5 ms·

Vera Rubin's primary mirror gets its first reflective coating

157 points by bikenaga 2y ago

cududa 2y ago
That video was pretty wild. I didn’t understand at first what had happened when the silver layer got laid down and it was suddenly reflective
- dylan604 2y ago
  The video was very cool. The arm swings around and hello! Just like that, it's a shiny mirror. Very impressive effect to be sure
  - keitmo 2y ago
    That was breathtaking.
- Loughla 2y ago
  God the work that went into that one shot. Not just camera work, but all the engineering for the mirror. It's astounding.
  - prpl 2y ago
    The mirror is the easy part. In fact, it was finished years ago - just not coated.
    - dylan604 2y ago
      lulz. Easy. Okay. It might be the easiest part of this build (I have my doubts to the accuracy of that), but let's not get carried away with slinging words like "easy" around things that are measured to microns accuracy and took years to build.
      - prpl 2y ago
        Maybe not “easy” but it was straightforward in comparison to much of the other things on the project. It was done in the U of A mirror lab and it’s a pretty well-defined process to build these mirrors, with a small exception for this mirror being that it’s a combination M1/M3 with two different grinds, but it’s about as close to a factory process as you can get for this project. Before the mirror lab, Corning would often build the big mirrors, both for science and defense. The 300T telescope movement assembly which can move the 300+ Ton telescope 3.5 degrees in 5 seconds, and the 3.2 GP camera especially are quite a bit harder to build, engineer, and integrate.
    - OldGuyInTheClub 2y ago
      Here's how they cast an 8m class mirror at the University of Arizona. This is for the Giant Magellan Telescope, not the VRO, but it also did that one. \ https://www.youtube.com/watch?v=c-lBKuHqHk0 https://www.youtube.com/watch?v=c-lBKuHqHk0 Grinding and polishing is its own art. https://www.youtube.com/watch?v=ebqeygLdBYc https://www.youtube.com/watch?v=ebqeygLdBYc None of this looks easy to me.
dylan604 2y ago
"The VRO’s design allows the camera to capture a large area of sky the size of 7 full moons across in a single image." Wow, that's a lot of sky for a telescope. Around 3.5° at once. I purchased a telescope specifically to view DSO, and it is ~3 full moons. When attaching my older DSLR, a full moon isn't even 50% of the captured image. Of course, mine's only ~150mm instead of 8.4m. Just putting size in perspective of my own use instead of using school buses or football pitches or basketball type nonsense.
- nullc 2y ago
  It's about the same angle of view that a 400mm lens on a full frame camera has vertically. (wide field astrophotography can be fun: https://nt4tn.net/astro/horse4.jpg https://nt4tn.net/astro/horse4.jpg ) VRO has a 10313mm focal length (vs 8.4m diameter, so f/1.234-- though with a huge central obstruction) illuminating a 630mm focal plane.
- mfranc42 2y ago
  That angle of view corresponds to a lens with 588mm focal length on a full frame camera (horizontally), and 392mm (vertically). Funny that astronomers and photographers mean very different things by saying wide angle. ;-)
  - dylan604 2y ago
    People get stuck into trying to apply A to B when B != A. Just because the same words are used does not mean they have the same use and meaning as applied. Also, "space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is."
    - bingbingbing777 2y ago
      It's a full frame equivalent. I don't think anyone is trying to use this on a full frame camera.
    - mfranc42 2y ago
      Well, if you never compare A to B, you won't believe it for sure. :-/
nullc 2y ago
The AAT has a pretty good video on aluminising with a lot of exposition, https://www.youtube.com/watch?v=fztRqJerfOk https://www.youtube.com/watch?v=fztRqJerfOk VRO's process is considerably more sophisticated, as they do the silicon nitride protective coating. I believe the specific approach on VRO is similar to the one pioneered for gemini: https://www.gemini.edu/files/docman/websplash/websplash2004-23/GN_silver_coating.pdf https://www.gemini.edu/files/docman/websplash/websplash2004-... Fancier protected metal (and dialectic enhanced) coatings have been common in smaller reflectors... but it's quite a big difference doing a fancy sputter coating on a 150mm object vs a 8m one! :P
mrbluecoat 2y ago
Very cool! HD video: https://rubin.canto.com/v/gallery/album/HDSNU?display=curatedView&viewIndex=2&column=video&id=l0mmphnind6krdvmp52e3mqf0l https://rubin.canto.com/v/gallery/album/HDSNU?display=curate...
ramijames 2y ago
It's so, so cool that humanity builds these kinds of things. I'm so grateful that there are people who dedicate their lives to this. I wish that I had.
- dylan604 2y ago
  you can always switch careers. you'll just have to switch the lifestyle to which you have become accustomed as well. I'm guessing the average bit banger around these parts earns a much higher salary than the average person in astronomy. You do however have the potential of actually helping humanity understand things rather than droning away on ad tech and social platforms which is definitely not pushing humanity in any thing resembling a positive direction.
  - somenameforme 2y ago
