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Crazy mirror box idea that might just work

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#1 Dick Jacobson

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Posted 01 August 2020 - 10:58 AM

Start with a thin spherical mirror resting in a flimsy square frame. Set a round post on top of the center of the mirror. Attach the top of the post to the four corners of the frame with wires. Tighten the wires. Now the force of the post pushing on the center of the mirror parabolizes it, while the wires stiffen the frame. You end up with an ultra-cheap mirror in an ultra-light mirror box. The center post has no effect on the image since it is in the shadow of the secondary. The wires create a small amount of diffraction similar to a spider.

 

I believe this could work since people have produced excellent "flex mirrors" by pulling on the back of spherical mirrors. The pulling systems have to be carefully designed but are not hard to build. I don't know whether anyone has tried a center-push instead of a center-pull system. Maybe it would be better to drill a hole in the center of the mirror and extend the post to the back. I believe it would be possible to build a back-support cell that would correctly support the mirror so it would approximate a paraboloid. Unlike a PLOP design, the support would be biased towards the center.

 

I came up with this maybe-not-crazy idea while thinking about ways to stiffen an ultralight mirror box. One just-plain-crazy idea was to build a pyramid in front of the mirror, which would create a lot of light blockage and diffraction.

 

For more about flex mirrors see https://www.cloudyni...nto-a-parabola/.

 

Also https://skyandtelescope.org/wp-content/uploads/Adler-MirrorFlex.pdf.

 

Also https://groups.google.com/forum/#!topic/sci.astro.amateur/Qh05z1ZOgoU.


Edited by Dick Jacobson, 01 August 2020 - 11:04 AM.

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#2 Pinbout

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Posted 01 August 2020 - 11:18 AM

the mirror needs a round aperture to be pressed against, or it will cause flat spots if you really mean a square box...



#3 Astrojensen

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Posted 01 August 2020 - 11:20 AM

The problem is that in order for the flex to work properly, the force needs to be distributed over a wide surface and not a center point. Alan Adler points out in his article that this was a major flaw in the first designs.

 

And how are you going to prevent the lightweight frame from flexing and creating astigmatism?

 

How are you going to ensure the wires are equally taut? If they're not, the center post will not push straight down, creating astigmatism. 

 

How do you ensure the long, thin wires are going to stay taut in in different temperatures, while maintaining the same pressure on the mirror?

 

How do you adjust the mirror pressure? 

 

 

Clear skies!
Thomas, Denmark



#4 mark cowan

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Posted 01 August 2020 - 12:05 PM

Aside from those things mentioned (none of them trivial) you don't really get to start with a "cheap" mirror because the starting figure has to be a very accurate smooth conic, and that's never cheap even for a sphere.  This is why flex mirrors really never took off IMHO.  :shrug:



#5 Dick Jacobson

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Posted 01 August 2020 - 01:54 PM

The problem is that in order for the flex to work properly, the force needs to be distributed over a wide surface and not a center point. Alan Adler points out in his article that this was a major flaw in the first designs.

 

And how are you going to prevent the lightweight frame from flexing and creating astigmatism?

 

How are you going to ensure the wires are equally taut? If they're not, the center post will not push straight down, creating astigmatism. 

 

How do you ensure the long, thin wires are going to stay taut in in different temperatures, while maintaining the same pressure on the mirror?

 

How do you adjust the mirror pressure? 

 

 

Clear skies!
Thomas, Denmark

I understand that the force would be too concentrated in the center if you just had a center push and rim support in the back. It seems to me that it would be possible to design a back support that compensates for this. Maybe too complicated to be practical, though. The frame would be lighter than a conventional mirror box though not "flimsy" (bad word choice). The center post would have a screw type tension adjustment, not the individual wires.

 

Many potential problems, many possible solutions. It's interesting, though, that in the third reference Gary Seronik says that his flex mirror is the best he has ever made and probably the best he has ever tested.



#6 KLWalsh

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Posted Yesterday, 08:40 AM

Aside from those things mentioned (none of them trivial) you don't really get to start with a "cheap" mirror because the starting figure has to be a very accurate smooth conic, and that's never cheap even for a sphere. This is why flex mirrors really never took off IMHO. :shrug:


Agree.
And by the time you’ve expended the effort to get a spherical mirror surface, you might as well take the extra bit of effort to make a parabola.


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