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Mirror grinding

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Re: Mirror grinding

#51

Post by xyzzyz »

gregfjohnson wrote: post #43

My dad (Tom Johnson) founded Celestron. His key idea was a way to create the corrector plate at the front of a Schmidt-Cassegrain telescope. The corrector plate compensates for spherical aberration (the difference between a parabolic main mirror and a spherical main mirror). This is a very complicated curve, and the traditional way to make it was to have an optician make it by hand. This was an arduous and time-consu…

I had some trouble understanding the exact procedure described above, but then I read the patent[1] for the technique, which was very helpful.

[1] - http://www.freepatentsonline.com/3837124.pdf

Re: Mirror grinding

#52

Post by vibrolax »

somebodynew wrote: post #50
gregfjohnson wrote: post #43

My dad (Tom Johnson) founded Celestron. His key idea was a way to create the corrector plate at the front of a Schmidt-Cassegrain telescope. The corrector plate compensates for spherical aberration (the difference between a parabolic main mirror and a spherical main mirror). This is a very complicated curve, and the traditional way to make it was to have an optician make it by hand. This was an arduous and time-consu…

Do you know what this process is called? This description makes it sound like the glass has to bend significantly under at most one atmosphere of (negative) pressure, which sounds strange to me. I'd like to read more about it.

The process is called stress polishing. Here's an article about it: http://bobmay.astronomy.net/vacpan/vacpan.htm

Re: Mirror grinding

#53

Post by nialo »

somebodynew wrote: post #50
gregfjohnson wrote: post #43

My dad (Tom Johnson) founded Celestron. His key idea was a way to create the corrector plate at the front of a Schmidt-Cassegrain telescope. The corrector plate compensates for spherical aberration (the difference between a parabolic main mirror and a spherical main mirror). This is a very complicated curve, and the traditional way to make it was to have an optician make it by hand. This was an arduous and time-consu…

Do you know what this process is called? This description makes it sound like the glass has to bend significantly under at most one atmosphere of (negative) pressure, which sounds strange to me. I'd like to read more about it.

I think the trick is that the glass actually doesn't need to bend much. Notice that the caption here: https://en.wikipedia.org/wiki/Schmidt_corrector_plate#/media... describes the plate as being hard to tell apart from a flat disk visually.

Re: Mirror grinding

#54

Post by vibrolax »

I'm a software engineer working in the precision optical fabrication and metrology industry since the late 80's. I've written a lot of software advancing the art in interferometry and computer controlled optical fabrication. It's always a thrill to visit optical shops all over the world and see my software still running on machines I helped build 20 years ago. I really have appreciated the opportunity to build software of enduring value, and to work on the enabling technologies for fab and test of microlithographic, aerospace, medical, research, and photographic optics.

Re: Mirror grinding

#55

Post by dom0 »

jacquesm wrote: post #48
semi-extrinsic wrote: post #32

Earlier quoted context omitted.

I'm guessing these are done with wet sand paper for the coarser grits ( Edit: from TFA: "Always work wet! Sprinkle some water on the grit before you start grinding! Glass dust is very dangerous and can cause silicosis, a serious lung disease if inhaled!"

> for the coarser grits ( I love it how you casually throw that out there. For the un-initiated, outside of grinding lenses: grit 3000 is approximately 6 micron particles and very fine indeed but for this purpose (and gem polishing) it is still considered 'coarse'. The finest polishing grits go to 100,000, ~0.25 u across.

And for knife sharpening people: 3000 grit (and a leather strip) is enough to almost reach razor sharpness ... with good steel, that is :)

Re: Mirror grinding

#56

Post by tejtm »

chillydawg wrote: post #3

I don't really understand how a perfect shape can come out of that process. Can someone explain how it's a product of the grinding technique?

if you rub two things together, there are only two shapes which could possibly remain in constant contact. the first is perfectly flat which is very hard with only two surfaces (easier with three) the second is hemispheres and if you do the easiest thing possible rubbing two things together they will wear one another into matching convex and concave hemispheres. for some optics that is close enough (small diameter long focal length e.g. a 6" F8) otherwise instead of a spherical surface you may need a paraboloid (or other aspheric)in which case you need to preferentially deepen some part of the curve (~70% of the radius) a few millionths of an inch. but when you do it wrong you go back to doing the easy thing and make it spherical then try again... and again ... and again ... (but I'm not bitter). Till you decide it is good enough!

