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Equipment Discussions >> Reflectors

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Pinbout
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5625286 - 01/15/13 10:00 PM

Don thinks a slow moving air helps produce a laminar flow, and his intuitions are good cause if you reduce the pressure it reduces the Reynolds number which lowers the turbulence.

Bobs needs the front fan to create turbulence to help create shear strength to break up the thick boundary layer.

I had a really crazy idea, what if you put suction cup dart right in the center of the mirror to help disrupt the thick boundary layer. And on the dart have some spiral vanes. It will lower the area of the boundary layer lowering the amount of shear needed to break it up.

Another really nutty idea, how about rifling inside the tube walls to help create a spiral of air moving up instead of chunks curling back on themselves.


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nevy
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5625325 - 01/15/13 10:18 PM

Quote:

I don't think laminar airflow ever occurs in a telescope tube anyway. Too many points of turbulence. Air blowing to the back if the mirror produces a giant eddy on the other side (hence the need for another fan) and from there it tumbles all the way up the tube. I don't think laminar airflow is possible at all here really. What's key in the face of that however and what nulls it out is the relative uniformity of temp of the air coming off the optics after proper equilibrium allowed. It still oodles of eddies but uniformity of airtemp its relatively transparent to invisible.

Pete



Won't the eddies curling round the front of the primary help push a little cooler air onto the face of the mirror and help with the boundary layer problem


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5625469 - 01/15/13 11:48 PM

That may be what was happening with Pons' scope--the fan behind the mirror blew up the tube and small eddies that may have formed in front of the mirror broke up the boundary layer. Because of the circular nature of the air movement, and the lower pressure of the more rapidly moving air by the tube wall, the heated air moved like a cyclone to the inside of the tube wall, where positive pressure forced the air up the tube.

Because the dramatic improvement of the images from a mirror that could not possibly have been cooled down yet showed that the rear mounted fan blowing up the tube made a HUGE difference--for the better.

I should note that there was a lot of clearance between the mirror and tube wall--more than any commercial scope I've seen--probably 1.5". That may have kept a lot of the air-up-the-tube movement out of the optical path.


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nevy
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5625825 - 01/16/13 09:15 AM

I have noticed with my 16" light bridge that planetary views gets sharper with the standard rear blowing fan on & I also noticed one day when I was viewing Venus just after I set the scope up in twilight ( the mirror not cooled down proply) the fan had been running for about 5-10 minutes but Venus was very sharp in fact it was one of the best views of it I've had.

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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5625851 - 01/16/13 09:31 AM

Quote:

Quote:

I don't think laminar airflow ever occurs in a telescope tube anyway. Too many points of turbulence. Air blowing to the back if the mirror produces a giant eddy on the other side (hence the need for another fan) and from there it tumbles all the way up the tube. I don't think laminar airflow is possible at all here really. What's key in the face of that however and what nulls it out is the relative uniformity of temp of the air coming off the optics after proper equilibrium allowed. It still oodles of eddies but uniformity of airtemp its relatively transparent to invisible.

Pete



Won't the eddies curling round the front of the primary help push a little cooler air onto the face of the mirror and help with the boundary layer problem




Didn't do anything. The thermal shadows viewed in out of focus stars was there, a little lessened but still there. There is a ledge folks put in front if the mirror that actually overlaps it slightly and this helps to vector the air over the boundary layer. At least from what I've seen it looks effective. Without this ring the boundary layer is untouched. It just sits there boiling away as the eddies are too weak and the heat off the mirror to strong. Even when my boundary layer fan was just above the mirrors surface - I mean millimeters - it did totally nothing. There was no vortice helping to peripherally suck the layer away, it just sat there. The surface needed direct air blowing the boundary layer off. Close wasn't good enough at all. You'd think the slightest zephyr would blow away this tenuous layer but its a heat property of the glass that needs treating.

Pete


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Pinbout
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5625871 - 01/16/13 09:41 AM

Quote:

There is a ledge folks put in front if the mirror that actually overlaps it slightly and this helps to vector the air over the boundary layer. At least from what I've seen it looks effective.




that creates a plenum which changes the pressure to help create the shear strength needed to get reduce the massive boundry layer.

yes very effective.


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Mark Peterman
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5625890 - 01/16/13 09:59 AM

Quote:

There is a ledge folks put in front if the mirror that actually overlaps it slightly and this helps to vector the air over the boundary layer.




This ledge was used with a fan that was sucking air out the bottom end of the tube. Was any test done with the airflow reversed?

In that scenario (reversed), it seems like the air would be forced over the face of the mirror before turning up, around the edge of the ring, to exit the front end of the tube, pulling warmer air with it.


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Pinbout
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Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5626029 - 01/16/13 11:21 AM

Quote:

Was any test done with the airflow reversed?






