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Why are high-quality mirrors so expensive?

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#1 Dragonwatcher

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Posted 05 March 2013 - 04:20 PM

This is probably a dumb question, but I'm sure many can tell me why it is. As I understand it, many mirror makers use or will soon be using robotic equipment. Now, to an uneducated person like me, this sounds like they take a great blank, put it into a slot, switch on the power, and presto - out comes a newly-minted mirror ready for coating. I know they must inspect & test somehow.

Now, why can't they churn out dozens a day? Are materials scarce? Does the grinding take a long time? Are there many points where humans intervene to check or modify?

I can understand where the equipment may be expensive and costs need to be recouped. I can understand where QC might be critical and labor-intensive.

But what accounts for the cost difference between Chinese robotic manufacture & US robotic manufacture? Chinese mirrors may be terrific or poor. I get that. A Zambuto will never be anything but terrific. Is that strictly a QC thing? Does he have better robotics? Or does he keep tweeking as many runs as he needs to get a great mirror while the Chinese simply pull them off the assembly line after a single run?

So why am I naive and misunderstanding the mirror making process. Are we getting closer to the day when robotics will be able to consistently produce Zambuto-like mirrors routinely, consistently & in large quantity over short time spans?

#2 csrlice12

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Posted 05 March 2013 - 04:23 PM

Unlike the Chinese mirrors where "close is good enough", If a Zambuto mirror doesn't meet Carl's specs....it makes a good ashtray, Carl won't put it in a scope.

#3 KerryR

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Posted 05 March 2013 - 04:31 PM

Some, if not most, of the difference in cost also relates to the hourly wage difference between high end mirror makers and Asian workers. The issue compounds-- more time spent at a higher cost.

#4 Deep13

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Posted 05 March 2013 - 05:13 PM

labor and skill intensive, economy of scale (small in this case)

#5 David Knisely

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Posted 05 March 2013 - 05:20 PM

While mass-produced mirrors are definitely getting better, their smoothness and precise figure will rarely match that of some of the better custom mirror makers like Zambuto or Lockwood. Then, the main factor is time. Time (and a fine touch) is needed to get the mirrors to the very high level of surface accuracy and smoothness that makes these mirrors really work well for the demanding observer. With machine grinding and quick figuring, you can put out a lot of mirrors of acceptable quality in a shorter period of time, but these mass-producing mirror manufacturers just can't afford to take the extra time needed to really make darn sure that the surfaces are simply as good as they can be (time is money after all). It is that hand figuring and repeated measurement of the professional optician that gets the quality of the custom mirrors to a very high level. Is the extra quality worth the extra cost? Well, maybe yes and maybe no, but for my money, the mass produced mirror I got was not even of acceptable quality. Now that Lockwood has refigured it to premium specs, I am secure in the knowledge that the optics of my telescope are simply as good as they can ever be. Clear skies to you.

#6 Starman1

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Posted 05 March 2013 - 06:07 PM

David's right, it's time spent, AND materials.

It might be OK if polished for a certain period of time. If you double that time, the surface may be even smoother, but you've just cut your production in half. And, if I understand it right, the time spent to make a just OK surface into a superb surface cuts production to a small fraction of the quantity. Since time = money, excellence = expensive.

And there is also a difference in materials. Many inexpensive mirrors are made of plate glass or BK7 or some less-expensive material. High end mirrors are made of Borosilicate glass or fused quartz or ULE materials or zero-expansion glass types, which can literally be 10-20X as expensive (and more).

There is even a difference in roundness, wedge, quality of edge bevel, and annealing between production telescope mirrors and the premium stuff.

Is the difference worth spending? Well, that's a personal matter each of us decides. For me, the very small incremental difference in visual quality going from a premium mirror to an ultra-premium mirror was worth it, even though the difference was less than going from a production mirror to a premium mirror. I could see a difference, and as long as I could see a difference, I was not going to rest until I had the best of the best.

