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: 510
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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: 2258
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: 211
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: 2103
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
Reged: 12/26/08
Posts: 211
Loc: P. R. of New Jersey
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Post deleted by polaraligned
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3481
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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: 2103
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: 620
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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)
Reged: 06/03/05
Posts: 2103
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
Reged: 07/08/07
Posts: 152
Loc: Golden State
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polaraligned
sage
Reged: 12/26/08
Posts: 211
Loc: P. R. of New Jersey
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Post deleted by polaraligned
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3481
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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
Reged: 12/26/08
Posts: 211
Loc: P. R. of New Jersey
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Post deleted by polaraligned
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3481
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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: 211
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
Posts: 3481
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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: 521
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: 395
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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