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Equipment Discussions >> Reflectors

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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5622796 - 01/14/13 02:35 PM

How do you know? Look at the vent holes around the meniscus of high-end Cats. They don't appear to be much if any larger than 1/4".

However, if the holes I've drilled are ineffectual, I can always make them bigger or drill more. No big deal.

Mike


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rockethead26
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5622861 - 01/14/13 03:14 PM

Quote:



However, the fans run at full speed at 12V, and I do not yet have a power control connected that allows me to slow the fans down to a slower speed.
It's ironic, since the fans get their power through a Kendrick Control box that would allow me to slow the fan down with the simple push of a button. I merely need to plug the cables into another port on the box.
At the moment, the control cables are a little too short to do that, so it will entail my getting a short extension for the RCA cable so I can do that.

It's quite possible that, slowed to a mere gentle push, the scrubbing of the boundary layer would not result in a build up of turbulence in the image and return the image quality immediately, or even allow me to run the fans continuously. I need to perform a further experiment.






Don,

Asa Teeter owner, I had the same issue, where I would wait for the boundary layer to build up, stop observing and blow it off with the full-speed side fans, switch them off and then begin observing again.

That led me to do two things back in March last year.
1) Find quieter, low vibration fans and
2) Install a speed control

I described the project in another thread here.

I can run the fans full speed for cooling and then reduce them to 20-25% speed and allow them to run while observing. At the slower speed, it is only a gentle push of air and I can only assume that there is no turbulence buildup at the escape holes. The fans actually show no vibration issues at full speed at 200x (the highest I've been able to use here in the mid-west), but after the majority of the cooling is done, there is no need to run them that fast.

All I know is that it works really well. The benefits are
1)I do not have to interrupt my observing to clear the boundary layer and
2) the boundary layer does not return.


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Pinbout
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5622871 - 01/14/13 03:19 PM Attachment (9 downloads)

Quote:

What's fun about these forums is reading all the different ways people have solved various problems in contemporary scopes.






just a thought cause someone did mention about dyson.

one thing about james is he'll tell how much conventional fans just chop the air in chuncks and never moves the air smoothly even at low rpm's the fans are still sending air in chucks.


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5622887 - 01/14/13 03:30 PM

Quote:

Don,

Quote:

And what works for smaller mirrors may not work at all for larger mirrors.




Yes, exactly. That is why I keep trying to steer the thread back to more moderate-sized, solid-tube Dobs - which is, after all, what the OP was concerned with. What works for a 20" truss scope will not necessarily - probably won't - work for an 8" or 10" solid-tube. But speaking about all these different examples does carry the discussion along and might stimulate ideas for solutions for Dobs of various types and scales.

Quote:

And the demands of cooling could be quite different than the demands of scrubbing the boundary layer. It's why I'm beginning to believe fans need to have a variable speed control.




Yes and yes.

Quote:

I could tell you about the scary "swiss cheese" SCT I saw--not only open tube, but the tube was reduced to only a very open lattice work. He completely solved the thermal issues, but injected a healthy dose of fragility.




Here ya go:

The Holey Telescope

And I'm reluctant to put just one moderate-sized hole in the side of my OTA!


Mike



This guy's SCT had roughly 4" x 4" holes with 1" of metal in-between all the way around the tube! He covered the holes with black felt once the scope cooled down (I imagine it cooled quickly!). Scary.


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Starman1
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Re: Thermal Issues and Fans Successes new [Re: rockethead26]
      #5622911 - 01/14/13 03:41 PM

Jim,
Thanks. I think the vibration from the fans won't be noticeable if I slow them down, so it's merely a matter of running the fan cable into an adjustable port in the Kendrick controller.
I still don't understand why the fan vent holes have grilles on them, but if they don't cause a slow-down of the air exiting the box, I'll leave them on.
It's another case of experimenting with them on and off to see which is more effective.


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rockethead26
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5622921 - 01/14/13 03:47 PM

Quote:

Jim,
Thanks. I think the vibration from the fans won't be noticeable if I slow them down, so it's merely a matter of running the fan cable into an adjustable port in the Kendrick controller.
I still don't understand why the fan vent holes have grilles on them, but if they don't cause a slow-down of the air exiting the box, I'll leave them on.
It's another case of experimenting with them on and off to see which is more effective.




Will the Kendrick controller allow you to do that? I never experimented with it because Rob told me that wouldn't work. My understanding is the the "adjustable" ports, simply cycle the power at variable rates, but do not reduce the voltage. I never would have installed the controller if that indeed works. You'll have to experiment and see. I think I even remember Kendrick at NEAF in 2011 telling me they were working on a new controller that would add that capability. I could be wrong...

I haven't experimented with removing the grills yet, either. A future test may be in order.


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demiles
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Re: Thermal Issues and Fans Successes new [Re: rockethead26]
      #5622950 - 01/14/13 04:01 PM

Most dew controllers will not work for controlling fan speeds. Use a pwm fan and a controller.

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johnnyha
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Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5623106 - 01/14/13 05:27 PM

Quote:

1/4" holes won't do squat....the holes will have to be bigger for air to get out or it will just bounce around in the tube.




In a passive system without fans though, I think 1/4" holes would be beneficial in letting warmer air vent.


