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That's a bit like saying that forks are superior to knives. They're good at different things. Ground-based telescopes have a number of very significant advanta

That's a bit like saying that forks are superior to knives. They're good at different things. Ground-based telescopes have a number of very significant advantages over space-based telescopes. You can build much larger and heavier telescopes and instruments on the ground. If you want to observe faint objects, you need more photons. To capture more photons, you need a larger primary mirror. Space-based telescopes used to give higher resolution, but adaptive optics undo much of that advantage. In fact, because the diffraction limit is dependent on the size of the primary mirror, ground-based telescopes can achieve better resolution than space-based telescopes. You can attach large, heavy instruments, such as massively multiplexed spectrometers, to ground-based telescopes. And you can switch instruments out and do periodic upgrades and maintenance. Space-based telescopes have advantages in certain specific areas. They can observe wavelengths that Earth's atmosphere absorbs or emits at (such as the ultraviolet and infrared). They can achieve much better calibration, because there's no atmosphere to calibrate out. They can achieve high resolution across a wide field of view (adaptive optics negates the effects of the atmosphere in a small field of view). Sometimes you need these particular advantages, so space telescopes are critical. Often you don't, and ground-based telescopes are superior.

marcus_holmes 6y ago
disclaimer: I know almost nothing about telescopes. Isn't there a way of multiplexing lots of small telescopes together to gather more photons and effectively emulate a larger telescope in software? If so, could you launch 1000 small telescopes into space and effectively have a (set of <1000) larger telescope(s)?
- Gibbon1 6y ago
  Multiple mirror telescopes are a thing. They can be used to gather more light or increase the angular resolution. See European Southern Observatory's very large telescope.
- zajio1am 6y ago
  AFAIK you can do that with radiotelescopes, but cannot easily with optical telescopes. That is because optical sensors lose phase information of incoming signal. There are telescopes formed from arrangement of smaller telescopes (like mentioned Very Large Telescope of European Southern Observatory), but these have to be connected optically (by a system of precise mirrors), not just digitally.
  - DiogenesKynikos 6y ago
    That's correct. For optical telescopes, you have to physically combine the signals in real time to do interferometry. The VLT can do optical interferometry, and can create baselines as long as 200 meters (reaching nearly 100x the resolution that the Hubble Space Telescope achieves).[1] If you just combine the images from different telescopes, without doing interferometry, you only get one of the benefits of a larger telescope (you can see fainter objects), but you don't get all the benefits (e.g., higher resolution). Interferometry is much easier with radio telescopes. You can record the waveform at each dish, and then digitally combine the signals afterwards (there is specialized "correlator" hardware that is purpose-built to do this very efficiently).[2] That means you can put receivers anywhere (on Earth or even in space), as long as you are able to synchronize the timestamps in the data well enough to digitally combine the signals afterwards. One of the major technical challenges faced by the Event Horizon Telescope, which imaged the black hole at the center of M87,[3] was synchronizing clocks at stations on opposite sides of the Earth. They had to physically bring atomic clocks from one location to another. After they took their observations, they flew hard drives with the recorded signals to a centralized location to do the correlation. 1. https://www.eso.org/public/teles-instr/technology/interferometry/ https://www.eso.org/public/teles-instr/technology/interferom... 2. https://www.aanda.org/articles/aa/full/2007/05/aa4519-05/aa4519-05.html https://www.aanda.org/articles/aa/full/2007/05/aa4519-05/aa4... 3. https://eventhorizontelescope.org/press-release-april-10-2019-astronomers-capture-first-image-black-hole https://eventhorizontelescope.org/press-release-april-10-201...
    - marcus_holmes 6y ago
      Interesting, thanks :) The article on interferometry implied that this is "just" a matter of computational difficulty - the higher frequency means that more accuracy and computing power is needed to resolve an image. Is that right? If so, surely this is a Moore's Law problem, solved by waiting a few years for the needed computing power to be cheap enough to use?
HeadsUpHigh 6y ago
Shouldn't 2( or more) smaller telescopes 1 positioned on the North pole and 1 on the South both on polar orbits pointing at the same spot solve that issue? To me it seems like the moment we got a proper space telescope( Hubble) there was a ton of low-hanging fruit to discover at such a degree that so many years later we are still finding out new stuff. Compared to that Earth based telescopes seem to have to go through massive efforts to make headline discoveries( not that any other less discussed paper isn't important, it's just that... how would we know about e.g. the acceleration of the universe from an Earth based observatory?).
- DiogenesKynikos 6y ago
  > Compared to that Earth based telescopes seem to have to go through massive efforts to make headline discoveries That's a function of how the media works. Ground-based telescopes are extremely important for astronomy, and I would even venture to say are involved in most major discoveries. Space telescopes are also extremely useful, but they're also much flashier than ground-based telescopes. > how would we know about e.g. the acceleration of the universe from an Earth based observatory? By observing type-Ia supernovae (SNe Ia), for example. Not only is it possible to do this with ground-based telescopes, but ground-based telescopes are the primary way it is done. Most of the SNe Ia observations that went into the discovery of the acceleration of cosmic expansion were done from the ground.

