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Equipment Discussions >> ATM, Optics and DIY Forum

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thinairart
professor emeritus


Reged: 11/18/09

Loc: Denver, Co
Re: 14" Hubble Optics Project new [Re: sopticals]
      #5360670 - 08/09/12 08:47 PM

Creative way to handle the different platter diameters! I'm very interested in how those edge supports perform under the stars. Do you find that the edge supports keep good contact regardless of orientation of the mirror (rotating mirror in the cell) ?

You'll definitely have to push on towards finishing the scope now that you are without a scope. That sharp looking Ethos is hungry for photons!


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Ian Robinson
Post Laureate


Reged: 01/29/09

Loc: 33S , 151E
Re: 14" Hubble Optics Project new [Re: thinairart]
      #5360786 - 08/09/12 10:09 PM

Quote:

Creative way to handle the different platter diameters! I'm very interested in how those edge supports perform under the stars. Do you find that the edge supports keep good contact regardless of orientation of the mirror (rotating mirror in the cell) ?

You'll definitely have to push on towards finishing the scope now that you are without a scope. That sharp looking Ethos is hungry for photons!




Me too.

Very interesting how you've tackled edge support.

Q How heavy is the 14" Hubble mirror ?

Q will the timber used for edgesupport be strong enough, wont it tend to compress a bit over time throwing colimation off and making the mirror loose ?

Would teflon or solid aluminium lathed to the correct diameters for the upper and lower platters be better ?


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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]
      #5361310 - 08/10/12 09:04 AM

Quote:

I cant imagine being "scopeless"




Stephen, its been mostly cloudy since I sold it so I don't miss it, but I know I will the first clear still night....


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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]
      #5361318 - 08/10/12 09:09 AM

Quote:

Do you find that the edge supports keep good contact regardless of orientation of the mirror (rotating mirror in the cell) ?



Yes, they do seem to be in good contact regardless of rotation. The two plates although different in diameter, seem to be centered in relation to each other. Is your's similar or are the top and bottom off-set?


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

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: Ian Robinson]
      #5361344 - 08/10/12 09:23 AM

Quote:

Q How heavy is the 14" Hubble mirror ?



My 14" is 17 lbs 11 ozs

Quote:

Q will the timber used for edgesupport be strong enough, wont it tend to compress a bit over time throwing colimation off and making the mirror loose ?



Good question, I don't know the answer, the dowels I used are older and dried out minimizing shrinkage but I think a more solid material would be a better choice.
Quote:

Would teflon or solid aluminium lathed to the correct diameters for the upper and lower platters be better ?



I think with the difference in diameters of upper and lower platters, something that can be adjusted would be more ideal. I had thought about nylon bolts threaded through a bigger upright support bolt, one nylon bolt for upper plate, one for lower. I may try that later if the dowel idea fails.

There are several challenges with these mirrors, not only the individual edge supports but also there is really no way to find COG...at least that I know of. Also my upper plate is quite a bit thicker than the lower one
And to top it off, the Hubble is about 25% lighter than a conventional solid mirror causing balance issues


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: 14" Hubble Optics Project new [Re: starman345]
      #5361485 - 08/10/12 10:50 AM

Quote:

Good question, I don't know the answer, the dowels I used are older and dried out minimizing shrinkage but I think a more solid material would be a better choice.





paint the wood and put some teflon on them.

grey pvc rods are tyically used at those locations when doing a sling, to stop the sling from sliding side to side.

I like your solution btw.


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

Reged: 07/06/10

Loc: New Brunswick, Canada
Re: 14" Hubble Optics Project new [Re: Pinbout]
      #5361639 - 08/10/12 12:33 PM

Quote:

paint the wood and put some teflon on them.




I don't follow? teflon on the dowels? Pieces of teflon? Or something else?


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: 14" Hubble Optics Project new [Re: starman345]
      #5361682 - 08/10/12 01:02 PM

Quote:

teflon on the dowels? Pieces of teflon?