    When I [briefly] looked into this, it seemed that positions in astronomy were extremely competitive, extremely limited, and generally had a requirement of PhD and the usual toppings alongside. Love to know if I was wrong somehow? Astronomy has always been a passion, but I never pursued it because of certain irritating things in cosmology that left me somewhat jaded.
    - ramijames 2y ago
      Same. I'm 44. No chance I'm going to be able to have a career in astronomy now.
    - OldGuyInTheClub 2y ago
      Not an astronomer but have an advanced degree in physical science, very experienced with optics and engineering, and incredibly interested in the science. I looked into technical positions at a couple of giant telescope projects (not VRO) and got nowhere. I did speak with someone informally about it and was told that the science community was pretty decoupled from the instrument and facility builders.
    - ripap 2y ago
airstrike 2y ago
as much as I'd like to read this, the mobile website is hot garbage due to the number of ads.. if anyone has a better source, I'm all ears
- Larrikin 2y ago
  https://www.firefox.com https://www.firefox.com https://github.com/dhowe/AdNauseam https://github.com/dhowe/AdNauseam
  - renjimen 2y ago
    Use Brave if you're on iOS. Firefox on iOS doesn't block ads IIRC
  - airstrike 2y ago
    no adnauseam on ff on iOS
- mrbluecoat 2y ago
  Via works great: https://viayoo.com/en/ https://viayoo.com/en/
npunt 2y ago
Vera Rubin was quite an astronomer [1] both in her discoveries like the rotation of galaxies (confirming dark matter), and for being a trailblazer for women in astronomy. I love reading bios of people like this. Glad to see her get recognized with such a great observatory [2] continuing her legacy of studying dark matter. [1] https://en.wikipedia.org/wiki/Vera_Rubin https://en.wikipedia.org/wiki/Vera_Rubin [2] https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory
- jpizagno 2y ago
  Fritz Zwicky discovered Dark Matter. Rubin thought she discovered Dark Matter, because she wasn't aware of Zwicky's paper written in German many years before Rubin's paper. edit: I have a PhD in Astronomy, with a focus on Dark Matter, and I measured rotation curves of galaxies myself.
  - deleted 2y ago
    [deleted]
  - david-gpu 2y ago
    Wikipedia link for those who, like me, where unfamiliar: https://en.wikipedia.org/wiki/Fritz_Zwicky https://en.wikipedia.org/wiki/Fritz_Zwicky
  - vo2maxer 2y ago
    I have not read anywhere that she discovered Dark Matter but rather that her observations confirmed its presence as “first proposed by Fritz Zwicky in 1933 but rejected by most scientists.” [1] https://www.nsf.gov/news/special_reports/medalofscience50/rubin.jsp https://www.nsf.gov/news/special_reports/medalofscience50/ru...
JohnMunsch 2y ago
This telescope and its mission to catch changes in the sky over time is going to help discover so many things.
- TheBlight 2y ago
  The reaction will be interesting.
- dylan604 2y ago
  I love that we're also remembering to take less scoped in to see a wider view. I wonder how many times they will complete the full sky survey. Will there be enough to provide a timelapse from multiple surveys, or will it take the entire scope to complete the full sky? I find some of the timelapse of various objects quite fascinating now that we've been recording observations long enough. Seeing the movement around SagA* is amazing. Just the other day, we saw Cassiopeia A over a couple of decades. Some part of my brain knows things are constantly changing in the universe, but at the time scale it seems strange things are noticeably different within our own lifetimes.
  - sapiogram 2y ago
    They will survey the entire sky every week or so. At the end, you can create a nice 60 fps timelapse of many objects in the southern sky.
    - deleted 2y ago
      [deleted]
unfamiliar 2y ago
Dumb question, but why don’t they just use a single more highly-curved mirror rather than a series of 3 bounces?
- Sharlin 2y ago
  More curved means more optical aberrations. The Rubin is a so-called three-mirror anastigmat design that minimizes astigmatism, coma, and spherical aberration. (Chromatic aberration is not a problem in reflectors because dispersion only occurs when light is refracted.) A two-mirror design couldn’t be used in such a wide-field telescope without severe image quality issues, at least comparatively speaking. https://en.wikipedia.org/wiki/Three-mirror_anastigmat https://en.wikipedia.org/wiki/Three-mirror_anastigmat
- deleted 2y ago
  [deleted]
euroderf 2y ago
Watching all that monster-size equipment makes me think of the Olympics: is a lot of money being poured into a single use ? Was this equipment pre-existing or purpose-built ? If the latter, will any get re-used/repurposed for other endeavors ?
- ccgreg 2y ago
  There are a bunch of Vera Rubin-sized mirrors in orbit, that's why this one is the size that it is.
  - prpl 2y ago
    No there’s not -JWST is largest mirror in orbit and it is a few meters smaller, despite being many smaller mirrors. The mirror size is largely due to a tunnel size on the way to the summit in Chile, and applies to the other nearby telescopes. It’s also really hard to ship mirrors much larger than this on a boat.
    - ccgreg 2y ago
      Apologies, I was thinking Roman not Rubin.
Synaesthesia 2y ago
This telescope has a lot of interesting details. The CCD is massive! The largest ever made, and it's going to be downloading so much data it's actually insane. Way more than can be processed by a human.