The minimum standard for good enough is a under a quarter wavelength in the short end of the spectrum of interest . this is because an extra quarter down plus the same quarter back up and you are a half wave out with another part of the mirror which when combined by your eye results in destructive interference of both parts of the mirror.

one last point on the final smoothness, the interplay of glass, water, pitch, metal oxide and mechanical force is imperfectly understood. crudely it may be closer to planing than grinding but that does not explain the oxide particles which are found beneath the surface of a figured mirror. another thought is the an atom in the glass is "stretched" up and snaps back into a lower energy configuration which is more atomically flat

Re: Mirror grinding

#57

Post by nomel »

dekhn wrote: post #38
digler999 wrote: post #11

Earlier quoted context omitted.

I'd imagine the math to be off-the-charts complicated, but I'm interested in the signal processing software people have created for correcting optical aberration (and how effective it is). I believe they pioneered this technology for the hubble's mirror defect. If it's effective, then manufacturers could reduce costs by not even needing to try for perfection, just staying within the limits of what software can fix.

The approach most advanced people use now is deformable mirrors. They work in concert with a laser that emits from the detector, bounces off the atmosphere, and produces a real time map for the deformation (you have to solve an inverse problem here IIRC). You can also use a wavefront sensor. For small problems, you can just buy these things off the shelf: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=32..…

The history behind wavefront sensors is pretty fascinating. It was developed in the 60's by Hartmann, in order to better image satellites from earth, and then largely ignored by the astronomy community, even though he presented it to them, until the 80's.

I can't find the original paper that I read, but here's a bit of history [1].

[1] https://pdfs.semanticscholar.org/d1ed/a97dd2cf70f54f24b85abc...

Re: Mirror grinding

#58

Post by X86BSD »

gregfjohnson wrote: post #43

My dad (Tom Johnson) founded Celestron. His key idea was a way to create the corrector plate at the front of a Schmidt-Cassegrain telescope. The corrector plate compensates for spherical aberration (the difference between a parabolic main mirror and a spherical main mirror). This is a very complicated curve, and the traditional way to make it was to have an optician make it by hand. This was an arduous and time-consu…

Greg,

Thanks for sharing this! As I get older and try to record as much family history I can, I really enjoy you and others sharing what you can about family/company history for the future. Stories like this from first hand accounts are getting harder to find and eventually we may lose these great stories and historical accounts for generations to come.

Thanks again!

Re: Mirror grinding

#59

Post by wycx »

hinkley wrote: post #45
semi-extrinsic wrote: post #32

Earlier quoted context omitted.

I'm guessing these are done with wet sand paper for the coarser grits ( Edit: from TFA: "Always work wet! Sprinkle some water on the grit before you start grinding! Glass dust is very dangerous and can cause silicosis, a serious lung disease if inhaled!"

Even with the wet grinding, eventually the slurry dries out, right? So wouldn't you have to keep your work area wet or mop up regularly during the process?

Unless you are a complete slob, you will clean your work area when you are done for the day, right?

In my experience grinding and polishing samples for petrography, Because the grit etc was once wet, it gets caked-on to everything once it dries. So caked-on that it can be hard to clean everything once it is dry, and if you want to remove it you have to wet it all again. Unless you are stirring up the air with a fan, or trying to remove caked-on grit with compressed air, I do not expect much dust will get airborne. So, clean up whilst it is still wet, and there will be no problems.

Re: Mirror grinding

#60

Post by mhb »

chanderson0 wrote: post #6

My parents run a business making telescope mirrors, and I grew up around this exact process. Measurement is the biggest difference between this and what they do. Take a look at https://en.wikipedia.org/wiki/Interferometry#Engineering_and... if you want to know more about how it's done in industry. The best metaphor I got from them about how precise a shape they make in glass is that if you took a typical 1 meter mirr…

What impressed me when I was polishing an 8" mirror was that you could easily see the effect of the thermal expansion of the glass where you touched it briefly with a finger.
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