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12" f/6 28mm thick mirror CELL DESIGNS?

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#1 Earthbound1

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Posted 24 March 2019 - 09:51 PM

Trying to understand why a solid metal surface with 3, 6, 9, or more raised points of contact wouldn't be as good as a compliant whiffle tree, or just plain ol RTV points of contact. What about 9 PTFE or Delrin points of contact? All of this assumes a basic 3, 6, or more edge support tabs with RTV. Consider this mirror as going into an EQ mount. I'm trying to make the simplest, easiest, non-pinching, cell possible. Does COE matter if it's a 2.5mm thick steel, or aluminum base, with either PTFE, HDPE, Delrin, or Silicone points of contact on the mirror? They ALL have DIFFERENT "coe's" and different levels of friction My point is, does an aluminum base with PTFE points and Siliconed edge supports really differ that much from one with Delrin instead? Or what about 57 microdots of Silicone used to support the mirror per PLOP? Inquiring minds want to know...

Edited by Earthbound1, 24 March 2019 - 10:19 PM.


#2 Earthbound1

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Posted 24 March 2019 - 09:57 PM

By all means, post pictures of anything relating to this topic. PLOP images, your custom cell, your basic cell, your Ronchigrams, interferometer readings, what have you, etc... Pictures of pinched optics before and pictures after remedying said optics. I may as well have called this thread SHOW US YOUR MIRROR CELL... Lol. All information is useful! Clear Skies!!!

Edited by Earthbound1, 24 March 2019 - 09:59 PM.


#3 Earthbound1

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Posted 24 March 2019 - 10:15 PM

Consider this design... A 2.5mm solid aluminum surface covered in a continuous 1mm thick teflon sheet with edge supports made the same way. No silicone touching the mirror. All acclimation concerns aside. Now, envision this same design with a 90mm or 120mm fan centered with 27 teflon points 2mm(or slightly bigger)thick, spaced per PLOP to allow for airflow... It SEEMS to me that this design would be simple, allow some cushioning for travel have very little friction and some level of active acclimation. An even simpler CHEAPER way would be to take 6 UHMW Polyethylene strips 1/4" thick 1" wide or thereabouts, radiating from the fan port to the edge. What's NOT to love about such a design? And while I'll concede that this particular design, even with refinement, MAY be not as good as a more complicated and accepted design, I HARDLY think that it could be any WORSE than a piece of plywood with 3 leather pads... Make of that, what you will.

Edited by Earthbound1, 24 March 2019 - 11:57 PM.


#4 mark cowan

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Posted 25 March 2019 - 12:18 AM

Because PLOP designs whiffletree cells and from what you're saying I don't think that's a whiffletree, just a plate with a whole bunch of contact points on it. 

 

Of course you can do that, and you can even just use bubblewrap to support the mirror.  Or memory foam, which has also been used (and works, albeit with poor airflow).  You could use astroturf, that would be a semi-compliant self conforming cell, all those little whiskers of fake grass like a bed of very thin nails.

 

 But the thing about it is that you'll never know exactly which points are actually supporting the mirror and by how much, so it won't necessarily perform like what the PLOP design predicts.


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#5 Earthbound1

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Posted 25 March 2019 - 12:33 AM

Thanks Mark! Fair enough.I also forgot to mention that this type of cell is NOT for a meniscus but perhaps some of the principles discussed here could be adapted to such a mirror. This particular thread is for flat backed mirrors primarily although as I said, all ideas and thoughts are welcomed so that innovative designs may be derived from this discussion. Thanks for clarifying that PLOP is exclusively for whiffle tree cells and NOT traditional ones.

#6 Earthbound1

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Posted 25 March 2019 - 12:35 AM

As for meniscus mirrors, has anyone made a negative mold of their mirror and then used it to form a CF meniscus cell? What do you think about that idea?

#7 mark cowan

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Posted 25 March 2019 - 12:43 AM

It doesn't matter what sort of mirror its for - flat back or curved back - it all works the same.  The curved back just sometimes requires different sort of side support.  I would just make an ordinary (albeit well made) cell that has the ability to curve around the back when put together.  Not too hard.  

 

I don't see what kind of "traditional cell" you're talking about here - all multi point cells are whiffletrees of some sort.  A 3 point is one too it just doesn't have to whiff as the full contact plane is described by the 3 points.  Everything else is just a variation on that.


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#8 Earthbound1

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Posted 25 March 2019 - 12:44 AM

What I'm also interested in is collecting as much data on this subject as has been somewhat alluded to by Mike Lockwood for an in depth comprehensive scientific study of mirror behavior in particular cell designs. Nearly an infinite set of variables to wade through, BUT, perhaps a trend or pattern could be discerned and used to further a very wide open subject. I have read and seen VERY dissimilar approaches to the same end and apparently all parties involved were happy with their results... Which is about as subjective and un-scientific as it can get lol! Unless these parties are WELL trusted and with established credentials I'd take their satisfaction with their results with a grain of salt... Nonetheless, perhaps we can all learn some cool things here in this thread about reaching the same ends by different means! Clear skies!!!