bloopernova 2y ago
From this page[1] linked from the article: "The Rubin Observatory will produce, on average, 15 terabytes of LSST data per night, yielding an uncompressed data set of 200 petabytes. Dedicated facilities will process the image data in near real time." And "The camera’s 3.2-gigapixel focal plane array comprises 189 4Kx4K CCD sensors with 10 µm pixels. The sensors are deep depletion, back-illuminated devices with a highly segmented architecture that enables the entire array to be read out in two seconds." Wow that is a huge amount of data from some amazing hardware. This is going to be amazing! [1] https://kipac.stanford.edu/research/projects/vera-rubin-observatorys-legacy-survey-space-and-time https://kipac.stanford.edu/research/projects/vera-rubin-obse...
hinkley 2y ago
Wait. I thought we were doing large diameter telescopes as a bunch of individual, tractably-sized mirrors pointing at a common collector. When the idea was presented to me they made it sound like it was impossible to make larger monolithic mirrors because we’d hit a wall. How did monolithic mirrors get out ahead again?
- hinkley 2y ago
  Answering my own question: Rubin will not be the largest aperture telescope in the world. It’s 8.4m, while Keck is 10m and no longer the largest segmented mirror. Rubin will be the largest single mirror main reflector, beating 8.2m for the previous largest telescopes. So it’s incremental improvement not retaking a crown.
- OldGuyInTheClub 2y ago
  8.x m is about the largest single mirror that can be cast with current technology. Above that, segmented wins. Giant Magellan[0] will use multiple 8m mirrors to get to ~25m total aperture. The Thirty Meter Telescope[0] and the European Extremely Large Telescope (39m) will be segmented. The VRO picked the single mirror approach for lower complexity and the ability to lock the primary and tertiary mirrors into one monolithic structure. For this, they accepted some higher fabrication risk. Source: A friend on the VRO project. [0] GMT and TMT are at budget risk and TMT faces stiff opposition from Hawaiians who say Mauna Kea is sacred and shouldn't have anything at the summit.
  - knodi123 2y ago
    manufacturing challenges aside, isn't there a practical limit to what terrestrial optical telescopes can achieve, given the interference from our atmosphere? I thought things like JWST was the necessary next step in improvement.
    - hinkley 2y ago
      I’ve been binging Goldratt lately, so my brain is awash in Theory of Constraints. It’s okay to use “old” equipment to offload from a bottleneck. Space based telescopes are going to be a bottleneck for the foreseeable future. They have severe maintenance restrictions and the more you task swap them the more frequent that maintenance. So from a constraint perspective, you use ground based observation to do anything that can be done without a space telescope. Including trying to determine which patches of sky justify time on JWT.
knodi123 2y ago
I'm not asking this skeptically, but out of curiosity and ignorance- how the heck does a 3 mirror setup make sense? Can't a single mirror always accomplish the same goal? (for that matter, why mirrors, with their blind spot where the sensor is, instead of lenses?)
- JumpCrisscross 2y ago
  > how the heck does a 3 mirror setup make sense? Can't a single mirror always accomplish the same goal? Easier to make three smaller mirrors than one big one. > why mirrors, with their blind spot where the sensor is, instead of lenses? "Mirrors don't cause chromatic aberration and they are easier and cheaper to build large. They are also easier to mount because the back of the mirror can be used to attach to the mount" [1]. (And there are off-axis designs that avoid obstructing the incoming light.) [1] https://lco.global/spacebook/telescopes/reflecting-telescopes/ https://lco.global/spacebook/telescopes/reflecting-telescope...
  - knodi123 2y ago
    That's not intuitive to me, but I recognize that the problem is on my end. Thanks for explaining!
- hooper 2y ago
  The image from a single curved mirror or lens will have distortions that can be reduced by adding additional curved mirrors/lenses. It's also harder to make and use large diameter lenses than mirrors. Each lens has two surfaces that need to be aligned without the possibility of post-fabrication calibration. Weight and sagging are a bigger problem for lenses. Mirrors can be thinner, partially hollow, and can have mechanical support behind them without blocking light. There are further considerations that might favor mirrors, like material cost and reaction to temperature changes. If nothing else, bending the light path back and forth with mirrors means the telescope can be shorter, easier to point, and will fit in a smaller building. The largest exclusively lens based ("refractor") telescopes got up to about 1 meter diameter before the trade offs caused a shift to mirrors for larger apertures. Even so, it's common to have lenses near the focal plane of a mirror based ("reflector") telescope to improve the image. Vera Rubin is like that, including a 1.5 meter lens (among others) near the sensor. The sensor doesn't actually form a blind spot in the image, because it is severely out of focus. Obstructions do affect the pattern of light a star forms on the sensor, but it's all relative, and no mirror or lens can produce perfect images.

