mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Super! I'll need to check out the available bits and get an idea of possible radii and the like.
Best,
Mark
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polaraligned
sage
Reged: 12/26/08
Posts: 226
Loc: P. R. of New Jersey
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Post deleted by polaraligned
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Houdini
professor emeritus
Reged: 07/13/07
Posts: 523
Loc: Europe
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Post deleted by Houdini
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16" f/4.9 motorized alt-az, 25" f/5 Dobson, 43" f/4 alt-az under construction
Mirror Edge Support Calculator
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Don W
Postmaster
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Posts: 14615
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Ok, people. It's not always possible to tell when someone is kidding and when they are not on the internet. Not being face to face limits the ability to discern the intent of some statements. With that in mind, let's all step back from the keyboard and settle down. Please take care of what you say and how you say it.
The Moderator Team
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Houdini
professor emeritus
Reged: 07/13/07
Posts: 523
Loc: Europe
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I've further discussed this with Scott by PM, and it's probably useful that I add some explanations about my previous comment made in the "twice as stiff" conversation above.
I wanted to demonstrate that the same engineering fact (a mirror being "twice as stiff", whatever that means) can be represented in different ways, either as a feature "wow, twice as stiff" or as a failure "the weight could have been reduced instead". Which message you choose is basically a marketing decision.
An additional fuzzy element is that it's unclear what is meant by "stiffness" of the cellular mirror without specifying what load case has been used (vertical forces, lateral forces, ...), and what deformation has been computed: Peak-Valley, RMS, Zernike coefficient, with or without refocusing and tilting. Without providing this information the "twice as stiff" claim is meaningless.
And finally, one must consider that increasing the stiffness is not always useful. If your mirror keeps its shape with an RMS error of less 2 nm, there's no point in doubling the stiffness to make it good to 1 nm.
This last point also answers in part the starting question of the original poster of this thread. Mirror cell designs have become that optimized (by use of PLOP, Mirror Edge Support Calculator etc) that thinner, more flexible mirrors can nowadays be used without compromising the image quality.
Cheers,
Robert
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16" f/4.9 motorized alt-az, 25" f/5 Dobson, 43" f/4 alt-az under construction
Mirror Edge Support Calculator
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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"Twice as stiff" used in advertising...hmmm...now where have I heard that before??? Comparing 2" to 1.6" large monolithic blanks, me thinks. PLOP PLOP, MESC, oh what a relief... 
Best,
Mark
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 653
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nice 28inch blank congrads .be nice working with a lighter glass/if the glass weighs less, it doesnot need to be as stiff/i think// kevin
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 653
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also when you think a mirror may last for ever. Being lighter is a big plus / shiping a heavy mirror is not cheap back problems are not cheap/kf
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3528
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i dont think weight is a big draw for cellular mirrors, i think cooldown is
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 653
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i donot see alot of advanages in small sizes but big glass it may be worth it . last year i ground a 26.25 in in my bacement and carried it up two sets of steps to test/ up stairs / the guy working the glass likes light and stiff kevin
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Dave,
Thinking and talking this over recently, I do see weight being a big factor. Look at it this way for example (I don't have real numbers to show on stiffness or weight, these are just guesses):
Suppose I take a 1.7" thick 20.25" f/4 quartz mirror (43 lbs) and lightweight it by 50%, reducing the face thickness to perhaps .5" and leaving plenty of ribs for the rest of the structure. Now I have a 21.5 lb mirror, equivalent to a .85" thick monolithic blank in weight, but quite obviously a lot stiffer than that would be. FYI I'm planning a 20.25x1.1" f/3.5 solid mirror later this year, but that's the thinnest I'd go at that size and f/ratio in quartz. It will weigh about 26 lbs (a guess here, due to the steeper curve), but you're right, the cooldown is better and the light weight doesn't hurt. If the lightweighting project comes along quickly I'll go with one of those instead, no question.
Pushing it along a little further, though, Greg Rhode and I are planning to lightweight some 20.25x1.5" quartz blanks for fast mirrors, they should get down to about 20 lbs or so. Would that not be sweet? Think about the weight of the structure needed to support them and draw your own conclusions about the weight advantage now.
Keep in mind though, here I'm not comparing, say, a 20x3" monolithic mirror with a lightweighted one - I'm comparing at most a 20.25x1.1" thin mirror with a slightly thicker lightweighted one - either will provide excellent thermal performance.
I'd also point out that matching the face-plate curve to the front pays off handsomely for mirrors below f/4, as it reduces the weight very appropiately, while the ribs are still providing the stiffness. Plus the face is now equal thickness, so the cooling is truly as a plate and doesn't cause distortions as you'd see with differential thicknesses.
Once a mirror gets over 40 lbs it becomes just a little tougher to flip it over and still hang onto by your fingertips - not to mention you actually have to think about how to handle them in the larger sizes...
