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Equipment Discussions >> ATM, Optics and DIY Forum

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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: 14" Hubble Optics Project new [Re: careysub]
      #5481234 - 10/21/12 12:21 AM

Quote:

Although I cannot rule out the possibility that this will work - I would bet against it. For one thing, the support has to be (effectively) very, very close to the COG plane of the mirror. I cannot think of a reliable way of even determining where that is[...]

The only reason I have confidence in the 90 degree two-point schemes are the FEM and experimental data to back them up.




Yes, it's likely it won't work; but it seems simple enough to try, given that other solutions appear non-trivial, and these mirrors represent a great opportunity.

The basic assumption is that hanging a solid mirror at 2 points does at least as well as resting the mirror on two points. Intuitively, that seems a reasonable assumption to me, worth checking by FEM.

Like you, I want to know if anyone had modeled a hanging (solid) mirror, testing for sensitivity close to 45 degrees and front-to-back. If it's not too touchy, then proceed with an empirical test.

Practically, put the cable anywhere repeatable front-to-back and star test - potato-chip over the entire surface is easy to distinguish from other errors; it goes from oval in one orientation to oval 90 degrees away on the other side of focus (I assume that would be the main error when off f-t-b CoG). Adjust as needed.

Or not; it just seemed like an easy solution to build solidly if it worked out.


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: 14" Hubble Optics Project new [Re: derangedhermit]
      #5481924 - 10/21/12 01:14 PM

Quote:



The basic assumption is that hanging a solid mirror at 2 points does at least as well as resting the mirror on two points. Intuitively, that seems a reasonable assumption to me, worth checking by FEM.

Like you, I want to know if anyone had modeled a hanging (solid) mirror, testing for sensitivity close to 45 degrees and front-to-back. If it's not too touchy, then proceed with an empirical test.




Seems like it would even simpler to try a 90 degree support that directly engages the posts. Cut a semi-circular notch in a support strut of some kind. Due to the somewhat disorderly positioning of the posts they would have to be custom cut for each side. The problem of finding the COG plane would be the same (though adjusting strut positioning might be easier than cables).

But when supported a sheet mirror at the edge, you are only putting two edge points of a continuous strong, rigid body under local compression. With the posts you are putting a shear load on the post attachment points that they were never intended to take, which is then transferred to two points at the back of the actual mirror surface, sort and extreme case of a "glued" mirror. Very different.

The post attachment points might not even be strong enough to take that load.

For a sufficiently small Hubble Optics mirror one would be very tempted to simply glue the mirror to the cell and avoid these edge support issues entirely.

My 18" mirror is too large to do this, even cranking it up to 18 points of support (6 is fine with edge support), gluing in gives me a Strehl reduction of 0.90 at 45 degree tilt, and 0.82 at 80 degrees.

(Who observes 10 degrees above the horizon? I do, when chasing those elusive southern latitude objects, like Omega Centauri. Not the greatest viewing, but I would rather see what the atmosphere allows without my telescope getting in the way).


Edited by careysub (10/21/12 02:43 PM)


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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: careysub]
      #5482047 - 10/21/12 02:29 PM

Quote:

I'll send you a sketch of my plan for the vertical whiffle-tree edge support, with a few variations. Not really very complicated.




Thanks, much appreciated!


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: 14" Hubble Optics Project new [Re: starman345]
      #5482081 - 10/21/12 02:54 PM

Quote:

Quote:

I'll send you a sketch of my plan for the vertical whiffle-tree edge support, with a few variations. Not really very complicated.




Thanks, much appreciated!




How thick is your 14"?

Have you looked into how well you can do with adding support points and gluing into the cell? It doesn't work well enough with my 18" (thickness ratio about 9:1) but it might for yours.

Assuming your mirror is 40mm thick (same ratio) and you built an 18-pt cell and glued it in the Surface RMS at a 60 degree tilt is 9 nm, giving a Strehl reduction of 0.95. Not too shabby. Even at 75 degrees tilt it is 10.1 nm, or S=0.938.


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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: careysub]
      #5482152 - 10/21/12 03:41 PM

My mirror is 40.45mm thick giving a TR of 8.8:1 I'd never thought of glueing to the points. Glueing always seemed counter intuitive to me but I see what you are proposing might be a viable alternative. If the supports at 45 degrees are a failure I can easily unbolt them from the cell, remove them and glue each of the 18 points to the mirror surface. I've noticed that Dennis Steele does this with his hubble mirrors.
Thanks! this would simplify things for sure


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: 14" Hubble Optics Project new [Re: starman345]
      #5482406 - 10/21/12 05:44 PM

Using your actual mirror thickness, and secondary size, my tweaked calculation gives 9.86 nm RMS at 75 degree tilt, and S=0.94 (also S=0.95 at 60 degrees, and S=0.97 at 45 degrees).

Looks good.


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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: careysub]
      #5483475 - 10/22/12 11:39 AM Attachment (30 downloads)

I made a jig for drilling holes through the 1" tube for connection of my finder scope, bolted the base to the tube with two 1 1/2" 8x32 screws and nuts. I was afraid I might not be able to tighten the nuts enough to hold the heavy finder steady without crushing the 1" tube but everything turned out fine.