Oui. you don't want the painted wood giving friction to the edge.


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Gene7
scholastic sledgehammer


Reged: 06/10/10

Loc: Mid Ohio, USA
Re: 14" Hubble Optics Project new [Re: Pinbout]
      #5362410 - 08/10/12 08:32 PM

Starman:
Oh yes, very nice scope and beautiful machine work. I also have a Hubble 14 inch mirror that is not yet in a scope. It looks similar, but the two disks are about the same size but are off-set by perhaps a 1/16 inch. Have been figuring how to best support them. Sorry, I am not at all comfortable with what I have seen as solutions so far. I think it is necessary that the weight be divided equally, as all other support components of the mirror are.

My current plans are to shim the edges of the mirror so they are both the same height at the support points and devise a simple lever clip that slides on a support cable to keep the load distributed equally. Perhaps 5 or 7 clips would be used.

Another solution for your support method would be to use a medium soft rubber dowel section, a different size for each side. Thus you could get very close to equal support for each slab, which can not be done with rigid wood or plastic dowel sections. Thanks again for posting the lovely work. Gene

Edited by Gene7 (08/10/12 08:34 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: Gene7]
      #5362972 - 08/11/12 08:18 AM

Hi Gene, thanks for your comments, all ideas are very welcome.

Quote:

It looks similar, but the two disks are about the same size but are off-set by perhaps a 1/16 inch.



It seems these mirrors are not all the same, I think Aaron(thinairart) mentioned his is similar to yours, mine on the other hand is not off-set, but there is a difference in platter diameter.

Quote:

Sorry, I am not at all comfortable with what I have seen as solutions so far.



Me either, but for lack of a better idea I'm forging ahead with an eye to re-doing things later if need be

Quote:

I think it is necessary that the weight be divided equally, as all other support components of the mirror are.




Yes, I agree here, but how to do that? My upper platter is significantly thicker than the lower one and just a bit larger in diameter...how to figure where COG is? Where and how much pressure do you put on each platter to equalize and how do you accomplish that accurately?
Pictures I've seen of the Hubble Optics UL16 shows they use a simple sling spanning both platters, nothing fancy, so I wonder if all the fussing I'm doing over this support issue is warranted at all.

Quote:

My current plans are to shim the edges of the mirror so they are both the same height at the support points and devise a simple lever clip that slides on a support cable to keep the load distributed equally. Perhaps 5 or 7 clips would be used.



This sounds interesting, can you give more detail? Are you talking about a cable sling? I'm not sure what you mean by lever clips?

Quote:

Another solution for your support method would be to use a medium soft rubber dowel section, a different size for each side. Thus you could get very close to equal support for each slab, which can not be done with rigid wood or plastic dowel sections. Thanks again for posting the lovely work.




Would soft rubber deform over time and introduce friction?
I ordered 1" PVC rod yesterday to replace the wood dowel, I thought that would be less likely to deform and not so susceptable to humidity and temperature changes.
Thanks for the comments Gene, gives me food for thought


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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]
      #5478788 - 10/19/12 12:55 PM Attachment (36 downloads)

I'm trying to decide how to mount my Stellarvue finder to the secondary cage. Looking at pictures of various setups here and on other forums most people seem to use some type of post installed just for the finder. I'm using 1" hollow tube to separate the cage rings, same material I'm using for the truss tubes..why couldn't I drill a couple of holes in the tube and mount the finder directly to the tube nearest the focuser? I'm just wondering if there is a down side to doing this, since I dont' see it being done. Opinions? I just have the finder leaning against the tube in the picture, trying to show where I'd like to position it.

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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: 14" Hubble Optics Project new [Re: starman345]
      #5479055 - 10/19/12 03:47 PM

Please tell me why you decided to orient the focuser knobs as they are.

There is an alternative for your secondary cover, especially popular in the southern US. It costs three times as much, but some people find the expense worth it. The product is called Blanton's.


