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https://en.wikipedia.org/wiki/Extremely_Large_Telescope https://en.wikipedia.org/wiki/Extremely_Large_Telescope The 39-meter primary mirror has 798 1.4-meter s

https://en.wikipedia.org/wiki/Extremely_Large_Telescope https://en.wikipedia.org/wiki/Extremely_Large_Telescope The 39-meter primary mirror has 798 1.4-meter segments, each individually adjustable. The scaffolding required to hold all the segments is "significant" and must be assembled in orbit. We would stick this out next to the James Webb at L2, which means we would need the ability to travel to L2 to construct this mythical orbital extremely large telescope. With our current technology, we can build ELT on Earth for a fraction of the cost of putting it in orbit.

feoren 2y ago
Why would you assemble it at L2 instead of assembling it in LEO and then sending it out on its own?
- rickydroll 2y ago
  It is not impossible. But you would have to design the telescope to be a rocketship complete with motors, fuel tanks, and all that other fun stuff. That would significantly increase the telescope's complexity to withstand the force and vibration of a rocket engine. Theoretically, one could use ion drives, but the engineering challenges of creating ion drives capable of moving a few hundred tons of telescope are significant. It is much simpler to build the telescope equivalent of IKEA flatpack furniture and assemble it on-site than it is to build it and then move it.
  - Asraelite 2y ago
    > the engineering challenges of creating ion drives capable of moving a few hundred tons of telescope are significant. Why? Just take existing ion engine designs but use a lot of them.
_dain_ 2y ago
For one thing, before this decade is out we will have the lift capacity to get an ELT-worth of mass to L2. And the lack of need to correct for atmospheric distortion would simplify the design considerably. But it's beside the point. Such a singular mega-telescope would no longer be the only way to do astronomy in space. A world of radically lower launch costs gives more possibilities. We could have a fleet of thousands of independent small/medium sized space telescopes. That way, we wouldn't have to carefully ration imaging time between competing astronomy projects anymore. High quality data would become cheap and abundant, procured on-demand.
- rickydroll 2y ago
  It doesn't simplify the design; it increases its complexity. All the problems of telescopes on Earth become significantly more problematic in space: temperature, flexure, warping, backlash, pointing models, optical alignment, and collimation are some of the issues. Electronics, sensors, and consumables fail or run out, which means you need to be able to get there to fix things. Then, as research projects change, you need to change instruments. Yes, one could have thousands of 1-5 m telescopes. A terrestrial scope is about an order of magnitude cheaper than an orbiting telescope. So, the big question is who will pay to replace the thousands of scopes in that size range that are already operating on Earth. Rationing will still needed (Research projects always exceed the number of available scopes). High-quality data, will not be as cheap or abundant as you think it might be. With the number of scopes we are talking about and the sensors that astronomers use, we can expect hundreds or thousands of petabytes a second of data. How will we get that data down from orbit? A practical example this volume of data comes from the event horizon telescope. I could describe the firehose of firehoses but this paper from supermicro does a much better job of it https://www.supermicro.com/white_paper/white_paper_Black_Hole_Event_Horizon_Imaging.pdf https://www.supermicro.com/white_paper/white_paper_Black_Hol... This is just one project. Could you imagine a thousand of these astronomical fire hoses running at the same time in orbit? The next issue is where these telescopes should be placed. The more we learn about running telescopes in orbit, the more we realize that they need to be placed very far from Earth to increase the observable sky and the length of time one can study a chunk of it. As I said above, consumables need to be replaced, sensors need to be changed for the observing program, boils down to how you are going to get a repair crew out to wherever the telescopes are.
  - deleted 2y ago
    [deleted]
  - labcomputer 2y ago
    > Electronics, sensors, and consumables fail or run out, which means you need to be able to get there to fix things. Then, as research projects change, you need to change instruments. So launch another. Launch capacity is getting cheap... let's use it. Consider that 1% of SpaceX's annual launch capacity is enough to put one Hubble in orbit every year. Instead of sending astronauts to fix the fucked-up mirror, you just launch one with a not-broken mirror. We as a planet don't even build that many ground-based telescopes with a 2.5m+ mirror each year. Think about how astronomy would change if you could just take every telescope we build today and put it in space.
labcomputer 2y ago
> With our current technology, we can build ELT on Earth for a fraction of the cost of putting it in orbit. That's going to change soon. The reason the ELT needs individually adjustable panels is to provide corrections for refraction in the atmosphere as it swirls around. A space-ELT doesn't need that (well, maybe some cheap slow actuators to set the focus on a large non-rigid mirror). As the cost of launches drop, we will hit a point where it is cheaper to put the telescope in space where you don't need those expensive atmosphere correctors.
- DiogenesKynikos 2y ago
  Every 30-meter-class telescope uses segmented mirrors, because nobody knows how to make a single, monolithic 30-meter-diameter mirror. It's only barely technically feasible to construct these 30-m telescopes on the ground. The idea that they can be constructed in space is just fanciful.
  - labcomputer 2y ago
    Yes, but in space the segments don't need real-time corrections for atmospheric effects (and the whole structure doesn't need to be built to withstand the strong winds at the top of a mountain). The idea that you can't build that in space is just naive.

