
Mirror grinding - Tomte
http://www.scopemaking.net/mirror/mirror.htm
======
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!

~~~
somebodynew
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.

~~~
nialo
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...](https://en.wikipedia.org/wiki/Schmidt_corrector_plate#/media/File:Schema_lame_de_Schmidt.svg)
describes the plate as being hard to tell apart from a flat disk visually.

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chanderson0
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...](https://en.wikipedia.org/wiki/Interferometry#Engineering_and_applied_science)
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 mirror and scaled it up to the size of
the United States, the biggest deformity from a perfect curve would be less
than you are tall - far less than your ability to perceive.

The other thing that was impressed me is that up until recently, the last
stage of high-precision mirror making was literally done by hand. My dad would
literally rub on a mirror with very, very fine grit to take out bumps on the
order of microns. Recently, they've switched to machines in that last step to
make it faster and more accurate, but for many applications the traditional
way worked just fine.

~~~
digler999
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.

~~~
dekhn
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...](https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3258)
"only" $17.5K

~~~
nomel
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...](https://pdfs.semanticscholar.org/d1ed/a97dd2cf70f54f24b85abc320915cab1be4a.pdf)

~~~
dekhn
WHen I started grad school in Biophysics (1995), a microscope professor
mentioned adaptive optics and asked the incoming students if they thought
similar processes could be done in microscopy. If you said yes, he let you
join his lab (they were already working on this).

~~~
digler999
I wonder if it would be possible to make a deformable mirror using a
combination of liquid mercury chemically bonded with a ferromagnetic metal (if
mercury isn't already magnetic). Then theoretically, you could use an electric
current in a coil to shape the liquid into a precise shape.

~~~
dekhn
Maybe? I wouldn't touch mercury for any reason, though.

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jonathansizz
I know a few keen amateur astronomers who make extra cash by producing
astronomical equipment for other amateurs. Some of them are very skilled, and
could probably start a fully-fledged business if they wanted to, but they
prefer to keep it as a hobby.

I paid one guy $400 to build me an equatorial platform for my 12-inch Dob. The
value it provides me is way higher than the financial cost, and he could
probably charge more for his work, but I think his main motivation comes from
the satisfaction of building things he's proud of and that others can enjoy.
Certainly, when I occasionally donate my time and knowledge at public
astronomy outreach events, I find it very rewarding, especially if I can
inspire children's interest.

N.B. By far the biggest weakness of Dobsonian (or any alt-azimuth mounted)
telescopes is the lack of tracking, and thus having continuously to nudge the
tube to keep objects centred. An equatorial platform eliminates this weakness,
and I strongly recommend one to anyone who's frustrated with their alt-az
scope.

~~~
Florin_Andrei
It's a very small market. And at the higher volume lower end (cheap scopes for
newbies) you can't compete with chinese mass production.

> _An equatorial platform eliminates this weakness, and I strongly recommend
> one_

Agreed. You own a dob, just get a platform already. It's even useful for
astrophotography.

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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.

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chillydawg
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?

~~~
digler999
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 reflective metal atoms. I'd wager that
process fills in the ~last nanometers~ or so of imperfections. Or at least
averages them out enough to not affect the optical performance.

~~~
Florin_Andrei
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 be very hard to
make mirrors.

Source: I make telescope mirrors.

~~~
Avshalom
And indeed x-ray mirrors are a bitch

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

------
mrob
This appears to be incomplete. It explains coarse and fine grinding but that's
not enough to make a usable mirror. You need to continue with polishing and
figuring before getting it aluminized. Here's an explanation of the whole
process:

[http://www.bbastrodesigns.com/JoyOfMirrorMaking/Intro.html](http://www.bbastrodesigns.com/JoyOfMirrorMaking/Intro.html)

------
Florin_Andrei
A much more extensive guide is at Stellafane:

[http://stellafane.org/tm/](http://stellafane.org/tm/)

Keep in mind this is a long term project. If you're looking for something to
do with your hands, a physical object in the real world, that would keep you
busy for months (assuming you're not working on it full time) - you've found
it.

Keep at it and you'll make a very high quality instrument. My first parabolic
mirror came out at lambda/25 precision (and no ripples, no turned edge),
whereas many commercial mirrors are only lambda/4 or 6 and the edge is
sometimes questionable.

