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Equipment Discussions >> ATM, Optics and DIY Forum

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davidpitre
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Reged: 05/10/05

Loc: Central Texas
Re: Boundary layer fan position relative to primary new [Re: Chuck Hards]
      #5842380 - 05/05/13 08:22 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?

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Chuck Hards
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Reged: 05/03/10

Loc: The Great Basin
Re: Boundary layer fan position relative to primary new [Re: davidpitre]
      #5842523 - 05/05/13 09:39 PM

Quote:

"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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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: Boundary layer fan position relative to primary new [Re: Chuck Hards]
      #5842621 - 05/05/13 10:44 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.

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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Boundary layer fan position relative to primary new [Re: Pinbout]
      #5842706 - 05/05/13 11:59 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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Chuck Hards
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Reged: 05/03/10

Loc: The Great Basin
Re: Boundary layer fan position relative to primary new [Re: don clement]
      #5842765 - 05/06/13 12:32 AM

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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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Boundary layer fan position relative to primary new [Re: Chuck Hards]
      #5842805 - 05/06/13 12:52 AM

Quote:

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.




I disagree on everything you’ve stated. First this is not the same compliant mechanism as is used to couple mis-aligned shafts. This mechanism allows for tip-tilt but not for axial movement.
Second it is truly floating w.r.t. to tip-tilt. Another type of solid machined compliant mechanism replaces the Nylon mounting pads that allows for sideway “floating” without stiction or need for lubricant such as graphite. In addition there is no need for a perfictly smooth and flat mirror back in order to float since all lateral movement is inside the solid and does not depend on a smooth flat surface to slide against.

This compliant mechanism works as well on small mirrors as on larger mirrors. Guess compliant mechanisms don’t fit your paradigm of how stiction works to inhibit movement at the nano level in the real world.

BTW I only responded to your sidetracking of the Ops post.

Don Clement


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Chuck Hards
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Reged: 05/03/10

Loc: The Great Basin
Re: Boundary layer fan position relative to primary new [Re: don clement]
      #5843128 - 05/06/13 09:18 AM

Well, good morning to you too. I really did mean to complement you on your design, so I hope you don't disagree with that.

That said, if I've misunderstood your mechanism I apologize. However I do maintain that stiction in this instance simply isn't an issue in a smallish Newtonian, at least not as far as a star-test of a mirror mounted such can reveal to my eye. If you say it works on small mirrors, I'm not going to disagree with you.
I have no idea who "strock" is, I tend to dip in and out of this board, and the hobby-at-large, at irregular intervals when life gets in the way of the fun stuff. I can't remember my original CN screen name or password from years ago, thus my recent re-reg.

Most everything I've posted was related to fan use for removing the boundary layer, specifically how in a closed tube on a smaller scope a rear-mounted fan can accomplish it. The mirror cell details and float detail sketch were posted in response to a direct request by others in the thread, so it's not as if I just volunteered them out of the blue.

If you have detailed drawings of your device, I would love to look them over. Have you posted details elsewhere on the forum? I'm not married to my ideas. Change my mind. I look forward to exploring it more fully in a dedicated thread.

Thanks, and have a great day.


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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Boundary layer fan position relative to primary new [Re: Chuck Hards]
      #5843196 - 05/06/13 10:08 AM

Chuck,

Most of the really good small mirrors less than 14” that I have seen don’t even use a whiffletree support but are conical and/or ribbed and supported by a central arbor and then the central arbor is a compliant mount.

Back on topic: An electrostatic air mover would work to help homogenize the boundary layer without the vibration of a mechanical fan. Here is a crude example of an electrostatic air mover run from 12V battery http://www.youtube.com/watch?v=zkAJQ0Ggh_Y

Don Clement


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Chuck Hards
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Reged: 05/03/10

Loc: The Great Basin
Re: Boundary layer fan position relative to primary new [Re: don clement]
      #5849922 - 05/09/13 12:22 PM

Don, please bear with me. I'm an old-timer with a basement full of optics and components that are earmarked for projects. I don't keep up with the high-tech end of telescope design and the latest concepts, mostly due to lack of time. My equipment is state-of-the-art...for the 1970's, lol. A deeper involvement simply has to wait until retirement, unfortunately. At least 8 more years.

The electrostatic air mover is interesting, thanks for posting the link. My questions are, could the high current draw be lessened for a portable setup, and if used for boundary-layer purposes, what would the ion bombardment do to the mirror coating over time, if anything?


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