astrolabio49
newbie
Reged: 06/04/09
Posts: 3
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Since the arriving of Dobsonians,all the mirrors seems to have gone to be thinner even the ones not intended for this type of telescopes.Does it means that the old 12.5 " and 60 mm thick mirrors like the mine are excessive and could be substitued by a lighter one with better results ? Where has gone all the advertisements on astigmathism of thin glass plates? has changed the kind of glass and now we can get the same quality with a glass plate?
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Owen
professor emeritus
Reged: 06/21/07
Posts: 513
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Plate has been replaced in the main by float glass - much more uniform, and most importantly, much fewer bubbles for the unwary. Add in that its easily cut, and available upto 3/4 thick, and you have a recipe for much atm use...
As a result, skills and techniques have improved, and shared to reduce the issues experienced by previous groups suh as astigmatism, to make the process laborious, but relatively straight forward...
The quality of the result will be determined by the skill of the mirror maker - and to really push the boat out in terms of performance, you have to be pretty good at making mirrors 
Your mirror is probably fine - just add a fan or two to assist with cool down, and you should be good to go. 
Owen
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 2269
Loc: Kent, UK
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The classical 6:1 full thickness mirror does take longer to cool than the modern thinner float version but I feel they are far more versatile. They are much more forgiving when it comes to cell design and tend to keep their figure during temperature changes. Barring accident a mirror may last several lifetimes aquiring several different tubes and configurations as technology progresses but thin quartz modern mirrors are perhaps a step forward and they will stand the test of time I think.
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Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
PST
DSI 1
and a curious mind
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polaraligned
sage
Reged: 12/26/08
Posts: 226
Loc: P. R. of New Jersey
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Open back cellular is clearly the best substrate out there. You are going to pay the price for that fast cooldown time.
I have a 28" currently being cast and it will be 2x stiffer than 2" mono, and have 4x the surface area. The face is .3" thick. I have no doubt that once they reach equilibrium, which should be fast, they will outperform even the ULE materials.
Scott
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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But why will the cellular outperform a ULE at equilibrium? Factor in changing temps and you do describe an advantage - but a 28x2" monolithic blank is a thin mirror at 14:1 aspect ratio, at least by traditional measures. 
Here's where you need excellent support in any case, cellular or monolithic. And obviously the cellular needs to be well designed and well annealed to perform correctly. Like the .3" face!
Best,
Mark
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polaraligned
sage
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Loc: P. R. of New Jersey
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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had they done any other 28" mirror blanks before yours?
what did the opticians who have worked these blanks have to say about the anneal?
what is the stated largest inclusion allowed before the blank is rejected?
cellular mirror blanks can even be less expensive than conventional blanks, because they use less material:
http://www.embeddedrf.com/Lookinglass_Webpage_Files/Webbed_Backed/Webbed_Blank.html
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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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Scott, I wondered if it might be them, certainly at .7m - the design looks superb. Can you mention the price of the blank, as they don't list any on their website? I think we know this is a custom order, BTW. Mike Lockwood recently mentioned them but SFAIK he hasn't worked with any, a good place to ask is here on Yahoo.
I do agree about the finishing/blank ratio for large mirrors (mine is right around 2:1 typically, excluding coatings), it's what drives me to use quartz exclusively but consider cellular as my size range increases (though there's hope for larger quartz blanks). That is to say, the price of the blank is a fraction of the price of the mirror, and improvements in performance from using a better substrate are an easy sell.
What do they mean by the gravity loading aspect, though? Is that referring to the FEM design for ribbing and support?
Dave, I contacted "embeddedrf" last year and they eventually told me they weren't sure if they were going to be going back into production - those prices are really old (see wayback machine for 2002) and no longer valid...
Is this a thread hijack in process? Maybe should start a new one.
Best,
Mark
Edited by mark cowan (07/04/09 02:57 PM)
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 657
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when you grind the back on a cellular flat do you have to bevel all those ribs so they don/t chip?? nice blank // kevin
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Stahi
super member
Reged: 07/08/07
Posts: 152
Loc: Golden State
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No, because the weight of the mirror is evenly distributed along the ribs when you push from above/across with your tool.
You would bevel the top edge because they are much easier to chip accidentally with your tool when you are grinding/polishing and are putting a lot more force/area as you move towards the edge.
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mark cowan
Vendor (Obsidian Optics)
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Posts: 2156
Loc: salem, OR
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I certainly hope the back of the cellular is already flat. For what they cost they should be trued and ready to work. Especially so as the blank is cast with approximate ROC built in - otherwise that .3" face thickness isn't going to work in a 28"!!