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5623182 - 01/14/13 06:19 PM

Quote:

I wonder if there is any difference in effectiveness for scrubbing the boundary layer depending on whether a fan pulls air directly onto the surface of the primary from above or across the surface from the side? It seems the former would result in air striking the center of the primary's surface and flowing radially out toward the sides, while the latter produces a laminar flow from one side of the surface to the other. But wouldn't air striking directly at the primary's surface result in more turbulence? Wouldn't the side fan produce a more stable image?

Mike




Well Mike, the benefit of the Sid fan is its blowing outside immediate ambient temp air into the mirror. A fan inside the tube is blowing down what's more than likely already been own over the mirror itself which more than likely isnt absolute am isn't air temp.

The question is though : does it matter? And to that end I do t think so - not in an measurable way. The radial airflow across the face would seem better as its so even handed but for all that I've looked the swath of air coming off the 80mm across a 203mm mirror is just as even handed. There isn't a lane or path of good air surrounded by bad air observed ever. The complete face of the mirror is scrubbed clean of boundary air with temperature homogenous flow.

I think both methods are excellent with larger mirrors demanding radial cooling from the center .

Pete


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5623189 - 01/14/13 06:25 PM

Quote:



Yes, exactly. That is why I keep trying to steer the thread back to more moderate-sized, solid-tube Dobs - which is, after all, what the OP was concerned with. What works for a 20" truss scope will not necessarily - probably won't - work for an 8" or 10" solid-tube. But speaking about all these different examples does carry the discussion along and might stimulate ideas for solutions for Dobs of various types and scales




I agree Don. The moment the glass gets big, thick and then trussed and what have you the dynamic changes. Some fan is better than none but different solutions for different size and designs are certainly in order.

Pete


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Sarkikos
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Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5623401 - 01/14/13 08:45 PM

Pete,

That was my quote about "trying to steer the thread back."


Mike


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azure1961p
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Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5623465 - 01/14/13 09:33 PM

Noted!

P.


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De Lorme
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Re: Thermal Issues and Fans Successes new [Re: rockethead26]
      #5623784 - 01/15/13 03:08 AM

Hi Jim , I'm De Lorme, What size are your side and rear fans and how much cfm? Also where did you find your speed control?
Thanks De Lorme


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rockethead26
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Re: Thermal Issues and Fans Successes new [Re: De Lorme]
      #5623959 - 01/15/13 08:13 AM

Hi De Lorme,

I only have side fans installed. Rob came out with the bottom fan option after my scope was built. I'm considering its addition after I return to my home in Flagstaff. If you follow the link in my earlier post and the follow the link to the fans I used, all the specs are there.

I found the speed control on eBay. The link in my original project posting goes to an "ended" auction with no picture. However, it was exactly like the one in the following link, except with three knobs to control up to 3 fans. I'll use the third connection when I add the rear fan.

http://www.ebay.com/itm/Computer-CPU-3-Pin-Fan-Cooler-Speed-Controller-12V-DC...

Sorry for continuing to discuss truss dob cooling here, but the fan and controller info may be useful to the OP and others.


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johnnyha
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Re: Thermal Issues and Fans Successes new [Re: rockethead26]
      #5624026 - 01/15/13 09:04 AM

Does this make sense?

With tube dobs, one popular scenario has the rear fan(s) blowing against the back of the mirror and up through the tube exiting the top in a laminar flow, with or without side fans/vents.

With a truss dob and mirror box however, because of the mirror box top baffle, and because the bottom of the mirror box is usually open, the laminar flow of the tube dob is best duplicated by reversing the flow of air - having a fan blowing on/across the top of the mirror and a rear fan that is exhausting air out the bottom of the mirror box. Make sense?


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Pinbout
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Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5624046 - 01/15/13 09:23 AM

in either scenario what is causing the air to move in straight even flow of parallel layers [laminar]?

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johnnyha
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Re: Thermal Issues and Fans Successes new [Re: Pinbout]
      #5624052 - 01/15/13 09:26 AM

Quote:

in either scenario what is causing the air to move in straight even flow of parallel layers [laminar]?



The lack of obstacles?

In the tube, it's open on the top end and once getting past the mirror the air is free to escape out the top.

In the mirror box however there is a baffle on top that prevents the free laminar flow of the air out the top, whereas at least in the case of my Obsession, the bottom of the mirror box is open and air can flow freely out the bottom.


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Pinbout
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Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5624188 - 01/15/13 11:03 AM

this is cool and has little to do with this, almost.

but you know the textured walls of the tube we so love for absorbing light are not so good a lot for the movement of air


http://www.youtube.com/watch?NR=1&feature=endscreen&v=WG-YCpAGgQQ


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Pinbout
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Re: Thermal Issues and Fans Successes [Re: johnnyha]
      #5624192 - 01/15/13 11:07 AM

check out this turbulence

http://www.youtube.com/watch?v=vQHXIHpvcvU

http://www.youtube.com/watch?v=0H63n8M79T8



Edited by Pinbout (01/15/13 11:10 AM)


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azure1961p
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Re: Thermal Issues and Fans Successes [Re: Pinbout]
      #5625198 - 01/15/13 09:04 PM

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


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