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Comment by dr_orpheus | Hacker News Reader

It still amazes me that one, the mirrors are sensitive enough to the deformation of Earth's gravity to have an effect and two, that we are now very good at compensating for this now.

Also consider that the mirrors on James Webb space telescope need to account for both deformation from gravity when they are manufactured but also that they are manufactured at room temperature and will be operated at cryo temperatures. But JWST is also special since they have actuators to put stress on the mirror segments to be able to reshape them slightly on-orbit to focus.

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My same grandpa that worked on the Hubble mirror also worked on anti-ICBM lasers back in the 70s. Apparently these devices had a tendency to melt their own mirrors, and that led to the development on self-deforming mirrors to deflect heat from hotspots.

(He also worked on the TEAL AMBER and TEAL BLUE satellite surveillance systems, and probably others that he never even talked about.)

Is there a good book I can read about all of this cold-war era tech? I find it endlessly fascinating the number of programs and technology advancements that happened during this period.

I don't have a good book to recommend offhand, but wandering a bit offotpic here, I know someone who worked on a military orbital program in the late '70 that was transmitting data from orbit at 800MB/s.

Some congressmen had not heard of this technology and were working to fund a completely different (iirc, civilian) program intending to develop essentially the same capabilities. They had to go give a few TS briefings to the congressmen to dissuade them from writing the redundant funding into the next defense spending bill.

Not a book, and not Cold War, but I think this talk does a good job outlining just how much of the silicon revolution came directly from WW2 military research. It also does a decent job of explaining how technologically advanced the aerial part of that war really was.

https://youtu.be/ZTC_RxWN_xo

Thank you.

There's a book called "Skunk Works" by Ben Rich about the making of different spy planes. There's some mentions in the book about satellites and what existed at the time how projects were carried out.

Skunk Works is a great read. Highly recommend it for people interested in the defense technology.

You may (or may not :-) find it surprising that there is a vibrant ecosystem of research and development on both defensive and offensive systems on going to this day. The IEEE used to give a talk about "Black Silicon Valley" which was not about people of color and their contributions (although that would be an awesome talk too) but about the origins of the technology focus in Silicon Valley with regard to radio and RADAR development starting in WW2 and continuing on to this day. When I first moved to Sunnyvale my neighbor worked at Lockheed and all he could tell me was that he worked with electricity :-). He said he hoped that some of the stuff he had worked on was disclosed in the future so that he could tell is kids and/or grand kids about it.

Having gone to school in LA where, at the time, ALL of the EE jobs were in the Defense sector, I specifically moved up to the Bay Area to work on cool stuff I could actually talk about with others[1].

One thing of note is that ageism is MUCH less of an issue in jobs that require security clearances. So a number of engineers who are suddenly perceived as "too old" to contribute to the latest hot startup find themselves recruited by the Raytheons and General Dynamics type companies that recognize their skills are valuable.

[1] As an new college grad [NCG] I was not yet aware of the myriad ways in which employers would use non-disclosure agreements to thwart such conversations.

>the origins of the technology focus in Silicon Valley

it's sad to me that most SV focus now is about how to sell more stuff to people whether that's through direct advertising or creating "social" platforms where people get to "influence" you into buying something or paying for something

Check out the Hiller Aviation Museum in San Carlos if you live near Silicon Valley. It has some amazing military tech from the 50s and 60s. Some of the aircraft have to be seen to be believed.

Just wanted to say your grandpa sounds pretty damn cool

Definitely sounds like a very cool grandpa!

Hey I saw self deforming mirrors as a missile defense intern circa 15 years ago! I thought they were dang cool at the time myself. I recall (for these designs) self deformation being used to help with tracking and compensate for, ya know, the shock of a little steering boost motors lighting off and such, in some cases.

> It still amazes me that one, the mirrors are sensitive enough to the deformation of Earth's gravity to have an effect and two, that we are now very good at compensating for this now.

I make telescope mirrors.

This problem was well understood since the time of Herschel. We just have better solutions now.

The fact that this is a space scope is irrelevant to the fact that gravity will cause issues. For any telescope, you have to account for differences in the way the mirror is deformed in manufacturing vs in actual usage, and the ways the deformation will change in usage as the scope is leaning at different angles. That is always a thing.

What is particular to the Hubble is that the load in usage is zero (which is unusual), so you have to think about it that way in manufacture. But deformations in manufacture are always an issue you have to account for somehow.

Look at it this way: there is an ideal shape that the mirror needs to have, usually a revolution surface of some conic section (parabola, hyperbola, ellipse, circle). The performance of the mirror will track the difference between the ideal surface and the actual mirror. The error allowance depends on the wavelength λ of the observed radiation.

Telescopes where the error is greater than λ/4 just suck, and are unusable. Good performance begins around λ/8. A great mirror may do better than λ/20.

For visible light, λ/4 is 100 nm, or 0.1 microns. That's 10 thousand times less than 1 mm. On that scale, the mirror is made of jelly. If you put it on a rough surface, it will deform. If you put your thumb on it, hold it for a minute so it heats up from your skin, then pull away, there will be a "mountain" left behind on the mirror, under your thumb, until it cools off again.

In many cases, the support structures for large mirrors are complex mechanisms that ensure the force is uniformly distributed across a large number of points on the back of the mirror. Even amateur telescopes built using the Dobsonian template use passive self-balancing support with 3, 6, 9, 18, or even more points, depending on the size / thickness ratio.

> thumb [...] heats [...] "mountain"

Telescope mirrors might be a nice setting for teaching a lot of interwoven physics and material science stories.

A lot of engineering is like that. I wonder how we might start systematically collecting and organizing such stories, for eventual use in education?

Absolutely right.

The phenomena are pretty intricate. E.g. in the initial stages, grinding the mirror is a physical process: you use some very hard material to carve the glass into the desired shape.

But when it gets to polishing (smoothing the surface to optical perfection), it's far more complex. The granules of the agent are microscopic, and the process is in that grey area between physics and chemistry. You end up slicing off only a few layers of molecules at each pass.

It goes on and on like this. It's very addictive if you like Physics. :)

It might be possible to handle the size aspect of such multiscale storytelling.[1] Perhaps even down through primary school.

And given a firm grasp of size, and of atoms, it might perhaps be possible to support an integrated and broad modern material science foundation. Maybe... but I'm not being very successful at finding opportunities for exploratory discussion of that. :/

It seems like it could be great fun though.

[1] a couple of old test videos https://twitter.com/mncharity/status/1377384282633596935

To be fair, Kodak made a sister mirror that wasn't chosen to go up, and when the issue on Hubble was found out, Kodak's mirror was found to be perfect.

Great tidbit -- found this in the NYT archive (likely paywall): https://www.nytimes.com/1990/07/18/us/hubble-has-backup-mirr...

It's fun to think that there is a bullet list of directions that would take me from where I'm typing this comment to standing in front of that exact mirror.

I worked in the Commercial and Govt Systems group at Kodak in the mid 90s and took a class on image compression from two guys who were part of the team that figured out how to mathematically correct the early, flawed Hubble images. Was really cool!

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