Because we're talking very small differences in money, here. Compare a Hyundai with a Lamborghini. That difference in price, both in dollars and percentage is WAY more than the difference between a cheap mirror and one of the best made. Sure, it depends on your economic position in life, but, the way I look at it, it's all about priorities. The very best eyepieces and the very best mirror cost small fractions of even a cheap boat. As hobbies go, this is a relatively cheap one. And that goes a long way toward explaining why so many people have cases full of eyepieces. :lol: My high-end dob, with a high-end mirror still cost less than the camera a close friend uses on his scope, and he paid almost as much for his scope and mount as my wife and I spent on new cars---combined! It's all about priorities.

David did it an economical way: get a production scope, have the mirror refigured to a premium level, and still pay a fraction of the price of a decent astrophotography rig. I'm sure he will be tinkering with the mechanicals of his scope (or already has), but the idea is good--it's the optics that we look through, not the wood in the base or the tube material or the bearings or the mirror cell. In fact, if you're good at woodworking, you could build a scope wherein the mirrors were 75% of the overall cost. Most of us aren't machinists or carpenters, though.

But, after the optics, the rest is just details.

#7 Achernar

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Posted 05 March 2013 - 06:45 PM

Unless you are dealing with meter class optics, good mirrors still require a good deal of work by hand. While machines do very well for excavating the curve and the polising, they fall short for the critical figuring process. It's very difficult to make a good large mirror, and opticians who do this work often have to put many hours into the figuring and testing, itself requiring a lot of skill to carry out. Yes you can get a good mirror that is mass produced, but a truly great mirror has to be at least partially handmade. The glass blanks are also very expensive, and so is the equipment required to produce optics commerically.

Taras

#8 Dragonwatcher

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Posted 05 March 2013 - 06:56 PM

So, even if someone uses robotics to do the work, hand figuring is required? What does that mean? The glass comes out of the robotics, tested, found slightly off, and is polished by recalibrating the robotics or done somehow manually? I am completely uneducated here - does figuring = grinding out imperfections? So robotics get the glass close, then in comes the human and somehow manually finishes the final polishing after extensive testing?

I can certainly understand that better grade of materials being used by premium mirror makers upping the price.

#9 nirvanix

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Posted 05 March 2013 - 07:12 PM

A nominal hourly rate for such work is likely 10 or more times higher here than Asia.

#10 Jon Isaacs

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Posted 05 March 2013 - 07:24 PM

So, even if someone uses robotics to do the work, hand figuring is required? What does that mean? The glass comes out of the robotics, tested, found slightly off, and is polished by recalibrating the robotics or done somehow manually? I am completely uneducated here - does figuring = grinding out imperfections? So robotics get the glass close, then in comes the human and somehow manually finishes the final polishing after extensive testing?

I can certainly understand that better grade of materials being used by premium mirror makers upping the price.


Jackie:

It is probably worth thinking about the accuracy that is required for the surface of a mirror. Generally optics are measured in terms of lambda, the wave length of light. For visual optics, typically lambda is 550nm, the wave length of green light. To get a feel for how small that really is, a human hair is about 200 times lambda.

Mirrors are measured in fractions of lambda, an average mirror might be 1/4 wave. I believe that is wave front error, a reflection doubles the surface error so that's a measure of the surface error over the entire mirror, it is perfect within something like 1/1600th the thickness of human hair. A good mirror is probably 1/10th wave, at the surface about 25nanometers.

So there you have a piece of glass maybe 10, 15, 20 inches in diametar, with a surface that is parabolic to within maybe 20 nanometers....

Such precision requires attention to each and every detail. Just the heat generated by a few quick polishes must be allowed to equilibrate before the mirror can be tested. In the book "the Perfect Machine", the story of the Hale 200 inch at Palomar, the final figuring was done in a basement that was temperature controls. Each week the mirror would be tested, polished for a very short while and then allowed to wait a week for the next test.

Amateur astronomers make their own mirrors. You could make one too.