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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: 14" Hubble Optics Project new [Re: derangedhermit]
      #5479074 - 10/19/12 04:01 PM

It looks to me like an easy alternative for this type mirror would be to hang it, instead of sitting it on something. Has anyone checked to see if two hanging points, 90 degrees apart, might work? I'd want to consider small cable loops around two of the interior posts on the mirror, attached to two posts.

EDIT: I mean for edge supports.


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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]
      #5479084 - 10/19/12 04:11 PM Attachment (23 downloads)

Quote:

Please tell me why you decided to orient the focuser knobs




You are right, the focuser is upside down, I guess, if there is a "right side up" but it really doesn't matter, knobs still work. If I mount it the other way I don't have clearance for my upper truss connector cam action lever.


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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]
      #5479152 - 10/19/12 05:12 PM Attachment (23 downloads)

I went with 1" solid PVC instead of wood dowel that I had used in the beginning. I don't know how hanging the mirror from the internal posts would work. It might be fine.
Any opinions on connecting my Stellarvue finder to the 1" tube?


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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: 14" Hubble Optics Project new [Re: starman345]
      #5479870 - 10/20/12 02:58 AM

If you're sure that's where you want it, I don't see anything wrong with drilling holes and mounting it there. At worst you move it later and have a few holes, or replace the tube section.

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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]
      #5480411 - 10/20/12 01:34 PM

Yea, I think I'm going to try it on the tube. I really don't want to add another piece to the secondary cage it is already pretty heavy.

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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: 14" Hubble Optics Project new [Re: derangedhermit]
      #5480531 - 10/20/12 02:48 PM

Quote:

It looks to me like an easy alternative for this type mirror would be to hang it, instead of sitting it on something. Has anyone checked to see if two hanging points, 90 degrees apart, might work? I'd want to consider small cable loops around two of the interior posts on the mirror, attached to two posts.

EDIT: I mean for edge supports.




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 in the post-filled gap between the mirror plates.* Even if you knew it, could you actually secure a cable in the correct position? And would you have any confidence that supporting the mirror weight at two interior posts (located somewhat randomly, not precisely designated) would create an astigmatism free stress pattern in the mirror surface?

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

I suggest the best may to support these mirrors is a vertical whiffle-tree, supporting the front and back plates so that the mirror will level out on its own. For my 18" HO mirror I really have no choice but to pursue this option since the front and back plates have different diameters, but I think it might be the most reliable method for all large HO mirrors.




*Well I can, but it would be difficult. Determine the volumes of the glass for front and back plate, and post layer using Euclid's method of displacement, then mathematically model the mirror and determine where the mid-plane must be.


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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]
      #5480961 - 10/20/12 08:33 PM

Quote:

Well I can, but it would be difficult. Determine the volumes of the glass for front and back plate, and post layer using Euclid's method of displacement, then mathematically model the mirror and determine where the mid-plane must be.



The longer I ponder on how to support this mirror the more I realize how complex it could be to do it properly, if that is possible at all. I was looking over my 14" HO mirror tonight, it has one plate bigger in diameter than the other, that being the "mirror" plate but of course it has a curve in it so probably doesn't contain any more glass (or much more) than the flat thinner plate on the back. The plates have 96 posts separating them, varying from 15mm to 16mm in diameter, at least that is what my calipers say on the ones at the edge that I can get at. That is a lot of calculation right there, and you would have to assume all the remaining out of reach posts are the same average size.
I thought about a wiffletree type support, it seems logical with all the variables that it would be best just to arrange something to let the mirror come to its own balance point..but at my stage of telescope building experience(baby stage) it seems beyond me. I've found myself laying in bed at night trying to figure out how to support this d**n mirror. Life needs to move on, I really miss my scope so need to get this functional...so, I'm going with the PVC supports at 45 degrees for now. If you design something for your 18" HO I would be very interested in seeing it. One good thing about being an ATM'er....you can always change things down the road


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: 14" Hubble Optics Project new [Re: starman345]
      #5481178 - 10/20/12 11:14 PM

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

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