Author Topic: Plan:Boundary fan suspended over 16" mirror.  (Read 152 times)

itupmenra

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Plan:Boundary fan suspended over 16" mirror.
« on: January 08, 2018, 04:23:33 AM »
So thats my plan! Its an OOUK 16" f4 with a tube thats not much larger than the mirror. Im convinced stirring the air up will help hugely - my boundary layer is certainly an issue. My impression is this does a better overall job than perhaps 2 fans in holes in the tube wall. Its also less damaging to the structure should I wish to get rid of it!

I've seen examples online but cant seem to find any now so would appreciate any advice or experience, in terms of positioning and suspension methods especially. And recommended cfm.

I'm planning on using something like a 50mm fan (so ~70mm diagonal - theres a 100mm secondary so plenty of shadow to sit in) suspended something like 6-8" above the mirror, with four thin wires, two of which would carry power. Zero chance of aligning perfectly with the spider but I think the extra diffraction is a price worth paying.

Please help!



Bill Pham

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #1 on: January 09, 2018, 03:21:48 AM »
Guitar strings! http://www.stevesmus...trings/electric

Guitar tuning machine heads. http://www.stevesmus...s/machine-heads
These can be purchased singly...

You can run the electricity to the fan through the guitar strings, too....

Dave

precaregmo

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #2 on: January 10, 2018, 10:12:54 AM »
I spent some time with this concept on my 16" f4.5 strut Dob. The difficulties I encountered were numerous enough that I gave up on the idea.

I was limited in the size/volume of the fan, even though my scope has a very large (4") secondary. I didn't want the fan to be out of this shadow, for fear that diffraction issues would null out the benefits of scrubbing the boundary layer.

I found vibration elimination very difficult to achieve-- the suspension, 1/8" thick 'bungee' material, would vibrate harmonically with the fan, and transmit to the scope and image. At the time, super-low vibration fans didn't exist, so perhaps that would've fixed the issue.

Observing the boundary layer on an out of focus star didn't show the effects of the fan having enough reach, so I came to the conclusion than the little fan I used just couldn't move enough air.

I think that numerous fans mounted to the side of the box (or tube) allows far more leeway for moving adequate air, though controlling vibration remains a crucial hurdle. This is easier, now, with the numerous super-quiet fans on the market.

In the end, I just went back to a single rear-mounted cooling fan, and called it good, limitations and all.

All this said, I'm sure the idea is workable. It'll just take more effort than I was willing to put into the project.

haigeoredis

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #3 on: January 11, 2018, 05:22:20 AM »
Thanks Cotts and Kerry, useful insights and ideas.

Anyone else got any experience?

Nathan Roberts

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #4 on: January 11, 2018, 07:31:09 AM »
Sorry, I don't have any ideas about how to make this work. It is a nice concept, but I'm wary of issues with suspending the mirror, any added diffraction effects, etc. How does one collimate with a fan in the way of the center spot? (Sometimes after viewing at 5 degrees off the horizon and then near vertical I need to touch up collimation as well...that would be a problem.)

I've thought about starting with a box design with a thin tapered duct shaped to blow down on the mirror laterally (from thefront sideof box)to sweep the surface both for boundary layer minimization and for cooling.This could work with a single/or double fan, but it isn't easy to lay it out with such short dimensions, and one would need to examine the interplay of the fan and duct to examine how uniform the resulting air flow was--it would likely take some trial and error to balance volumetric flow with the cross section of mirror area traversed.

An approach more in line with what you areconsidering would be adoughnut shaped mirror with a center tube baffle rising through it with a fan blowing air out radially in all directions across the mirror face. That isn't the norm forNewtonian primaries. Again, how would one collimate in the field with the center spot obstructed/missing?

ecidjapa

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #5 on: January 13, 2018, 03:13:52 AM »
I am not an expert on fans and newts but some people install two fans in the tube sides just above the primary. One that blows and one that draws air out of the tube.

raposttavers

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #6 on: January 18, 2018, 08:22:56 AM »
Have you seen the thread about the 'Comprehensive Boundary Layer Mitigation System?' The topic was the design and execution of a suspended front-blowing fan on a custom JP Astrocraft dob.

Edit: Here's the thread:http://www.cloudynig...djp-astrocraft/

stunfalriave

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #7 on: January 21, 2018, 11:35:52 AM »
Quote
Sorry, I don't have any ideas about how to make this work. It is a nice concept, but I'm wary of issues with suspending the mirror, any added diffraction effects, etc. How does one collimate with a fan in the way of the center spot? (Sometimes after viewing at 5 degrees off the horizon and then near vertical I need to touch up collimation as well...that would be a problem.)

Rats! I hadnt thought of the centre spot collimation issue! Cheers....