Best,
Mark
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Stahi
super member
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polaraligned
sage
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Loc: P. R. of New Jersey
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dave b
Post Laureate
Reged: 05/10/05
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thanks for the info scott!
i ask about the bubbles because guys in the club have been casting mirrors (they use a pottery kiln with different controller). they have found that the bubbles hide grit (even when blasted out with a waterpik) and contaminate the next polish. please keep us posted on how mike deals with this problem as he gains more experience.
"high temperature form material" is usually just 50% pottery plaster/ 50% silica flour. mirror blanks have been cast in this material for at least 100 years, probably much longer. guys in our club are forming the flasks then milling out the mold/ribs on a CNC router made by shopbot.
the only finished Lookingglass cellular i ever saw had u-clips siliconed to the ribs with a PLOP spacing (generated as if it were a mono). i could not find the link, ill look later.
speaking of "where is the engineering?", remind the guys at dream cellular that when you double the thickness of a material it becomes 8x as stiff - not 2x as stiff.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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polaraligned
sage
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dave b
Post Laureate
Reged: 05/10/05
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thats what i do for a living.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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polaraligned
sage
Reged: 12/26/08
Posts: 226
Loc: P. R. of New Jersey
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Quote:
thats what i do for a living.
So you should realize that 8x stiffer would only apply if you were comparing a 4" thick mono to a 2" thick mono. For any given cross section the I is going to be lower because it is ribbed not solid.
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dave b
Post Laureate
Reged: 05/10/05
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sorry, that is not how it works...
anyway, dont get defensive, cellular mirrors are interesting to many of us on this site. when we ask questions, its because we want to learn more of how others have made inroads .
keep helping us out and we will help watch your back.
thanks!
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Houdini
professor emeritus
Reged: 07/13/07
Posts: 523
Loc: Europe
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An alternative point of view is that the cellular mirror is 4 times less stiff than a 4" thick monolithic mirror.
Yet another point of view is that a 2" monolithic mirror has plenty of stiffness. One can argue that there's no need for the cellular to be twice as stiff, and that the cellular could perform perfectly well with only 3" thickness instead of 4". In other words it's not truly an optimized design .
The bottomline: all this is more marketing than science, don't let the engineers bamboozle you!
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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Cary
Vendor (Optic Wave Labs)
Reged: 07/07/04
Posts: 398
Loc: Rancho Cordova, CA
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Celluar optics really win in the thermal area. They can dump the heat many many times faster than a monolithic blank. In larger sizes (perhaps 25" and up?) they start to make sense from a mass perspective.
Where cellular optics lose big is in cost and lead time. It seems that no one wants to pay even close to the cost of what it takes to make a good cellular mirror. To the best of my knowledge there is no commercial company that has a proven track record of delivering apertures over ~12" in a reasonable time frame.
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Cary Chleborad
http://www.opticwavelabs.com
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polaraligned
sage
Reged: 12/26/08
Posts: 226
Loc: P. R. of New Jersey
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Quote:
sorry, that is not how it works...
anyway, dont get defensive,
No. Not defensive.
I would like to see the calculations that show that a 4" cellular is 8X stiffer than a 2" mono. My experience in beam calculations tells me this is a false statement.
Thanks,
Scott
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polaraligned
sage
Reged: 12/26/08
Posts: 226
Loc: P. R. of New Jersey
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Quote:
Yet another point of view is that a 2" monolithic mirror has plenty of stiffness. One can argue that there's no need for the cellular to be twice as stiff, and that the cellular could perform perfectly well with only 3" thickness instead of 4". In other words it's not truly an optimized design
Robert
The reality here Robert is that this mirror was originally designed as a replacement for a 2" mono mirror in dobs. I had Dream alter it and put the center hub in for me. The dob manufacturer wanted the cellular to be close to the weight of the mono mirrors. Dream will be perfectly happy designing a mirror to your specs. The weight is not an issue with me so I went with this design. Loosing 1" off the rear side would probably only save 10# or so.
Scott
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Houdini
professor emeritus
Reged: 07/13/07
Posts: 523
Loc: Europe
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Quote:
I would like to see the calculations that show that a 4" cellular is 8X stiffer than a 2" mono.
Dave's value of 8X would only apply to a monolithic 4" mirror. Whatever the exact value, for the normal gravity load a standard 18-point cell will be more than adequate.
More interesting is the analysis of the mirror and the mirror cell when looking at the horizon. How much does the mirror surface deform under the lateral gravity load?
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)
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Posts: 2156
Loc: salem, OR
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Thanks for the info!
