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We don't need to replicate Hubble exactly; I'm just suggesting we should put up some cheaper space telescopes with smaller mirrors than James Webb, if such a th

by elihu 5y ago

We don't need to replicate Hubble exactly; I'm just suggesting we should put up some cheaper space telescopes with smaller mirrors than James Webb, if such a thing makes sense. Maybe give them a variety of different focal ratios too. Usually with telescopes you need a huge aperture to see dim things, unless you're willing to wait a long time for an image to resolve. Not everything is super dim, though, so maybe having telescopes with an aperture much smaller than 6.5 meters would make sense for those tasks. I don't know if there's some other reason James Webb needs a huge aperture than being able to resolve and image quickly. (Maybe the lower wavelengths don't behave on small mirrors?) If that's it, though, it seems like having backup would be good.

hnaccount_rng 5y ago
The point isn’t dimness. It’s resolution (well angular, the number of pixels also doesn’t matter that much). If you want to resolve things that are close together you physically can’t separate the signals. And what is too close depends inversely on the aperature, so yeah you really need the size, for some observations. Fun fact for dimness: We are pretty much at “we can count every photon that comes” so if you want to pick up far away things, increasing the aperature is also one of the few things you can do. But at that point you’d probably prefer multiple telescopes if you can align them properly
- elihu 5y ago
  Is it just that infrared light doesn't focus as easily as visible light unless you scale up the size of the telescope accordingly? (For instance, at some point light stops behaving like light and starts behaving like radio waves.)
  - hnaccount_rng 5y ago
    The wave length of the photons also goes into it yes. I’d have to look up the precise formula. But it’s basically larger aperature -> smaller angle of separation. Longer wavelengths -> larger angle of separation. From a physics point of view there really isn’t much difference between light and radio waves. Both is just em radiation. But the absorption by matter (well really gas, there is nothing else) changes with wavelength too. And you’d optimize the ccds for whatever you want to look at. So that would be the primary consideration and keep in mind these things tend to also be build as tech demos (which also means you typically don’t get to realize all design capabilities in action). So “because it’s the most we could get away with” is a reasonable reason

by elihu 5y ago

hnaccount_rng 5y ago

The point isn’t dimness. It’s resolution (well angular, the number of pixels also doesn’t matter that much). If you want to resolve things that are close together you physically can’t separate the signals. And what is too close depends inversely on the aperature, so yeah you really need the size, for some observations. Fun fact for dimness: We are pretty much at “we can count every photon that comes” so if you want to pick up far away things, increasing the aperature is also one of the few things you can do. But at that point you’d probably prefer multiple telescopes if you can align them properly

elihu 5y ago
Is it just that infrared light doesn't focus as easily as visible light unless you scale up the size of the telescope accordingly? (For instance, at some point light stops behaving like light and starts behaving like radio waves.)
- hnaccount_rng 5y ago
  The wave length of the photons also goes into it yes. I’d have to look up the precise formula. But it’s basically larger aperature -> smaller angle of separation. Longer wavelengths -> larger angle of separation. From a physics point of view there really isn’t much difference between light and radio waves. Both is just em radiation. But the absorption by matter (well really gas, there is nothing else) changes with wavelength too. And you’d optimize the ccds for whatever you want to look at. So that would be the primary consideration and keep in mind these things tend to also be build as tech demos (which also means you typically don’t get to realize all design capabilities in action). So “because it’s the most we could get away with” is a reasonable reason