Quote

Have you seen the thread about the 'Comprehensive Boundary Layer Mitigation System?' The topic was the design and execution of a suspended front-blowing fan on a custom JP Astrocraft dob.

Edit: Here's the thread:http://www.cloudynig...djp-astrocraft/


Thankyou - interesting and exactly the sort of thing I need to read, though with a closed tube and other differences much of this isnt possible.

Hmm maybe this isnt the simple solution I had hoped for!

firorectve

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #8 on: January 21, 2018, 02:06:13 PM »
I guess you would not be able to easily reach down the tube to pivot the fan out of the way for collimation.

Perhaps your best best would be to drill two holes in the tube near the mirror and mount fans on an angled mount to blow on the mirror face.

finatissau

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #9 on: January 23, 2018, 12:06:45 AM »
Quote
I guess you would not be able to easily reach down the tube to pivot the fan out of the way for collimation.

Perhaps your best best would be to drill two holes in the tube near the mirror and mount fans on an angled mount to blow on the mirror face.


I think you're right - except I'm not sure there's enough space.

bolgsorchumsdea

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #10 on: January 23, 2018, 09:55:39 AM »
Hum, "stirring the air up will help hugely" may not be a good idea. One of the problems with tube currents ismixed air turbulence from the mirror and/or tube walls that will disturb the optical path and at times be seen as bad "seeing." There are some articles (S&T I think) around detailing how to use fans fro boundary layer air flow and while I have never done it, it seems like a workable solution.You sat the tune ID is close to the mirror diameter, how much so? A rule of thumb is to allow at least an inch on both sides of the primary for air flow. Just saying.

Alex Hart

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #11 on: January 23, 2018, 08:31:34 PM »
Quote
Hum, "stirring the air up will help hugely" may not be a good idea. One of the problems with tube currents ismixed air turbulence from the mirror and/or tube walls that will disturb the optical path and at times be seen as bad "seeing." There are some articles (S&T I think) around detailing how to use fans fro boundary layer air flow and while I have never done it, it seems like a workable solution.You sat the tune ID is close to the mirror diameter, how much so? A rule of thumb is to allow at least an inch on both sides of the primary for air flow. Just saying.


Yes, but the problem is having distinct bodies of warm and cooler air which then act as weak 'air lenses'. Mixing it removes this effect.

Probably less than an inch on all sides, but it is what it is! Im not rebuilding the entire scope at this stage! If this doesnt work, then side mounted fans and holes in the tube come next.

Chuck Johnston

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #12 on: January 26, 2018, 04:17:08 AM »
Back in the day after building and using my 12.5" Cass for several years I finally succumbed to friendly suggestions and rebuild the tube assembly. Replacing the heavy and heat retaining fiberglass tube with a thinaluminum one, installed small fans in the rear mirror cell and some other things I now forget, that scope was 1,000% better. Yeah, I really mean is was much better.

All you will accomplish with mixing the warm and cool air "lenses" is to createdsmaller warm and cool air"lenses." But, I am fairly new at this ATM business so what do I know?

Cesar Taylor

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #13 on: January 29, 2018, 09:26:20 PM »
Quote
So thats my plan! Its an OOUK 16" f4 with a tube thats not much larger than the mirror. Im convinced stirring the air up will help hugely - my boundary layer is certainly an issue. My impression is this does a better overall job than perhaps 2 fans in holes in the tube wall. Its also less damaging to the structure should I wish to get rid of it!

I've seen examples online but cant seem to find any now so would appreciate any advice or experience, in terms of positioning and suspension methods especially. And recommended cfm.

I'm planning on using something like a 50mm fan (so ~70mm diagonal - theres a 100mm secondary so plenty of shadow to sit in) suspended something like 6-8" above the mirror, with four thin wires, two of which would carry power. Zero chance of aligning perfectly with the spider but I think the extra diffraction is a price worth paying.

Please help!

I have the 8" version of your scope - here is what I did......

1. I made an aluminium plate for the back of the scope with holes for the four bolts that hold the fan, one for the power plug and a big central hole the same size as the rotating part of the fan.
2. Diassembled the rear of the OTA.
3. Put the aluminium plate between the fan and the rear mirror housing.
4. Put the fan back - REVERSED - so its sucking air out of the OTA.
5. Reassembled the rear of the OTA.

This causes no damage to the OTA,can be undone, and draws air from the front of the tube across the mirror and out the back - effectively dealing with the boundary layer......

Eric Guffey

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Re: Plan:Boundary fan suspended over 16" mirror.
« Reply #14 on: January 29, 2018, 11:14:25 PM »
What you have to do is bring everything to equilibrium. That means the more air the more faster the better. Turbulent air isn't a problem if it is all the same temperature. The only problem is differential refraction causes by differential temperatures.