Jon

#11 Dragonwatcher

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Posted 05 March 2013 - 07:33 PM

Jon,

Thanks. I guess I never thought about the substrate needing to "rest" between polishes to come to equilibrium. I suppose temperature, air particulates, material interactions, gases, etc all come into play. So robotics don't control all the variables, requiring human intervention...

I know some amateurs grind their own mirrors. I know a couple... I can understand the painstaking, time-consuming nature of that. It's a labor of love and pride often. But I wasn't understanding why automatic robotics needed tinkering with if well implemented. I am beginning to see now...

#12 GeneT

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Posted 05 March 2013 - 07:55 PM

Superior craftsmanship, quality control, good process, professional reputation--just to name a few.

#13 GeneT

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Posted 05 March 2013 - 08:00 PM

Don,
Good answer. If you buy the right telescope, it will last a lifetime. If on a limited budget, the Chinese made mirrors for the most part give good value for the money. However, I am at the time of my life when I will only buy premium mirrors for my telescopes. My goal is to spend all my money before I die. My kids believe in the survival of the fittest. So, why die and leave them rich? :grin:

#14 GlennLeDrew

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Posted 05 March 2013 - 08:03 PM

Getting that last 10% of improvement can take 90% of the time.

#15 Jason D

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Posted 05 March 2013 - 08:18 PM

There is a more basic answer and that is supply and demand. As long as the price convinces the master optician to stay in business and as long as the same price will attract enough buyers then that sets the “fair” price regardless of its amount.

#16 Cary

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Posted 06 March 2013 - 12:14 AM

I've quickly scanned the thread and have this:

There are a number of misconceptions:

#1 Mirror makers are using robotic equipment. Not so, not so, not so.... show me one mirror maker that caters to the amateur market that is using "robotic" equipment to figure mirrors. I only know of ONE person that claims so but has shown no evidence of doing so.


#2. Don Pensack made a statement of "time and materials". Nope... it's just time. My materials cost for figuring mirrors is insignificant. PITIFULLY SMALL. Its the time and therefore the overhead of my shop that drives the cost.

What drives the cost of a "good" mirror up is the added time to take it from what some consider acceptable, to what "high quality" mirror makers consider acceptable. To put this in understandable terms:

What you are paying for is the additional labor and overhead. Figuring a mirror from 1/4 wave to 1/8th wave or better is purely art that only a handful of people on earth have learned how to do on a commercial scale.

And... to top it off we all do it for a pitifully low wage when you consider our labor input and overhead. I'm sure many other mirror makers will chime in and back me up on the fact that we don't do it for the money alone.

#17 jpcannavo

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Posted 06 March 2013 - 06:32 AM

This is probably a dumb question, but I'm sure many can tell me why it is. As I understand it, many mirror makers use or will soon be using robotic equipment. Now, to an uneducated person like me, this sounds like they take a great blank, put it into a slot, switch on the power, and presto - out comes a newly-minted mirror ready for coating.


I think there is a misunderstanding here. Even for mass produced mirros, the above description is a qualitative oversimplification of a process that still requires a significant human involvement. I don't think - and anyone please correct me if wrong - we are anywhere close to such an entirely robotic process that yields a polished, figured surface for typical mass produced mirror. As such, the already required hands-on-human element becomes far more time-intensive when trying to obtain premium results. And so, price goes up accordingly.

#18 Dragonwatcher

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Posted 06 March 2013 - 09:23 PM

There are a number of misconceptions:

#1 Mirror makers are using robotic equipment. Not so, not so, not so.... show me one mirror maker that caters to the amateur market that is using "robotic" equipment to figure mirrors. I only know of ONE person that claims so but has shown no evidence of doing so.


Perhaps you are correct. I was under the impression that one NW mirror maker does, it's done in China, and when I go to the site for Veritas Optics I see the message "Veritas Optics is currently implementing robotic figuring/testing systems to produce the highest quality medium to large aperture primary mirrors. Availability expected mid 2013."