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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send me the dxf files and ill run it and tell you exactly.
you have to imagine that maybe 35% of the mirror is 4" thick, and another 40% may be functioning as a T beam (depending on how well it is designed): http://www.efunda.com/math/areas/TBeam.cfm
on the topic of cellular mirror nightmares, look at photo 7 (7th from the left) for some nice print through:
http://ngm.nationalgeographic.com/2009/07/telescopes/mcnally-photography
these are pics from this months issue.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Houdini
professor emeritus
Reged: 07/13/07
Posts: 523
Loc: Europe
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Quote:
send me the dxf files and ill run it and tell you exactly
Maybe you should start by defining what "stiffness" means in the context of a cellular mirror. What load case would you apply, where would you measure the deformation?
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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Darenwh
Pooh-Bah
Reged: 05/11/06
Posts: 1219
Loc: Covington, GA
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So, does anybody think this thread got a little off topic? 
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Daren
Covington, GA
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polaraligned
sage
Reged: 12/26/08
Posts: 226
Loc: P. R. of New Jersey
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The SolidWorks files are proprietary. It does cost money to have real engineers design these things.
Getting to "on the topic of cellular nightmares" is just showing the malicious and misinformed mindset of some in the amateur community. People are seeing things in pictures that are not there. I'll bet some members here can tell you the figure accuracy just by looking at the mirror surface.
I will not participate in this conversation anymore.
It has decayed and is off topic to the original posting.
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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I have been working on four of the types of blanks being worked, plate, Thin-Aspect ULE (6" , 10" 0.8" Fused Silica), standard thickness Pyrex (2 6" X 1" ; 8" X 1.25" ; 12.5" X 2.125") and cellular (one of those old EmbeddedRF 16" blanks).
The 6" plate was by far easier to grind out to f/4.7 and polish. The surface appears to be rough due to too fast a polish stroke, leaving the infamous 'dog-biscuit' pattern in testing.
The 12" Pyrex was another matter. It had been started by soemone and left with an off-center depression some 1/2" to 1" from reaching the edge. Four hours of an off-center MOT stroke and 65 wets roughed it out to f/4.9 to the edge.
I have now roughed out the 6" fused silica blank to f/6, taking nearly twice as much time and number of wets as the aforementioned plate mirror. Very tough material.
The 16" cellular does possess some issues. There are a number of hemispherical bubbles in the front surface up to 3/16" across. The link to LookingGlass above has a photo, and the number of defects they give is consistent with the one I have. The one I got had a rough surface for the ribs, which I have been working to correct. It had a curve roughly parallel to the cast f/5 curve in the front. I started working on this by using a 320 grit SiC stone and water. Progress was slow, so I switched to 46/70 grit and a steel plate. This got all but the outside 1 1/2" of the glass and left a few fine chips on the edges of the depressions in the back. I put wood shaving in the bottom half and topped them up with plaster and I will resume lapping the back flat. I also cleaned up the inside of the tapered holes with a wooden tool charged with a mix of epoxy and SiC.
Its is unclear what the glass is though. In spite of the greatly reduced area of glass on the back where I have been flattening it, progress is slow, so I don't think it is plate of any kind. It had a number of small glass 'bubbles' that were attached to the insides of the ribs. I saved a few to identify the glass if possible.
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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Here is a recent photo with the plaster poured in the holes and leveled. I got the idea and wondered if it was a smart one, and after reading the book about Palomar, found out they did the same thing when flattening the back of theirs.
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Olivier Biot
Amused
Reged: 04/25/05
Posts: 16327
Loc: 51°N (Belgium)
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How will you remove the plaster without damaging the blank?
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Prediction is very difficult, especially about the future. Niels Bohr
Tal-200K (#199) with JMI NGF-Mini2M focuser on GEM3 • Astro-Tech AT80ED • Orion Sirius EQ-G with wireless EQDIRECT • Astro-Tech Voyager • Celestron Regal LX 10x42 • Helios 15x70
ATM 14" f/5 (redesigning) • ATM 10" f/6 Portable Truss (polishing) • ATM 10" f/25 Dall-Kirkham (optics)
AstroForecast
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Mike Lockwood
Vendor
Reged: 10/01/07
Posts: 59
Loc: Usually in my optical shop
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Quote:
Mike Lockwood recently mentioned them but SFAIK he hasn't worked with any, a good place to ask is here on Yahoo.
Well, since you asked here I believe I can answer here if I don't name other businesses.
I have worked ~10 cellular blanks (from two suppliers) ranging from ~6" elliptical flats up to 28" diameter hyperbolics. (There is no secret about who the suppliers are, but I can't mention them here.)
There are several more cellular blanks coming up in my schedule very soon.
Thus far, strain has not been an issue. With care blanks polish out to figures of revolution and figure nicely. Inclusions have not been an issue. Bubbles sometimes appear, but are becoming much less common.
After years of watching 1.25"-2" thick hunks of glass cool before testing, I must say that the thermal performance is quite amazing.
