lookin

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Posted 30 April 2013 - 10:17 PM

and another pic.

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#15 StarStuff1

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Posted 01 May 2013 - 12:16 AM

I agree with Chuck. A small fan blowing on the back of the primary will do a good job. Years ago I stored my 8-in StarHopper in the garage where temps soared high and low. One summer night I took it out and the images were aweful. About 2 minutes with a box fan aimed at the primary fixed the problem. Since then a small computer fan suspended with rubberized hair bands does the job well.

#16 Chuck Hards

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Posted 01 May 2013 - 08:10 AM

I typically buy several fans of the same model, and test each for lowest vibration and current draw, with the rejects going in the swap-table box. A good source for fans is Marlin P. Jones & Associates.

http://www.mpja.com/

Some of them are takeouts, so be warned. Not necessarily a bad thing, but it may mean a shortened lifespan in the telescope.

#17 Chuck Hards

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Posted 01 May 2013 - 08:16 AM

The more I look at that setup, the more I'm liking it. The fan is completely shadowed by the secondary, though I'd use blackened fishing line instead of wires. Aligned with the secondary spider vanes, there is no increase in diffraction effects.

With an open mirror box, you're not even recirculating the same warm air.

Keep us posted on this effort.



I've been experimenting with this type
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#18 MessiToM

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Posted 02 May 2013 - 09:02 PM

It's blackened out.
Posted Image

I also have a rear fan that exhausts and both fans rpm can be adjusted

#19 demiles

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Posted 03 May 2013 - 06:45 AM

I have similar setup on my 15in. that serves a dual purpose. In general I have found that any boundary fan configurations success is almost entirely dependent on the temperature differential between the mirror and the ambient air (Delta T). If that Delta T is higher than 10-12 degrees running a front blowing fan still distorts the image. Once the Delta T gets down to the 8-10 degree and lower thats were the magic happens. YMMV. To think that any fan configuration will over come a high Delta T such as in winter bring out your mirror at 70 degrees to a 30 degree outside temp and getting a clean image, its not going to happen. Managing that Delta T is the probably the single best thing you can do to over come the boundary layer issue. So that fan is a cooling fan at the start of a session, then a boundary layer fan later on.

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#20 Chuck Hards

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Posted 05 May 2013 - 03:29 PM

Chuck, please do post.


Here you are, sorry it took me so long. It's busy season at the day job.

Here is a view of a 6-inch cell under construction. It is almost identical to the cells used on the "Woodshop" telescopes from S&T, March 99. Single small fan, centered on the rear of the mirror.
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A rear view. The aluminum cover on the left is the battery compartment. 8 AAA cells provide either a high or low speed, provided by the DPDT, center-off switch seen at upper right.
This cell design has been in-use since 1998 and works very well in a closed-tube. I've used it on 6" f/8, 6" f/5, and will be using this one on a 6" f/10.
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The six-inch cell fan clears-up image degradation within 2 seconds of turning on the fan, even with very warm optics that have been sitting in a closed, hot garage all day.

Here is the same concept in a 10-inch, 9- point floatation cell. A dummy plastic disk is sitting-in for the mirror in these pics. The three black plastic housings each hold 8 AAA cells.
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The back side of the cell. This one is also designed for a closed tube. Here you can see the access doors for the 3 battery compartments as well as the collimation knobs. The fan itself is mounted in a PVC block on a Sorbothane gasket. The PVC block is also separated from the aluminum back plate by another Sorbothane gasket.
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A side view of the cell. Each collimation bolt has 2 springs, one inside the other. The batteries will be wired in parallel to extend the run time of the fan. I'll probably use NiMH rechargeables and be able to charge them from the car 12V system enroute to the observing site, or at home with a wall wart type charger.
You can also see the stainless-steel ball-bearing pivot under one of the floatation pads. Each ball is loosely caged by the 4 SS allen-head screws that also hold the pads themselves. The balls nest in conical seats on both the back of the pads, and the cell back.
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Looking through the plastic dummy mirror at the "out" side of the fan. While this system works very well in the smaller 6" scopes, I haven't tried it yet on the 10" (12" OD aluminum tube), so I have a plan for a second fan blowing across the face of the mirror if this arrangement proves too anemic.
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Both the 6" and 10" cells were made using woodworking tools only, on thick sheet PVC and aluminum plate up to 1/2" thick.

Some "old school" stuff here, for sure. Be gentle and patient with me. ;)

#21 davidpitre

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Posted 05 May 2013 - 07:22 PM

"Be patient with" you? That is fantastic work and design. Thanks for sharing. I don't understand the relationship of the allen screws to the ball bearing pivot. Any link on the hardware used?

#22 Chuck Hards

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Posted 05 May 2013 - 08:39 PM

"Be patient with" you? That is fantastic work and design. Thanks for sharing. I don't understand the relationship of the allen screws to the ball bearing pivot. Any link on the hardware used?


Thanks for the kind words, David.

Here's a quick pencil sketch of the floatation mechanism. The steel ball is nested between two conical recesses, one on the bottom of the triangular floatation pad, the other on the main mirror carrier plate below it. Each ball is "caged" by four stainless allen-head screws. They are countersunk into the pad but the pad isn't threaded and they pass through. The pad holes are slightly oversized. They are threaded into tapped holes in the main carrier plate, under the pads (the big triangular plate with the mirror edge clips on it). I've only indicated one screw in the sktech, but there are four per pad. In practice, the pad can wiggle and rock a bit with no mirror on there, so it's a true floatation system. The ball can't fall out when the mirror is on-edge because it's constrained by the opposing cone seats and the pad itself is held by the four allen screws. They keep the pad from rotating around the ball, as well.

Hope this makes sense. I've got Solidworks on my office computer but not at home at present.

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#23 Pinbout

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Posted 05 May 2013 - 09:44 PM

thats similar to strock's design, but he glues the ball to the bottom plate and I believe his "screws" are pins that move freely up and down but stop the float from rotating.

#24 don clement

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Posted 05 May 2013 - 10:59 PM

My solid machined compliant pivot is a direct replacement for the BB pivot that also prevents triangle rotation but without the stiction of sliding pins or the stiction of the BB.

Don Clement

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#25 Chuck Hards

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Posted 05 May 2013 - 11:32 PM

That's a great idea for larger, more massive mirrors, Don. It's the same solution used for coupling slightly mis-aligned shafts. Not truly "floating", but it does allow some pivot motion.

Stiction doesn't seem to be a problem with these smaller mirrors. A little graphite keeps the float pad loose enough. A similar mechanical pivot on my old Novak 17.5" cell is still moving freely when it has to.

We've really sidetracked from the OPs question, lol.






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