As for the instrument itself (everything except the optics), it's not much
harder than making a simple cabinet, and in some ways it's easier. Go slow,
read a lot, think ahead, and you'll succeed.

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danbruc
There is also a 90 minutes film »Telescope building with John Dobson« [1],
inventor of the Dobsonian telescope [2], going through the entire process of
building a 16" telescope from scratch.

[1]
[https://www.youtube.com/watch?v=snz7JJlSZvw](https://www.youtube.com/watch?v=snz7JJlSZvw)

[2]
[https://en.wikipedia.org/wiki/Dobsonian_telescope](https://en.wikipedia.org/wiki/Dobsonian_telescope)

------
_rpd
From a previous HN thread for this article ...

> I grinded some mirror. Nice experience but at end monotonous and boring.
> Also grinding powder is lethal, I had my appendix removed since I eat some
> :-(

[https://news.ycombinator.com/item?id=6503667](https://news.ycombinator.com/item?id=6503667)

------
erikpukinskis
Random semi-related question I was thinking about last night:

Could you:

1) make a coarse "random" mirror that was bumpy at the large scale

2) buff out all the fine texture so you get clear fragments of an image all
across the lens at random focal planes

3) put it on a track with a LCD screen on one side and a camera on the other

4) use some sort of gradient descent algorithm to move the image and camera
around and reverse engineer the normal map for the lens

5) Use that displacement map to generate a light field from an image

6) Use the light field to generate any (clear, in focus) image you want in the
light field volume.

In other words, can you make a Very Bad but Smooth lens, and then make up for
it in software?

My guess: you could, but for any given focal plane you'd recover very little
resolution because probabalistically few of the microlens fragments are
focused there.

~~~
Balgair
1) Yes, ThorLabs sells them:
[https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=11...](https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1132)

2) You mean like a fun house mirror?

3) So you mean have a screen shine through a wonky lens at a camera?

4) Yes, this is commonly done with interferometers in many junior physics lab
classes, though the set-up is different.
[https://en.wikipedia.org/wiki/Optical_coherence_tomography](https://en.wikipedia.org/wiki/Optical_coherence_tomography)

5) So, try to figure out how the surface deforms the light image.

6) Yes, we have been doing this for many years now:
[https://www.edmundoptics.com/resources/application-
notes/opt...](https://www.edmundoptics.com/resources/application-
notes/optics/introduction-to-adaptive-optics-and-deformable-mirrors/)

Yes, that is considered a 'solved' problem, but the costs and market for such
devices limits their wide-spread use. Typically they are used for nanoscopes
and telescopes. There are a few purveyors, but add 2 0s to the end of whatever
you think the cost should be.

The limiting factor is the light coming in and having a 'known' value to
measure your noise against. In telescopes, this is typically a 'guide' star
that has a very well calibrated light spectrum and positions that you can
measure the noise values against.

Also, another cool thing youmay want to look into is electro-wetting lenses,
basically a digital lens:
[https://en.wikipedia.org/wiki/Electrowetting](https://en.wikipedia.org/wiki/Electrowetting)

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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.

------
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..

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analog31
I work in optics at my day job. For me, one of my secret pleasures of optics
is learning the ancient technologies, techniques, and measurements, that
remain viable even in modern times. It's a pleasurable counterbalance to the
"new framework of the month" syndrome, and helps keep me sane.

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ryanf323
I interned at a great company ([https://qedmrf.com/](https://qedmrf.com/))
that engineers magnetorheological fluid and machines for polishing / analyzing
optics. I can't imagine doing it by hand. Fascinating stuff.

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pstuart
If you live in the SF East Bay, there's a free workshop that meets every
Friday night at the Chabot Space Center:

[http://www.chabotspace.org/telescope-makers-
workshop.htm](http://www.chabotspace.org/telescope-makers-workshop.htm)

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chairmanwow
How is the curvature of the mirror measured during manufacturing? It's obvious
that the mirrors require extreme precision in order to function correctly.

~~~
digler999
[https://en.wikipedia.org/wiki/Spherometer](https://en.wikipedia.org/wiki/Spherometer)

~~~
Florin_Andrei
Spherometers are only used during the initial stages. Towards the end you
switch to optical testing.

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agumonkey
Needs the same for lenses. I'm very eager to try optics.

