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In astronomy a technique called lucky imaging can be used to reduce the effects of the atmosphere. You use a high frame rate and throw away all the frames where

by xioxox 6y ago

In astronomy a technique called lucky imaging can be used to reduce the effects of the atmosphere. You use a high frame rate and throw away all the frames where the atmospheric quality is poor. This technique produces a narrow field of view, however, and requires a lot of photons.

goodcanadian 6y ago
"Lucky imaging" is not a widely used technique in astronomy. There is generally no shortage of photons in earth observation, however, so it could work quite well on spy satellites. A much more common technique is adaptive optics. A wavefront sensor measures the incoming signal from a reference star (or laser guide star) and deforms the mirror to compensate. I wouldn't be surprised if adaptive optics is also used on spy satellites though I can't imagine what they would use for a reference. As other commenters have mentioned, you are always going to be limited by the diffraction limit of the telescope, which is a function of the mirror size and wavelength.
- balnaphone 6y ago
  > "Lucky imaging" is not a widely used technique in astronomy True, but it is very popular in astrophotography. Also, there are many recent advances in super-resolution imaging, compressive sensing, and sub-diffraction-limit pixels, often based on results from Donoho, Candes, and friends. Taking advantage of signal sparsity (i.e. that the set of physically possible signals is much smaller than the set of all possible signals, and physical systems have many constraints upon them) has led to a lot of surprising results over the past 15 years or so. Another approach that has yielded results is aperture synthesis, which has been applied to optical systems for at least the past 15 years. And in some certain circumstances for spysats, optical heterodyne detection can also be used to great effect. > you are always going to be limited by the diffraction limit of the telescope While the diffraction limit of a system is definitely a constraint, it's not as simple as this anymore.
  - goodcanadian 6y ago
    Aperture synthesis, aka interferometry, has been applied to optical systems for far more than 15 years. The Narrabri Stellar Intensity Interferometer[1] saw first light in 1963, for example. However, your resolving power is still fundamentally limited by the distance between your telescopes. The diffraction limit may be more of a fuzzy limit than a hard cut off, but it is fundamental physics. Optical interferometry is hard, and you don't get a image in the way that most people would think of it. To build up a detailed image with aperture synthesis requires measurements over a very large number of baselines. [1] https://en.wikipedia.org/wiki/Narrabri_Stellar_Intensity_Interferometer https://en.wikipedia.org/wiki/Narrabri_Stellar_Intensity_Int...
    - balnaphone 6y ago
      Yes, agreed. In fact, the first example I can think of is the Michelson stellar interferometer at the Mount Wilson Observatory, which was in operation in 1920. However, there has been a major push recently to circumvent the limitations of optically connecting the telescope arrays; there are a few different approaches, of which I'm not aware of any fatal flaws. Seems likely it's a matter of time and physics and engineering; very difficult, but achievable. See for example: https://dspace.mit.edu/bitstream/handle/1721.1/51042/Tegmark-2009-Fast%20Fourier%20transfo.pdf;sequence=1 https://dspace.mit.edu/bitstream/handle/1721.1/51042/Tegmark...
- extrapickles 6y ago
  If they know the shape of buildings (or vehicles) near the area of interest, they could use those to guide correction.
- exhilaration 6y ago
  "Lucky imaging" is not a widely used technique in astronomy. There was a major announcement on Friday using just this technique: https://www.gemini.edu/pr/gemini-gets-lucky-and-takes-deep-dive-jupiter-s-clouds https://www.gemini.edu/pr/gemini-gets-lucky-and-takes-deep-d... It might become more popular now.
  - goodcanadian 6y ago
    Yes, I saw it, and that was the first time I had heard the term in my years of studying and working in astronomy. Most astronomical sources of interest are not anywhere near as bright as Jupiter. It can take hours of integration to even detect many targets of interest. The technique is simply not possible for 99%+ of astronomy work.
    - xioxox 6y ago
      Did I use the word "often" in my comment above? Anyway, as an astronomer, I've heard about lucky imaging quite a few times - it likely depends what other astronomers you are exposed to. Of course, it can currently only be used on some bright targets, but it's a fascinating method. Similar techniques are also now being used at other wavelengths, such as for the LOFAR low frequency radio telescope.
- rbanffy 6y ago
  > I can't imagine what they would use for a reference All the orbits are well known (to all interested parties at least) so I'd assume a downwards pointing laser guide wouldn't be much of an issue.

