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Mirror grinding

scopemaking.net

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Re: Mirror grinding

#41

Post by Avshalom »

Florin_Andrei wrote: post #24
digler999 wrote: post #17

Earlier quoted context omitted.

Depends on what you mean by perfect. its very precise, but I'm sure if you examined the surface with an electron microscope you'd still see imperfections. Edit: was incorrect about what I said below. Leaving it for posterity It's important to remember that the final surface - the one that actually reflects photons - is created chemically by releasing a gas inside a vacuum that very evenly coats the surface with refle…

No, it's not needed. Nanometers don't matter, that's the atomic level, far below what visible light could resolve. The metallic coating doesn't change the shape. Once you're below 20 nanometers it basically doesn't matter. The wavelength of visible light is on the order of 400 nanometers. In a different universe where the wavelength of visible light would be comparable to, or smaller than, the size of atoms, it would…

And indeed x-ray mirrors are a bitch

(Although that's more because of destructive self interference than imperfections)

Re: Mirror grinding

#42

Post by imroot »

This is one of my favorite hobbies to take part in.

I've got a hot glass shop on my farm in Kentucky (where I will make blanks and other artistic pieces), and I have built a cold shop with some tools (my favorite is a mold that uses pennies instead of the porcelain that they use in the article) and then using a HD projector and a DSLR that I remotely trigger, I get a map on the screen of where the imperfections are relative to the "top" of the mirror (that I indicate with a dot on the side).

Once I've worked the imperfections out, I'll then take it to an observatory in Ohio to get mirrorized -- and generally speaking, I'll donate my last telescope to the observatory (for public use) once I get the one I'm building done.

Re: Mirror grinding

#43

Post by gregfjohnson »

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-consuming process. Tom's idea was to spend that time manually creating a block that has the OPPOSITE shape from the one desired, and carve narrow grooves in it. Then, take a normal optical-quality sheet of glass, and use a vacuum to deflect that glass down onto the block. Grind the exposed surface flat, which is a quick and easy thing to do accurately. Release the vacuum, and out pops a piece of glass that is a high-quality corrector lens. As a kid, I can remember many weekends when my dad was out in the garage grinding lenses and refining the idea. Eventually Celestron became successful enough to allow him to quit his day job!

Re: Mirror grinding

#44

Post by gregfjohnson »

From one of Jon Bentley's Programming Pearls books, the "Bumper sticker computer science" chapter:

[Thompson's Rule for First-Time Telescope Makers] It is faster to make a four-inch mirror and then a six-inch mirror than to make a six-inch mirror.

    Bill McKeenan
    Wang Institute
This is offered as wisdom for software development..

Re: Mirror grinding

#45

Post by hinkley »

semi-extrinsic wrote: post #32
siavosh wrote: post #30

Earlier quoted context omitted.

I'd love to grind my own mirror, but are there health concerns with the fine powder for the skin and respiratory systems?

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?

Re: Mirror grinding

#46

Post by wikibob »

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…

These kind of stories are why I come here. I'm sure we'd all love to hear more interesting history that you'd care to share.

Re: Mirror grinding

#47

Post by Stratoscope »

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…

Small world. I have one of your dad's scopes right here, and I always wondered how they made the optical components.

I will think of him the next time I use it.

Re: Mirror grinding

#48

Post by jacquesm »

semi-extrinsic wrote: post #32
siavosh wrote: post #30

Earlier quoted context omitted.

I'd love to grind my own mirror, but are there health concerns with the fine powder for the skin and respiratory systems?

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.

Re: Mirror grinding

#49

Post by ceejayoz »

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…

We've got one of the NexStar 6SEs here that my kids love to look at Jupiter and Saturn and whatnot with. Thank your dad for us - it's an awesome family activity!

Re: Mirror grinding

#50

Post by somebodynew »

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.
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