So what I'm wondering though is how thin to spec face-plate thickness, to avoid print-through effects. Experiences?
Best,
Mark
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3528
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as the mirror gets lighter, the mirror box has to become deeper to counterweight the difference (and of course the rocker box needs to become taller too to accommodate the deeper mirror box), so the scope gains some weight anyway
BUT im all for lighter optics just because they will have less thermal mass.
even at roughly .75" face thickness, guys in the club were seeing some print through and had to change the way they were polishing. im no mirror maker, so i cant offer much
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Quote:
as the mirror gets lighter, the mirror box has to become deeper to counterweight the difference (and of course the rocker box needs to become taller too to accommodate the deeper mirror box), so the scope gains some weight anyway
True in a traditional design, not much of a factor in true ultralight structure. I should be able to show something along these lines in about a month... 
Best,
Mark
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 653
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MARK do all the 20 inch quartz blanks have a hole in the center? thanks for the info kevin
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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No. Look at Greg's website www.quartzmirrors.com where he shows pictures. This material comes mostly as thick cylinders that he slices down to the desired thickness. If you want holes in the center it's easy enough.
Best,
Mark
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Owen
professor emeritus
Reged: 06/21/07
Posts: 513
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All these thoughts of print through for webbed mirrors... surely they can be addressed using a 'fill' substrate for the cells - someting like plaster of paris.?
They could then be gently scraped out and acid washed to remove all traces, once the sphere and preliminary parabolisation was done, and final figuring was about to commence..?
Just a thought
Owen
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polaraligned
sage
Reged: 12/26/08
Posts: 226
Loc: P. R. of New Jersey
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Quote:
So what I'm wondering though is how thin to spec face-plate thickness, to avoid print-through effects. Experiences?
Best,
Mark
Mark,
That is one of many reasons that proper engineering is so important. If the mirror is built right and the optician has a bit of experience with cellular, there will be no print through. My 28" has a face thickness of .3". Plenty thick. It has 6 different rib widths and heights. There are sub-ribs between the main ribs. Every rib was placed by FEA analysis. Nobody sat down and drew the rib pattern in 2 hours. The engineer who did the design has a PhD in mechanical engineering and specializes in FEA. Polishing displacements have been thoroughly analyzed. The faceplate has been made to be as stiff as possible for the optician.
The mirror has been flipped every which way 'till Sunday and analyzed in each position. FEA can have traps and pitfalls. It can lie in the wrong hands. It is not for the inexperienced.
Don't take my word for it. You know who has worked this cellular before. Talk to him.
Scott
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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Quote:
All these thoughts of print through for webbed mirrors... surely they can be addressed using a 'fill' substrate for the cells - someting like plaster of paris.?
They could then be gently scraped out and acid washed to remove all traces, once the sphere and preliminary parabolisation was done, and final figuring was about to commence..?
Just a thought
Owen
I don't think plaster would help all that much, given as how it is a great deal weaker, therefore less rigid than the glass around it.
I have filled in the holes in the back of my mirror but not for that purpose, but so I can do a nicer job of fine grinding the back where the mirror will be supported as I am planning leaving it as open as possible for cooling and asthetic reasons. Once I grind it a bit more to get the entire back flat and without wedge, I plan to coat it with a thin epoxy layer for the fine grinding.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Owen,
It sounds ok, but anything you do to the back of a thin mirror or faceplate after the mirror is figured affects the figure. A cellular mirror needs to be figured exactly as it will be used...
Scott,
Mike is quite pleased with the workability of these mirrors, from what I gather. I'm waiting for him to post back on his Yahoo group with more details.
What you say about engineering is true, whether it involves the inverse pattern to create the ribs with, or the removal pattern to lightweight by. In either case the tolerances of FEA analysis can be met by the fabrication methodology.
But "stiff" has another meaning for optical manufacture than it does for deployment in the field on some sort of support. "Stiff" in polishing/figuring assumes deformation/recovery in predictable ways, and for monolithic thin blanks quite large pressures can be used with no local effects. So called "print thorough" is what happens when the faceplate is stressed beyond elastic recovery locally, and the polishing action now produces local defects that mimic the pattern of the rib structure. Put a solid bump underneath a monolithic thin blank in polishing and you'll see the same kind of imprint. With a fraction of the thickness available in a monolithic blank polishing and figuring forces generally need to be lighter. Or so I understand.
Thanks for the input!
Best,
Mark
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polaraligned
sage
Reged: 12/26/08
Posts: 226
Loc: P. R. of New Jersey
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Yes Mark, as I said, polishing displacements have been thoroughly analyzed in the Dream blanks.
The only "stiff" I am referring to here is from the opticians perspective as I stated. Hence, the sub-ribs and all.
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