157 points by bikenaga 2y ago

That video was pretty wild. I didn’t understand at first what had happened when the silver layer got laid down and it was suddenly reflective

dylan604 2y ago
The video was very cool. The arm swings around and hello! Just like that, it's a shiny mirror. Very impressive effect to be sure
- keitmo 2y ago
  That was breathtaking.
Loughla 2y ago
God the work that went into that one shot. Not just camera work, but all the engineering for the mirror. It's astounding.
- prpl 2y ago
  The mirror is the easy part. In fact, it was finished years ago - just not coated.
  - dylan604 2y ago
    lulz. Easy. Okay. It might be the easiest part of this build (I have my doubts to the accuracy of that), but let's not get carried away with slinging words like "easy" around things that are measured to microns accuracy and took years to build.
    - prpl 2y ago
      Maybe not “easy” but it was straightforward in comparison to much of the other things on the project. It was done in the U of A mirror lab and it’s a pretty well-defined process to build these mirrors, with a small exception for this mirror being that it’s a combination M1/M3 with two different grinds, but it’s about as close to a factory process as you can get for this project. Before the mirror lab, Corning would often build the big mirrors, both for science and defense. The 300T telescope movement assembly which can move the 300+ Ton telescope 3.5 degrees in 5 seconds, and the 3.2 GP camera especially are quite a bit harder to build, engineer, and integrate.
  - OldGuyInTheClub 2y ago
    Here's how they cast an 8m class mirror at the University of Arizona. This is for the Giant Magellan Telescope, not the VRO, but it also did that one. \ https://www.youtube.com/watch?v=c-lBKuHqHk0 https://www.youtube.com/watch?v=c-lBKuHqHk0 Grinding and polishing is its own art. https://www.youtube.com/watch?v=ebqeygLdBYc https://www.youtube.com/watch?v=ebqeygLdBYc None of this looks easy to me.

