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I was lately reading about making pictures of ISS. The pictures are not really great from "home telescopes".

I was wondering what the usual altitude of espionage satellites is, given those pictures of i.e. cars are pretty detailed.

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  • $\begingroup$ Your question is a bit of a non-sequitur. The imaging system of a spy satellite is a good deal bigger than a home telescope. For example, the Hexagon satellites used in the 1970s-80s had an effective 20-inch aperture but even high-end home telescopes are more at the 10-inch range (which is a quarter of the light-gathering capacity, since that depends on the area, rather than the diameter). $\endgroup$ – David Richerby Feb 10 '17 at 13:39
  • $\begingroup$ Also, to add: when you click on the photo the amount of detail is actually quite astonishing. Just look at how clearly visible the space shuttle main engines are! $\endgroup$ – user2705196 Feb 11 '17 at 4:17
  • $\begingroup$ Actually 0,64m mirror is already quite big. The point is whether I can be able to view it with some small telescope. Which as answered should be imposible due the speed and cheap telescopes limitation. $\endgroup$ – Zveratko Feb 14 '17 at 5:47
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That's a great question! So your question is: why are images taken from observation satellites much better than the typical images of the ISS taken from the ground.

The answer is:

  • observation satellites generally have higher diameters and focal length than your typical amateur telescope.
  • observation satellites are very stable, so the image is not degraded by vibration. Plus they take pictures only when they are not rotating. On the contrary, taking ISS photos requires moving the telescope around quickly, which introduces a lot of vibrations in most amateur telescopes.
  • most observation satellites have a special sensor wich compensates for object movement when taking pictures (that's called time-delay integration). Ground sensor do not have that (that's why they have to move around to track the target).
  • the atmosphere degrades the images taken by ground telescopes much more than those taken by satellites
  • however observation satellites tend to orbit higher than the ISS (typically 600 to 700km, with the exception of a few spy satellites that fly lower)
  • the ISS has very high contrasts, so it's hard to have an image in which you can see details in both the brightly lit parts and the parts in darkness
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Reconnaisance satellites are at similar altitudes to the ISS. The ISS is at 330-400 km, recon sats are in elliptical orbits with a perigee on the order of 250 km.

The big difference is the optics. A photo recon satellite has a telescope with a mirror 2.4 m in diameter (dimensions similar to the Hubble space telescope). Ground-based telescopes that big would have a hard time taking photos of the ISS because they can't be moved quickly enough to keep the ISS in sight.

This is the best ground-based photo of the ISS I've seen yet. It was made with a 0.64 m telescope:

ISS

This site lists exposure times of around 1/250 s. Bigger telescopes will use shorter exposure times.

I suspect the main reason large telescopes haven't been used to take pictures of the ISS is that observation time is at a premium: every observatory has a waiting list of people doing actual science. Taking time out to take pictures just for fun wouldn't sit well with the telescope's users.

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  • $\begingroup$ Aside from the speed of targeting the mirror on the ground, are there any other limitation? $\endgroup$ – Zveratko Feb 10 '17 at 8:48
  • $\begingroup$ I don't know how long it takes to shoot such a picture( 1/50s?), in this case the scope could just wait for the shot, couldn't it? $\endgroup$ – Zveratko Feb 10 '17 at 8:54
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    $\begingroup$ ISS moves at around 8km/s. That means it travels 160 meters during 1/50th of a second. At 1/250s it's 32m. The whole ISS is a bit over 100m long. So no, the camera MUST trace it to make the photo with any reasonably low amount of motion blur. $\endgroup$ – SF. Feb 10 '17 at 10:15
  • $\begingroup$ Another limitation of taking photos of ISS is that they can be taken only near morning/evening. During the day, ISS will be invisible on the bright sky. At night it will be dark. Only briefly, when it's still lit by the sun, but sky above the photographer is already dark, it's easily visible and reflects enough light for good photos. $\endgroup$ – SF. Feb 10 '17 at 10:18
  • $\begingroup$ What about moon reflection? It wil probably lit the station from wrong side, right? $\endgroup$ – Zveratko Feb 10 '17 at 11:05

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