Bottom line - I am eager to have my own personal telescope with a cellular primary and secondary.
However, I'm currently busy with a 14.5" F/2.6 for my own experimental telescope (I'm an ATM, it is my own ATM project, not a commercial offering), so will be a while before I get to it. It WILL happen, though.
Quote:
Dave, I contacted "embeddedrf" last year and they eventually told me they weren't sure if they were going to be going back into production - those prices are really old (see wayback machine for 2002) and no longer valid...
Additionally, those blanks had flat faces I believe. They would not have yielded a deeply curved mirror and would have had non-uniform face thickness if generated to non-flat.
Mike Lockwood
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Mike Lockwood
20" F/3 MX Starmaster, MX = Mike's eXperiment
14.5" F/2.55 self-built Newtonian
Nine other self-built telescopes, 4.25" to 30"
TeleVue Paracorr, eyepieces
http://www.Loptics.com/
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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I had planned on using a dremel tool to drill a hole and then expand it close to the edge, then finish up with a brass bristle brush.
The EmbeddedRF blank I have, HAS a curve cast into it, allowing the blank to be used at up to an f/2.5 ration. I plan on going to f/2.75 as the first step of an 16 1/2" f/8.3 RC with a 32% CO.
Edited by JohnH (07/07/09 04:02 PM)
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 657
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WOW MIKE a f 2.6 It will take a lot of correction/any special coma corrector// kevin frederick
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BluewaterObserva
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F/2.6 is fairly typical of RC systems no? Sounds real nice to me.
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Owen
professor emeritus
Reged: 06/21/07
Posts: 513
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As a comment that could be close to heresey... On what are now seen as 'thick' mirrors, could we not grind the back using steel rod, grit and water to a pre-determined depth.
This presumes a well annealed blank, and would have to be done before grinding...
Just a thought...
Owen
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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Quote:
As a comment that could be close to heresey... On what are now seen as 'thick' mirrors, could we not grind the back using steel rod, grit and water to a pre-determined depth.
This presumes a well annealed blank, and would have to be done before grinding...
Just a thought...
Owen
the russians used to do something along those lines. those mirrors were said to be "cored" or "lightweighted"
coke can sized holes were bored into the glass to make it lighter and help cool the thick glass
sydor does a similar thing, but with a cnc router:
http://www.sydor.com/products-light-weight-substrates.asp
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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BluewaterObserva
Post Laureate
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Glass is decently expensive. Seems a ricky proposition to drill out material. Maybe not though, I don't really know what it takes to cut glass, or take out material in such a way at all.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Lightweighting can be done, but you need to devote the same kind of analysis to it as to the design of cellular mirrors, certainly if going to extremes (but why bother otherwise? ). My favorite would be single-crystal silicon, but now we're talking $10k a kilo, if memory serves.
Now just thinking out loud, as this thread has been hopelessly hijacked at this point:
Diamond boring tools cut through pyrex like butter, through quartz like, well, cold butter, and a fairly simple CNC boring machine could be built and well isolated, where the only byproduct is quartz dust mud - just the same as what comes off my generating machine. Certainly doesn't have to run at router speeds, a couple thousand RPM is totally sufficient. Not hazardous if kept wet. Greg Rhode built a diamond sectioning saw for quartz in a shower enclosure, basic idea.
With a little bit of design, that would allow one to pretty much halve the weight of a thin quartz (fused silica) blank while getting the face thickness down to 1/3", perhaps to 1/5" though it's riskier. You could lay it out to match the cell supports.
Fused silica comes extremely well annealed with no visible strain, so the material would take lightweighting very nicely, and the best part is? It's cheap, electrical costs have already been paid, and there are tons of it available! Suitable diamond tools are available from http://www.thk.hk with great service and fast shipping. I like it.
I'll build a machine to do the processing if I could get somebody to agree to do the modeling of the mechanical result, as I don't have the resources to pull that off. I was planning to build a CNC generator to for blanks larger than the 20" my current sine-table generator handles anyway, and this is an extension of that.
Best,
Mark
Edited by mark cowan (07/09/09 02:31 AM)
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Cary
Vendor (Optic Wave Labs)
Reged: 07/07/04
Posts: 398
Loc: Rancho Cordova, CA
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Mark,
Let me know what you want and I will design and do the FEA for you.
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Cary Chleborad
http://www.opticwavelabs.com
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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
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Houdini
professor emeritus
Reged: 07/13/07
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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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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: 657
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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: 657
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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
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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
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Posts: 657
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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)
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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: 3529
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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
--------------------
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: 657
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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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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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That's excellent, as the two forms of displacement (polishing vs usage) are different! I apologize for not quite understanding your comments.
Best,
Mark
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