dylan604 2y ago

"The VRO’s design allows the camera to capture a large area of sky the size of 7 full moons across in a single image." Wow, that's a lot of sky for a telescope. Around 3.5° at once. I purchased a telescope specifically to view DSO, and it is ~3 full moons. When attaching my older DSLR, a full moon isn't even 50% of the captured image. Of course, mine's only ~150mm instead of 8.4m. Just putting size in perspective of my own use instead of using school buses or football pitches or basketball type nonsense.

nullc 2y ago
It's about the same angle of view that a 400mm lens on a full frame camera has vertically. (wide field astrophotography can be fun: https://nt4tn.net/astro/horse4.jpg https://nt4tn.net/astro/horse4.jpg ) VRO has a 10313mm focal length (vs 8.4m diameter, so f/1.234-- though with a huge central obstruction) illuminating a 630mm focal plane.
mfranc42 2y ago
That angle of view corresponds to a lens with 588mm focal length on a full frame camera (horizontally), and 392mm (vertically). Funny that astronomers and photographers mean very different things by saying wide angle. ;-)
- dylan604 2y ago
  People get stuck into trying to apply A to B when B != A. Just because the same words are used does not mean they have the same use and meaning as applied. Also, "space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is."
  - bingbingbing777 2y ago
    It's a full frame equivalent. I don't think anyone is trying to use this on a full frame camera.
  - mfranc42 2y ago
    Well, if you never compare A to B, you won't believe it for sure. :-/

nullc 2y ago

The AAT has a pretty good video on aluminising with a lot of exposition, https://www.youtube.com/watch?v=fztRqJerfOk https://www.youtube.com/watch?v=fztRqJerfOk VRO's process is considerably more sophisticated, as they do the silicon nitride protective coating. I believe the specific approach on VRO is similar to the one pioneered for gemini: https://www.gemini.edu/files/docman/websplash/websplash2004-23/GN_silver_coating.pdf https://www.gemini.edu/files/docman/websplash/websplash2004-... Fancier protected metal (and dialectic enhanced) coatings have been common in smaller reflectors... but it's quite a big difference doing a fancy sputter coating on a 150mm object vs a 8m one! :P

mrbluecoat 2y ago

Very cool! HD video: https://rubin.canto.com/v/gallery/album/HDSNU?display=curatedView&viewIndex=2&column=video&id=l0mmphnind6krdvmp52e3mqf0l https://rubin.canto.com/v/gallery/album/HDSNU?display=curate...

ramijames 2y ago

It's so, so cool that humanity builds these kinds of things. I'm so grateful that there are people who dedicate their lives to this. I wish that I had.

dylan604 2y ago
you can always switch careers. you'll just have to switch the lifestyle to which you have become accustomed as well. I'm guessing the average bit banger around these parts earns a much higher salary than the average person in astronomy. You do however have the potential of actually helping humanity understand things rather than droning away on ad tech and social platforms which is definitely not pushing humanity in any thing resembling a positive direction.
- somenameforme 2y ago
  When I [briefly] looked into this, it seemed that positions in astronomy were extremely competitive, extremely limited, and generally had a requirement of PhD and the usual toppings alongside. Love to know if I was wrong somehow? Astronomy has always been a passion, but I never pursued it because of certain irritating things in cosmology that left me somewhat jaded.
  - ramijames 2y ago
    Same. I'm 44. No chance I'm going to be able to have a career in astronomy now.
  - OldGuyInTheClub 2y ago
    Not an astronomer but have an advanced degree in physical science, very experienced with optics and engineering, and incredibly interested in the science. I looked into technical positions at a couple of giant telescope projects (not VRO) and got nowhere. I did speak with someone informally about it and was told that the science community was pretty decoupled from the instrument and facility builders.
  - ripap 2y ago

airstrike 2y ago

as much as I'd like to read this, the mobile website is hot garbage due to the number of ads.. if anyone has a better source, I'm all ears