#9 photomagica

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Posted 25 March 2019 - 12:45 AM

Your want a rigid, low friction connection between the aluminum telescope cell and the mirror so that the whole thing will stay aligned and collimation won't drift with pointing attitude as the mirror changes its orientation minutely on the more flexible material. Floating mirrors on complaint material without defined points of contact is an inferior approach to mirror suspension. Whiffletrees are necessary to equalize the load on the points of contact. With the points on a rigid surface (in cases of more than 3 points) the load will not be equalized and the mirror will distort. Using somewhat complaint points of contact as you suggest, may help but it will not fix this.

I recommend taking some time to study and think through the engineering design theory that underlies PLOP and all good design for telescopes. A great deal of experience in telescope engineering underlies PLOP. The cell design that results from PLOP and the recommended materials is hard to beat. Incredibly hard.

Bill


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#10 Earthbound1

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Posted 25 March 2019 - 12:51 AM

Traditional cell, ie. A flat base with just 3, 6, or 9 points of contact. Some are open backed cast aluminum rings others solid plywood with no holes for tube currents to flow... I've even read about cells that had 2" circular silicone rings at about 40% from center for 10" mirrors several years ago... So, yeah, there's a LOT of ways to go about supporting a mirror and a LOT of mirror variables as well...

#11 mark cowan

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Posted 25 March 2019 - 01:06 AM

A flat base with just 3, 6, or 9 points of contact.

 

 

FWIW these "traditional" cells were also used with "traditional" mirrors at 1:6 (or thicker) aspect ratios.  Make the mirror thick enough and small enough you can pretty much put it on anything.  Make it large and thin and that just doesn't work well at all, for very good, very well understood reasons.  :shrug:


Edited by mark cowan, 25 March 2019 - 01:29 AM.

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#12 Earthbound1

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Posted 25 March 2019 - 01:39 AM

Well how good of a cell does the above mentioned mirror need? 12" f/6 1.1" thick. Flat POLISHED back.

Edited by Earthbound1, 25 March 2019 - 01:40 AM.


#13 mark cowan

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Posted 25 March 2019 - 02:09 AM

6 point will do nicely.  3 rocker arms and 6 contact pads.

 

Gets you to 1.3nm RMS surface error by 3D PLOP analysis:

 

 

temp.png


Edited by mark cowan, 25 March 2019 - 02:18 AM.

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#14 Earthbound1

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Posted 25 March 2019 - 02:24 AM

Thanks! Where exactly should those 6 points be? In the center of the blue I would assume, but what is the actual distance from the center? It looks like 50-60% from center... If so, then, 3" or so, all the way around.

Edited by Earthbound1, 25 March 2019 - 02:38 AM.


#15 CrazyPanda

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Posted 25 March 2019 - 02:49 AM

Thanks! Where exactly should those 6 points be? In the center of the blue I would assume, but what is the actual distance from the center? It looks like 50-60% from center... If so, then, 3" or so, all the way around.

PLOP will give you all of these dimensions (the radius of the three connection points for the three rocker bars, and the length of each bar). 


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#16 mark cowan

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Posted 25 March 2019 - 04:52 AM

Thanks! Where exactly should those 6 points be? In the center of the blue I would assume, but what is the actual distance from the center? It looks like 50-60% from center... If so, then, 3" or so, all the way around.

That's the error map not the layout. 

 

I just ran a standard 6 point cell using the automatic setup.  I didn't save the design but you can recreate it easily and it has all the details and layouts.


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#17 Earthbound1

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Posted 25 March 2019 - 06:54 AM

While PLOP is a fine tool, gentlemen, I've yet to find a version that runs on an iPhone 5s... Unfortunately, I can't tether this phone to my Windows 7 i7 laptop anymore due to AT&T's change in unlimited plans vs family plans. I'm EXTREMELY rural and thus, limited in AFFORDABLE internet provider options. Thanks for understanding!

Edited by Earthbound1, 25 March 2019 - 06:58 AM.


#18 Earthbound1

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Posted 25 March 2019 - 08:03 AM

Now, where are all the other members with THEIR information? Anyone?

#19 mark cowan

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Posted 25 March 2019 - 10:06 AM

While PLOP is a fine tool, gentlemen, I've yet to find a version that runs on an iPhone 5s... Unfortunately, I can't tether this phone to my Windows 7 i7 laptop anymore due to AT&T's change in unlimited plans vs family plans. I'm EXTREMELY rural and thus, limited in AFFORDABLE internet provider options. Thanks for understanding!