marcus_holmes 6y ago

disclaimer: I know almost nothing about telescopes. Isn't there a way of multiplexing lots of small telescopes together to gather more photons and effectively emulate a larger telescope in software? If so, could you launch 1000 small telescopes into space and effectively have a (set of <1000) larger telescope(s)?

Gibbon1 6y ago
Multiple mirror telescopes are a thing. They can be used to gather more light or increase the angular resolution. See European Southern Observatory's very large telescope.
zajio1am 6y ago
AFAIK you can do that with radiotelescopes, but cannot easily with optical telescopes. That is because optical sensors lose phase information of incoming signal. There are telescopes formed from arrangement of smaller telescopes (like mentioned Very Large Telescope of European Southern Observatory), but these have to be connected optically (by a system of precise mirrors), not just digitally.
- DiogenesKynikos 6y ago
  That's correct. For optical telescopes, you have to physically combine the signals in real time to do interferometry. The VLT can do optical interferometry, and can create baselines as long as 200 meters (reaching nearly 100x the resolution that the Hubble Space Telescope achieves).[1] If you just combine the images from different telescopes, without doing interferometry, you only get one of the benefits of a larger telescope (you can see fainter objects), but you don't get all the benefits (e.g., higher resolution). Interferometry is much easier with radio telescopes. You can record the waveform at each dish, and then digitally combine the signals afterwards (there is specialized "correlator" hardware that is purpose-built to do this very efficiently).[2] That means you can put receivers anywhere (on Earth or even in space), as long as you are able to synchronize the timestamps in the data well enough to digitally combine the signals afterwards. One of the major technical challenges faced by the Event Horizon Telescope, which imaged the black hole at the center of M87,[3] was synchronizing clocks at stations on opposite sides of the Earth. They had to physically bring atomic clocks from one location to another. After they took their observations, they flew hard drives with the recorded signals to a centralized location to do the correlation. 1. https://www.eso.org/public/teles-instr/technology/interferometry/ https://www.eso.org/public/teles-instr/technology/interferom... 2. https://www.aanda.org/articles/aa/full/2007/05/aa4519-05/aa4519-05.html https://www.aanda.org/articles/aa/full/2007/05/aa4519-05/aa4... 3. https://eventhorizontelescope.org/press-release-april-10-2019-astronomers-capture-first-image-black-hole https://eventhorizontelescope.org/press-release-april-10-201...
  - marcus_holmes 6y ago
    Interesting, thanks :) The article on interferometry implied that this is "just" a matter of computational difficulty - the higher frequency means that more accuracy and computing power is needed to resolve an image. Is that right? If so, surely this is a Moore's Law problem, solved by waiting a few years for the needed computing power to be cheap enough to use?

HeadsUpHigh 6y ago

Shouldn't 2( or more) smaller telescopes 1 positioned on the North pole and 1 on the South both on polar orbits pointing at the same spot solve that issue? To me it seems like the moment we got a proper space telescope( Hubble) there was a ton of low-hanging fruit to discover at such a degree that so many years later we are still finding out new stuff. Compared to that Earth based telescopes seem to have to go through massive efforts to make headline discoveries( not that any other less discussed paper isn't important, it's just that... how would we know about e.g. the acceleration of the universe from an Earth based observatory?).

DiogenesKynikos 6y ago
> Compared to that Earth based telescopes seem to have to go through massive efforts to make headline discoveries That's a function of how the media works. Ground-based telescopes are extremely important for astronomy, and I would even venture to say are involved in most major discoveries. Space telescopes are also extremely useful, but they're also much flashier than ground-based telescopes. > how would we know about e.g. the acceleration of the universe from an Earth based observatory? By observing type-Ia supernovae (SNe Ia), for example. Not only is it possible to do this with ground-based telescopes, but ground-based telescopes are the primary way it is done. Most of the SNe Ia observations that went into the discovery of the acceleration of cosmic expansion were done from the ground.