Larrikin 2y ago
https://www.firefox.com https://www.firefox.com https://github.com/dhowe/AdNauseam https://github.com/dhowe/AdNauseam
- renjimen 2y ago
  Use Brave if you're on iOS. Firefox on iOS doesn't block ads IIRC
- airstrike 2y ago
  no adnauseam on ff on iOS
mrbluecoat 2y ago
Via works great: https://viayoo.com/en/ https://viayoo.com/en/

npunt 2y ago

Vera Rubin was quite an astronomer [1] both in her discoveries like the rotation of galaxies (confirming dark matter), and for being a trailblazer for women in astronomy. I love reading bios of people like this. Glad to see her get recognized with such a great observatory [2] continuing her legacy of studying dark matter. [1] https://en.wikipedia.org/wiki/Vera_Rubin https://en.wikipedia.org/wiki/Vera_Rubin [2] https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory https://en.wikipedia.org/wiki/Vera_C._Rubin_Observatory

jpizagno 2y ago
Fritz Zwicky discovered Dark Matter. Rubin thought she discovered Dark Matter, because she wasn't aware of Zwicky's paper written in German many years before Rubin's paper. edit: I have a PhD in Astronomy, with a focus on Dark Matter, and I measured rotation curves of galaxies myself.
- deleted 2y ago
  [deleted]
- david-gpu 2y ago
  Wikipedia link for those who, like me, where unfamiliar: https://en.wikipedia.org/wiki/Fritz_Zwicky https://en.wikipedia.org/wiki/Fritz_Zwicky
- vo2maxer 2y ago
  I have not read anywhere that she discovered Dark Matter but rather that her observations confirmed its presence as “first proposed by Fritz Zwicky in 1933 but rejected by most scientists.” [1] https://www.nsf.gov/news/special_reports/medalofscience50/rubin.jsp https://www.nsf.gov/news/special_reports/medalofscience50/ru...

JohnMunsch 2y ago

This telescope and its mission to catch changes in the sky over time is going to help discover so many things.

TheBlight 2y ago
The reaction will be interesting.
dylan604 2y ago
I love that we're also remembering to take less scoped in to see a wider view. I wonder how many times they will complete the full sky survey. Will there be enough to provide a timelapse from multiple surveys, or will it take the entire scope to complete the full sky? I find some of the timelapse of various objects quite fascinating now that we've been recording observations long enough. Seeing the movement around SagA* is amazing. Just the other day, we saw Cassiopeia A over a couple of decades. Some part of my brain knows things are constantly changing in the universe, but at the time scale it seems strange things are noticeably different within our own lifetimes.
- sapiogram 2y ago
  They will survey the entire sky every week or so. At the end, you can create a nice 60 fps timelapse of many objects in the southern sky.
  - deleted 2y ago
    [deleted]

unfamiliar 2y ago

Dumb question, but why don’t they just use a single more highly-curved mirror rather than a series of 3 bounces?

Sharlin 2y ago
More curved means more optical aberrations. The Rubin is a so-called three-mirror anastigmat design that minimizes astigmatism, coma, and spherical aberration. (Chromatic aberration is not a problem in reflectors because dispersion only occurs when light is refracted.) A two-mirror design couldn’t be used in such a wide-field telescope without severe image quality issues, at least comparatively speaking. https://en.wikipedia.org/wiki/Three-mirror_anastigmat https://en.wikipedia.org/wiki/Three-mirror_anastigmat
deleted 2y ago
[deleted]

euroderf 2y ago

Watching all that monster-size equipment makes me think of the Olympics: is a lot of money being poured into a single use ? Was this equipment pre-existing or purpose-built ? If the latter, will any get re-used/repurposed for other endeavors ?

ccgreg 2y ago
There are a bunch of Vera Rubin-sized mirrors in orbit, that's why this one is the size that it is.
- prpl 2y ago
  No there’s not -JWST is largest mirror in orbit and it is a few meters smaller, despite being many smaller mirrors. The mirror size is largely due to a tunnel size on the way to the summit in Chile, and applies to the other nearby telescopes. It’s also really hard to ship mirrors much larger than this on a boat.
  - ccgreg 2y ago
    Apologies, I was thinking Roman not Rubin.

Synaesthesia 2y ago

This telescope has a lot of interesting details. The CCD is massive! The largest ever made, and it's going to be downloading so much data it's actually insane. Way more than can be processed by a human.

bloopernova 2y ago

From this page[1] linked from the article: "The Rubin Observatory will produce, on average, 15 terabytes of LSST data per night, yielding an uncompressed data set of 200 petabytes. Dedicated facilities will process the image data in near real time." And "The camera’s 3.2-gigapixel focal plane array comprises 189 4Kx4K CCD sensors with 10 µm pixels. The sensors are deep depletion, back-illuminated devices with a highly segmented architecture that enables the entire array to be read out in two seconds." Wow that is a huge amount of data from some amazing hardware. This is going to be amazing! [1] https://kipac.stanford.edu/research/projects/vera-rubin-observatorys-legacy-survey-space-and-time https://kipac.stanford.edu/research/projects/vera-rubin-obse...