So if I have jumped the gun in my assumption and robotics are not currently being used by many/any mirror makers, then I apologize for making an unfounded statement.

Also, I wonder if we could be dealing with a semantics issue? I may not have a clear understanding of robotics from an engineering/manufacturing point of view. Does largely automated fall short of the definition of robotics?

#19 mark cowan

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Posted 06 March 2013 - 10:12 PM

I've had to change that a few times already, as the difficulty involved keeps pushing my timeline back.

Best,
Mark

#20 Starman1

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Posted 06 March 2013 - 10:40 PM

I've quickly scanned the thread and have this:

There are a number of misconceptions:

#1 Mirror makers are using robotic equipment. Not so, not so, not so.... show me one mirror maker that caters to the amateur market that is using "robotic" equipment to figure mirrors. I only know of ONE person that claims so but has shown no evidence of doing so.


#2. Don Pensack made a statement of "time and materials". Nope... it's just time. My materials cost for figuring mirrors is insignificant. PITIFULLY SMALL. Its the time and therefore the overhead of my shop that drives the cost.

so far as I know, commercial scopes from China do not use fused quartz, Zerodur, ULE materials, and I'm not even sure they use borosilicate glass.
So there would, indeed, be a significant difference in materials cost.
Admittedly, though, materials are not a significant part of the difference in price between premium mirrors and those in commercial scopes from China.

#21 Jeff Morgan

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Posted 06 March 2013 - 10:49 PM

And there is also a difference in materials. Many inexpensive mirrors are made of plate glass or BK7 or some less-expensive material. High end mirrors are made of Borosilicate glass or fused quartz or ULE materials or zero-expansion glass types, which can literally be 10-20X as expensive (and more).


Even in things as innocuous as pitch and cerium oxide.

Or in letting a mirror set in the stand for three hours before testing, vs. 30 seconds.

#22 Achernar

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Posted 06 March 2013 - 10:54 PM

The final adjustments to the mirror's surface have to be done by hand, IF you want a mirror to be as smooth and well corrected as possible. Machines do very well for hogging out the curve, and the initial polishing. The figuring is best done by hand because when a person does it the figuring strokes are somewhat random, which keeps roughness and errors to a minimum. Figuring a large and or fast mirror is no small task, let alone the testing to see what needs to be done next or if the mirror is finished. That can take quite some time. Also, premium mirror makers can make your mirror from any sort of low-expansion glass or glasslike material you want. That alone is quite expensive, but worth it because they are easier for the optician to work on, and you won't have to deal with the mirror's figure shifting when the mirror is cooling down to ambient temperature.

Taras

#23 GlennLeDrew

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Posted 06 March 2013 - 11:53 PM

When I make 12.5" f/2 primaries of elliptical figure, getting that last 10% of finesse takes at least 80% of the time devoted to aspherizing. And that's in spite of the substrate being fused silica, which allows virtually instantaneous testing due to its low coefficient of thermal expansion. Initial removal of glass can proceed quickly, with robust action of the tool. But as the final figure is approached, the pace slows down considerably so as to bring about a smooth surface of good correction.

A graph of time spent vs volume of glass removed over time would show a curve which very rapidly deflects to near horizontal for most of the process, due to the relatively minuscule amounts of glass polished away during the last 80+ per cent of work on aspherizing.

#24 Jon Isaacs

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Posted 07 March 2013 - 07:24 AM

Perhaps you are correct. I was under the impression that one NW mirror maker does, it's done in China, and when I go to the site for Veritas Optics I see the message "Veritas Optics is currently implementing robotic figuring/testing systems to produce the highest quality medium to large aperture primary mirrors. Availability expected mid 2013."



"Highest quality" is probably subjective.. Reading that interferometer and deciding where to make the corrections would seem difficult to automate. If you want Roland Christen or Peter Ceravolo quality optics, someone like Roland or Peter or ... better be at the helm.

Jon

#25 azure1961p

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Posted 07 March 2013 - 07:51 AM

Time. It's very time intensive.

Pete






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