4 ms·

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 comp

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.

downrightmike 5y ago
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.
- eckmLJE 5y ago
  Great tidbit -- found this in the NYT archive (likely paywall): https://www.nytimes.com/1990/07/18/us/hubble-has-backup-mirror-unused.html https://www.nytimes.com/1990/07/18/us/hubble-has-backup-mirr...
- jcims 5y ago
  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.
- chiefgeek 5y ago
  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!
CWuestefeld 5y ago
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.)
- 23B1 5y ago
  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.
  - toss1 5y ago
    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.
  - dundarious 5y ago
    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 https://youtu.be/ZTC_RxWN_xo
    - 23B1 5y ago
      Thank you.
  - patrick0d 5y ago
    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.
    - panda88888 5y ago
      Skunk Works is a great read. Highly recommend it for people interested in the defense technology.
Florin_Andrei 5y ago
> 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.
- mncharity 5y ago
  > 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?
  - 5y ago

downrightmike 5y ago

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.

eckmLJE 5y ago
Great tidbit -- found this in the NYT archive (likely paywall): https://www.nytimes.com/1990/07/18/us/hubble-has-backup-mirror-unused.html https://www.nytimes.com/1990/07/18/us/hubble-has-backup-mirr...
jcims 5y ago
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.
chiefgeek 5y ago
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!

CWuestefeld 5y ago

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.)

23B1 5y ago
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.
- toss1 5y ago
  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.
- dundarious 5y ago
  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 https://youtu.be/ZTC_RxWN_xo
  - 23B1 5y ago
    Thank you.
- patrick0d 5y ago
  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.
  - panda88888 5y ago
    Skunk Works is a great read. Highly recommend it for people interested in the defense technology.

Florin_Andrei 5y ago

> 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.

mncharity 5y ago
> 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?
- 5y ago