hinkley 2y ago

Wait. I thought we were doing large diameter telescopes as a bunch of individual, tractably-sized mirrors pointing at a common collector. When the idea was presented to me they made it sound like it was impossible to make larger monolithic mirrors because we’d hit a wall. How did monolithic mirrors get out ahead again?

hinkley 2y ago
Answering my own question: Rubin will not be the largest aperture telescope in the world. It’s 8.4m, while Keck is 10m and no longer the largest segmented mirror. Rubin will be the largest single mirror main reflector, beating 8.2m for the previous largest telescopes. So it’s incremental improvement not retaking a crown.
OldGuyInTheClub 2y ago
8.x m is about the largest single mirror that can be cast with current technology. Above that, segmented wins. Giant Magellan[0] will use multiple 8m mirrors to get to ~25m total aperture. The Thirty Meter Telescope[0] and the European Extremely Large Telescope (39m) will be segmented. The VRO picked the single mirror approach for lower complexity and the ability to lock the primary and tertiary mirrors into one monolithic structure. For this, they accepted some higher fabrication risk. Source: A friend on the VRO project. [0] GMT and TMT are at budget risk and TMT faces stiff opposition from Hawaiians who say Mauna Kea is sacred and shouldn't have anything at the summit.
- knodi123 2y ago
  manufacturing challenges aside, isn't there a practical limit to what terrestrial optical telescopes can achieve, given the interference from our atmosphere? I thought things like JWST was the necessary next step in improvement.
  - hinkley 2y ago
    I’ve been binging Goldratt lately, so my brain is awash in Theory of Constraints. It’s okay to use “old” equipment to offload from a bottleneck. Space based telescopes are going to be a bottleneck for the foreseeable future. They have severe maintenance restrictions and the more you task swap them the more frequent that maintenance. So from a constraint perspective, you use ground based observation to do anything that can be done without a space telescope. Including trying to determine which patches of sky justify time on JWT.

knodi123 2y ago

I'm not asking this skeptically, but out of curiosity and ignorance- how the heck does a 3 mirror setup make sense? Can't a single mirror always accomplish the same goal? (for that matter, why mirrors, with their blind spot where the sensor is, instead of lenses?)

JumpCrisscross 2y ago
> how the heck does a 3 mirror setup make sense? Can't a single mirror always accomplish the same goal? Easier to make three smaller mirrors than one big one. > why mirrors, with their blind spot where the sensor is, instead of lenses? "Mirrors don't cause chromatic aberration and they are easier and cheaper to build large. They are also easier to mount because the back of the mirror can be used to attach to the mount" [1]. (And there are off-axis designs that avoid obstructing the incoming light.) [1] https://lco.global/spacebook/telescopes/reflecting-telescopes/ https://lco.global/spacebook/telescopes/reflecting-telescope...
- knodi123 2y ago
  That's not intuitive to me, but I recognize that the problem is on my end. Thanks for explaining!
hooper 2y ago
The image from a single curved mirror or lens will have distortions that can be reduced by adding additional curved mirrors/lenses. It's also harder to make and use large diameter lenses than mirrors. Each lens has two surfaces that need to be aligned without the possibility of post-fabrication calibration. Weight and sagging are a bigger problem for lenses. Mirrors can be thinner, partially hollow, and can have mechanical support behind them without blocking light. There are further considerations that might favor mirrors, like material cost and reaction to temperature changes. If nothing else, bending the light path back and forth with mirrors means the telescope can be shorter, easier to point, and will fit in a smaller building. The largest exclusively lens based ("refractor") telescopes got up to about 1 meter diameter before the trade offs caused a shift to mirrors for larger apertures. Even so, it's common to have lenses near the focal plane of a mirror based ("reflector") telescope to improve the image. Vera Rubin is like that, including a 1.5 meter lens (among others) near the sensor. The sensor doesn't actually form a blind spot in the image, because it is severely out of focus. Obstructions do affect the pattern of light a star forms on the sensor, but it's all relative, and no mirror or lens can produce perfect images.