by xioxox 6y ago

goodcanadian 6y ago

"Lucky imaging" is not a widely used technique in astronomy. There is generally no shortage of photons in earth observation, however, so it could work quite well on spy satellites. A much more common technique is adaptive optics. A wavefront sensor measures the incoming signal from a reference star (or laser guide star) and deforms the mirror to compensate. I wouldn't be surprised if adaptive optics is also used on spy satellites though I can't imagine what they would use for a reference. As other commenters have mentioned, you are always going to be limited by the diffraction limit of the telescope, which is a function of the mirror size and wavelength.

balnaphone 6y ago
> "Lucky imaging" is not a widely used technique in astronomy True, but it is very popular in astrophotography. Also, there are many recent advances in super-resolution imaging, compressive sensing, and sub-diffraction-limit pixels, often based on results from Donoho, Candes, and friends. Taking advantage of signal sparsity (i.e. that the set of physically possible signals is much smaller than the set of all possible signals, and physical systems have many constraints upon them) has led to a lot of surprising results over the past 15 years or so. Another approach that has yielded results is aperture synthesis, which has been applied to optical systems for at least the past 15 years. And in some certain circumstances for spysats, optical heterodyne detection can also be used to great effect. > you are always going to be limited by the diffraction limit of the telescope While the diffraction limit of a system is definitely a constraint, it's not as simple as this anymore.
- goodcanadian 6y ago
  Aperture synthesis, aka interferometry, has been applied to optical systems for far more than 15 years. The Narrabri Stellar Intensity Interferometer[1] saw first light in 1963, for example. However, your resolving power is still fundamentally limited by the distance between your telescopes. The diffraction limit may be more of a fuzzy limit than a hard cut off, but it is fundamental physics. Optical interferometry is hard, and you don't get a image in the way that most people would think of it. To build up a detailed image with aperture synthesis requires measurements over a very large number of baselines. [1] https://en.wikipedia.org/wiki/Narrabri_Stellar_Intensity_Interferometer https://en.wikipedia.org/wiki/Narrabri_Stellar_Intensity_Int...
  - balnaphone 6y ago
    Yes, agreed. In fact, the first example I can think of is the Michelson stellar interferometer at the Mount Wilson Observatory, which was in operation in 1920. However, there has been a major push recently to circumvent the limitations of optically connecting the telescope arrays; there are a few different approaches, of which I'm not aware of any fatal flaws. Seems likely it's a matter of time and physics and engineering; very difficult, but achievable. See for example: https://dspace.mit.edu/bitstream/handle/1721.1/51042/Tegmark-2009-Fast%20Fourier%20transfo.pdf;sequence=1 https://dspace.mit.edu/bitstream/handle/1721.1/51042/Tegmark...
extrapickles 6y ago
If they know the shape of buildings (or vehicles) near the area of interest, they could use those to guide correction.
exhilaration 6y ago
"Lucky imaging" is not a widely used technique in astronomy. There was a major announcement on Friday using just this technique: https://www.gemini.edu/pr/gemini-gets-lucky-and-takes-deep-dive-jupiter-s-clouds https://www.gemini.edu/pr/gemini-gets-lucky-and-takes-deep-d... It might become more popular now.
- goodcanadian 6y ago
  Yes, I saw it, and that was the first time I had heard the term in my years of studying and working in astronomy. Most astronomical sources of interest are not anywhere near as bright as Jupiter. It can take hours of integration to even detect many targets of interest. The technique is simply not possible for 99%+ of astronomy work.
  - xioxox 6y ago
    Did I use the word "often" in my comment above? Anyway, as an astronomer, I've heard about lucky imaging quite a few times - it likely depends what other astronomers you are exposed to. Of course, it can currently only be used on some bright targets, but it's a fascinating method. Similar techniques are also now being used at other wavelengths, such as for the LOFAR low frequency radio telescope.
rbanffy 6y ago
> I can't imagine what they would use for a reference All the orbits are well known (to all interested parties at least) so I'd assume a downwards pointing laser guide wouldn't be much of an issue.