What are you asking then?  There's only one best version of PLOP out there (3.0.5 from 2011) and it's a Windows program.  Do you want the program itself?   If you can get to someplace with free WiFi you can download it on the laptop.  Or it could be sent on a CD in the mail...   Or if all you want are the #s for the design I can post those here, as can any number of other people... 


Edited by mark cowan, 25 March 2019 - 10:32 AM.

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#20 Earthbound1

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Posted 25 March 2019 - 11:16 AM

Lol, thank you Mark. Yes, please, if you will, kind sir, post the measurements you derived for the PLOP plot you provided. I would be everso grateful for your time and effort. At some point I do intend to aquire a better internet connection for my laptop here at home. Our local library, which is rarely ever open had internet, but I fear the town budget cut their funding, so...

#21 Earthbound1

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Posted 25 March 2019 - 11:20 AM

Now, where are all the other members with THEIR information? Anyone?

This is for pictures of their cell designs, graphs, materials, what have you. A simple "Designing Cells" thread which would be somewhat redundant because of the compliant bearing thread could be in order... After re-reading my above posts, it is clear that I was vague in what I meant. Sorry for the confusion. Texting can be tough! Lol.

Edited by Earthbound1, 25 March 2019 - 11:24 AM.


#22 Bob4BVM

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Posted 25 March 2019 - 01:34 PM

This is for pictures of their cell designs, graphs, materials, what have you. A simple "Designing Cells" thread which would be somewhat redundant because of the compliant bearing thread could be in order... After re-reading my above posts, it is clear that I was vague in what I meant. Sorry for the confusion. Texting can be tough! Lol.

Why are you reinventing the wheel again ?  This has all been done before, and the results are in... 'the wheel will be round and have an axle thru the center'

 

As has been stated, PLOP is THE tool to use. Find someone with a computer and run your mirror numbers on it. (and please don't try to run it on a cell phone, least of all an iPhone ! ! )

 

As for all of our specifics on our cells, what you will find is pretty redundant, a lot of cells which all look very similar because they were all designed around PLOP methods. 

 

If you want a good treatise on cell design, no need to create a new one, there are some excellent ones out there. Start by reading everything Mike Lockwood says about mirror support on his shop site.

 

CS

Bob


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#23 Earthbound1

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Posted 25 March 2019 - 01:46 PM

Lol, least of all an iPhone... I hate 'em too! But it's the wife's and it has internet vs my Galaxy s4 that I still love. Not really trying to reinvent the wheel again... If I had an optical test bench I'd do some before and after Ronchigrams of some of these cells to compare the results. I'm limited for resources and simple is faster and cheaper and while not BEST, acceptable for the time being. This mirror will be fine in a 6 or 9 point "fixed" cell. The donor has assured me of that. I'm just trying to stimulate thought and outside the box/alternative thinking about what is possible. No one has the luxury of time, or never ending resources although they may erroneously believe so... And simplicity is sometimes best.

#24 Dale Eason

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Posted 25 March 2019 - 02:13 PM

This mirror will be fine in a 6 or 9 point "fixed" cell. The donor has assured me of that.

I know of no experienced ATM who would have said that for that size of mirror.  Fixed points might work but it is a gamble that the actual three points that do provide the support will be in the right locations to do so.

Right we don't have the luxury of time and if there is a existing and proven quick, and easy to implement solution; I use that.  For me that is PLOP and and wiffle trees.

 

For a first time telescope user or ATM fixed 3 point may be good enough if you don't have experience in looking through a scope at higher powers.  Eventually you can upgrade to better support  If you made it in the first place you can always tweek it to make it better.  


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#25 photomagica

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Posted 25 March 2019 - 02:17 PM

I seriously doubt the mirror will perform to its optimum or even be "fine" in a fixed point cell. The mirror will sit on three points "of its choosing" regardless of how flat or smooth you make the back or the force will be distributed unevenly among the points. You want it to sit with equal force on specific points of your choosing - likely 9 of them but PLOP will allow you to select the best number and location. It is not too hard to fabricate whiffletree cells and absolutely worth the effort. I built the cell components for a thin 17.5" mirror some years ago. It took a day to hand fabricate all the pieces from aluminum stock. I used a hacksaw, small drill press and a power sander to smooth the edges. It was completely worth the effort. The instrument performed brilliantly with no mirror support issues.

 

As a previous poster has said - there aren't many threads on cell design because it is settled technology. In a prior generation of telescope making just about every cell option permitted by materials was explored. Today we know that cells that operate outside the established technology give results ranging from obviously inferior at the eyepiece to just plain ugly. Current superior cell design and engineering (PLOP) has really enable the whole thin mirror world we live in today. Innovations need to build on this solidly established foundation.

Bill


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