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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: starman345]
      #5483479 - 10/22/12 11:40 AM Attachment (29 downloads)

And with the finder mounted. I wonder how it will hold up after I hit my head on it in the dark a few times?

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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: 14" Hubble Optics Project new [Re: starman345]
      #5483733 - 10/22/12 02:05 PM

If you have any leftover tube, you could cut a piece about twice the length of the distance between the two nuts. Cut that piece in half. Drill two holes matching the bolt holes to make a large saddle washer that goes under the nuts. That should prevent buckling or tearing.

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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: 14" Hubble Optics Project new [Re: careysub]
      #5483739 - 10/22/12 02:11 PM

Quote:

Seems like it would even simpler to try a 90 degree support that directly engages the posts. Cut a semi-circular notch in a support strut of some kind. Due to the somewhat disorderly positioning of the posts they would have to be custom cut for each side. The problem of finding the COG plane would be the same (though adjusting strut positioning might be easier than cables).

But when supported a sheet mirror at the edge, you are only putting two edge points of a continuous strong, rigid body under local compression. With the posts you are putting a shear load on the post attachment points that they were never intended to take, which is then transferred to two points at the back of the actual mirror surface, sort and extreme case of a "glued" mirror. Very different.

The post attachment points might not even be strong enough to take that load.




You're right, it would be easier, and in either case, the posts might break or break loose. I thought the small loops would reduce potential side loading. I don't see an easy way to do that at the bottom. Thanks for considering it.


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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: derangedhermit]
      #5483798 - 10/22/12 02:47 PM

Quote:

If you have any leftover tube, you could cut a piece about twice the length of the distance between the two nuts. Cut that piece in half. Drill two holes matching the bolt holes to make a large saddle washer that goes under the nuts. That should prevent buckling or tearing.




Brilliant!, I'm going to do just that, thanks


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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: starman345]
      #5485150 - 10/23/12 12:06 PM Attachment (34 downloads)

Done! that noticeably stiffened things up.

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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: starman345]
      #5487071 - 10/24/12 02:01 PM Attachment (34 downloads)

More progress today, I installed the Protostar flockboard or whatever its called inside the secondary cage. I was nervous about doing this. According to the K&B book put the Kydex in the freezer first..since this isn't Kydex I'm using but Protostar I didn't know if I needed to do that or not. It was quickly evident that wouldn't work. As soon as I took it out of the freezer condensation began to form on it so no way it would stick to the tape on the inside of the secondary rings. So..I wiped it down, by this time it had warmed a bit so I began...
I used carpet tape on the secondary rings, outdoor carpet tape, that stuff is really sticky, hope I never have to take it off.


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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: starman345]
      #5487074 - 10/24/12 02:03 PM Attachment (31 downloads)

Since I could find only 1 1/2" wide tape I had to trim it with my utility knife, got glue goo all over the blade and had to clean it a few times but got it done.

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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: starman345]
      #5487078 - 10/24/12 02:06 PM Attachment (31 downloads)

Finally the big moment, I had to un-roll the Protostar onto the sticky tape, had to get it right the first time as there is no way to un-stick that stuff without pulling the tape off the secondary rings. It went ok, I had to trim off about 1/8" in a couple of spots but I think it was more good luck than good management.

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sopticals
professor emeritus


Reged: 03/28/10

Loc: New Zealand
Re: 14" Hubble Optics Project new [Re: starman345]
      #5487495 - 10/24/12 07:05 PM

Hi Brian,

Your on the steep learning curve (gets a little "slippery" at times).Keep up the good work (looks like your winning).

Stephen.(Down under 44deg.S.)


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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: sopticals]
      #5487550 - 10/24/12 07:47 PM

Thanks for the words of encouragement. Yes, steep learning curve for sure, a lot of things I've had to re-do because I didn't do it right the first time. I'm going to have a lot of extra pieces and parts leftover when-if this telescope ever gets finished

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thinairart
professor emeritus


Reged: 11/18/09

Loc: Denver, Co
Re: 14" Hubble Optics Project new [Re: starman345]
      #5488651 - 10/25/12 01:39 PM

Great progress Brian! Your scope project is starting to come together. I think your solution for mounting the Finder will work great. I used a separate post and in retrospect I would mount directly to the nearest tube if I did it again, saves the extra work.

The Protostar stuff is really nice. I used contact cement to install mine, which was a little more forgiving then sticky tape, but not by much.

Looking forward to seeing the rest of this project come together.


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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: thinairart]
      #5488917 - 10/25/12 04:29 PM

Thanks Aaron, yes, the Protostar material is nice, it is really black, black on the inside. The K&B book said to cut the material a couple of inches too wide and trim it after install but I took a chance and cut it the actual depth of the secondary cage, it worked out nicely and I think I would do it that way again. The piece I received from Protostar was 21" wide, my secondary cage 10 deep so I now have a piece left over that is 11" wide so I can use it on another scope.
I know you had some improvements you were thinking of doing to your dob, have you moved ahead with any of them yet?


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starman345
Wait, I'm Thinking
*****

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: starman345]
      #5545599 - 11/29/12 02:00 PM Attachment (21 downloads)

The mirror box has been under construction. First I cut the panels from 1/2" baltic birch and cut the various dadoes for everything to interlock.

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