Yeah, some dude did a test blowing air in and with the baffle ring the smoke dissappear in a blink of an eye.

He didn't test it being sucked out.

but that doesn't really work for my very open bottom [no pressure change]



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SteveG
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Re: Thermal Issues and Fans Successes new [Re: Pinbout]
      #5626255 - 01/16/13 01:59 PM

Here's some videos I made 3 years ago, staring with the stock fan, then adding a rear baffle, baffle seal, then a ring in front. I need to do more testing visually before I can comment any further on the planetary performance.

http://www.youtube.com/watch?v=vslnABcyi8k&list=PL2BD34EF33CC80FF3


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Pinbout
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Re: Thermal Issues and Fans Successes new [Re: SteveG]
      #5626271 - 01/16/13 02:10 PM

what are you using for smoke?

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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Pinbout]
      #5626302 - 01/16/13 02:31 PM



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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5626303 - 01/16/13 02:32 PM



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Scanning4Comets
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5626588 - 01/16/13 05:02 PM

In video test #7 @ 0:49 you can see the smoke moving towards the center of the mirror which is where the boundary layer would be...so my take on the back completely covered and the ring baffle on the inside will work to scrub the boundary layer!

Great videos Steve!

Cheers,


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Bob S.
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5626628 - 01/16/13 05:22 PM

Absolutely great videos Steve, I would love to see how the smoke would work placed into the tube if you reversed the fan and made it a sucking fan rather than a blowing fan. Very nice demonstration of some of the phenomenon that we are dealing with. Bob

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Project Galileo
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Re: Thermal Issues and Fans Successes new [Re: Bob S.]
      #5626649 - 01/16/13 05:33 PM

Great videos Steve. I have seen these before and love them. Seeing the baffle/ring work is believing. I concur with Bob too. I would love to see how the smoke test would work with a sucking fan vs. blowing fan on a sealed back telescope with the baffle/ring.

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rlmxracer
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Re: Thermal Issues and Fans Successes new [Re: Project Galileo]
      #5626700 - 01/16/13 06:03 PM

Quote:

Great videos Steve. I have seen these before and love them. Seeing the baffle/ring work is believing. I concur with Bob too. I would love to see how the smoke test would work with a sucking fan vs. blowing fan on a sealed back telescope with the baffle/ring.




That is the setup I have that seems to work well.
If I can find out what he used for the smoke I could try to get some video of my setup in action.


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Project Galileo
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5626719 - 01/16/13 06:13 PM

In a previous post about this smoke test Steve states, "The smoke is created using a ventilation smoke tube, which is small plastic tube with chemicals and a bulb at one end. You break (2) glass ampules inside the plastic tube, and use the bulb to push the chemicals together and out the tube, creating white smoke. We use them in testing chemical fume hoods."

Here is that link to the other thread.


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5626726 - 01/16/13 06:16 PM

Again with the sucking fan issues...

When your reflector is pointed at the zenith or near it, and your face is against the tube essentially as you observe through the eyepiece, your warm breath rises straight up along the tube, over the end ring and down into the OTA. From there as it migrates down the tube it blurs your views all to hell. You will need to heat your secondary mirror here too since the ambient air being sucked by it chills it so few formation is now occurring.
Unless your reflectors secondary is waaaayyyyyy down the tube it's problematic on a number of accounts. An aperture ring will not deflect air across the boundary layer either if its sucking from behind rather than blowing.

It's simply not a winning combo .


Blow it. Period.

Pete


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Mark Peterman
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Re: Thermal Issues and Fans Successes [Re: rlmxracer]
      #5626729 - 01/16/13 06:19 PM

The annulus seems to work well, however, the smoke did seem to linger in the middle of the tube and not get 'blown' out the top very fast (and this was a short tube).

Just thinking outloud...

What if the annulus, instead of a 360 degree ring, was just 180 degrees (from 9 o'clock to 3 o'clock). The air blown from the annulus side would move across the mirror, and the air coming from the opposite side woud provide the propulsion to lead it faster, up the tube and closer along the top edge.


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Project Galileo
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Re: Thermal Issues and Fans Successes [Re: azure1961p]
      #5626744 - 01/16/13 06:27 PM

Pete,

I hate to pick at the issue but that is not what happens at all. I can say without exception I have never seen that happen with the sucking system I employ. I have never had problems with body heat, breath, or any heat entering the tube from the front with my sucking system causing problems of any kind. If it happens at all, the small amount that it may, it neither degrades the view nor causes dewing or any problems with the secondary. In fact, the opposite is what I find. The moving air keeps the secondary clear without heaters. It just isn't the case. I have employed this system for quite some time over thousands of hours viewing. It just doesn't happen that way in my experience.


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