feoren 2y ago

Why would you assemble it at L2 instead of assembling it in LEO and then sending it out on its own?

rickydroll 2y ago
It is not impossible. But you would have to design the telescope to be a rocketship complete with motors, fuel tanks, and all that other fun stuff. That would significantly increase the telescope's complexity to withstand the force and vibration of a rocket engine. Theoretically, one could use ion drives, but the engineering challenges of creating ion drives capable of moving a few hundred tons of telescope are significant. It is much simpler to build the telescope equivalent of IKEA flatpack furniture and assemble it on-site than it is to build it and then move it.
- Asraelite 2y ago
  > the engineering challenges of creating ion drives capable of moving a few hundred tons of telescope are significant. Why? Just take existing ion engine designs but use a lot of them.

_dain_ 2y ago

For one thing, before this decade is out we will have the lift capacity to get an ELT-worth of mass to L2. And the lack of need to correct for atmospheric distortion would simplify the design considerably. But it's beside the point. Such a singular mega-telescope would no longer be the only way to do astronomy in space. A world of radically lower launch costs gives more possibilities. We could have a fleet of thousands of independent small/medium sized space telescopes. That way, we wouldn't have to carefully ration imaging time between competing astronomy projects anymore. High quality data would become cheap and abundant, procured on-demand.

rickydroll 2y ago
It doesn't simplify the design; it increases its complexity. All the problems of telescopes on Earth become significantly more problematic in space: temperature, flexure, warping, backlash, pointing models, optical alignment, and collimation are some of the issues. Electronics, sensors, and consumables fail or run out, which means you need to be able to get there to fix things. Then, as research projects change, you need to change instruments. Yes, one could have thousands of 1-5 m telescopes. A terrestrial scope is about an order of magnitude cheaper than an orbiting telescope. So, the big question is who will pay to replace the thousands of scopes in that size range that are already operating on Earth. Rationing will still needed (Research projects always exceed the number of available scopes). High-quality data, will not be as cheap or abundant as you think it might be. With the number of scopes we are talking about and the sensors that astronomers use, we can expect hundreds or thousands of petabytes a second of data. How will we get that data down from orbit? A practical example this volume of data comes from the event horizon telescope. I could describe the firehose of firehoses but this paper from supermicro does a much better job of it https://www.supermicro.com/white_paper/white_paper_Black_Hole_Event_Horizon_Imaging.pdf https://www.supermicro.com/white_paper/white_paper_Black_Hol... This is just one project. Could you imagine a thousand of these astronomical fire hoses running at the same time in orbit? The next issue is where these telescopes should be placed. The more we learn about running telescopes in orbit, the more we realize that they need to be placed very far from Earth to increase the observable sky and the length of time one can study a chunk of it. As I said above, consumables need to be replaced, sensors need to be changed for the observing program, boils down to how you are going to get a repair crew out to wherever the telescopes are.
- deleted 2y ago
  [deleted]
- labcomputer 2y ago
  > Electronics, sensors, and consumables fail or run out, which means you need to be able to get there to fix things. Then, as research projects change, you need to change instruments. So launch another. Launch capacity is getting cheap... let's use it. Consider that 1% of SpaceX's annual launch capacity is enough to put one Hubble in orbit every year. Instead of sending astronauts to fix the fucked-up mirror, you just launch one with a not-broken mirror. We as a planet don't even build that many ground-based telescopes with a 2.5m+ mirror each year. Think about how astronomy would change if you could just take every telescope we build today and put it in space.

labcomputer 2y ago

> With our current technology, we can build ELT on Earth for a fraction of the cost of putting it in orbit. That's going to change soon. The reason the ELT needs individually adjustable panels is to provide corrections for refraction in the atmosphere as it swirls around. A space-ELT doesn't need that (well, maybe some cheap slow actuators to set the focus on a large non-rigid mirror). As the cost of launches drop, we will hit a point where it is cheaper to put the telescope in space where you don't need those expensive atmosphere correctors.

DiogenesKynikos 2y ago
Every 30-meter-class telescope uses segmented mirrors, because nobody knows how to make a single, monolithic 30-meter-diameter mirror. It's only barely technically feasible to construct these 30-m telescopes on the ground. The idea that they can be constructed in space is just fanciful.
- labcomputer 2y ago
  Yes, but in space the segments don't need real-time corrections for atmospheric effects (and the whole structure doesn't need to be built to withstand the strong winds at the top of a mountain). The idea that you can't build that in space is just naive.