I am wondering if there is a telescope in or outside the ISS that has been used for astronomical imaging? I have seen here that there is already a Celestron telescope on board but it is only used for Earth observation.
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The Celestron telescope is not stationary. It can be moved around inside the space station to look through any window.
It is however just used to observe the Earth, and the reason for that is the orbital velocity of the ISS. Moving at almost $8km/s$, the view change with an angular velocity of 3.88 arc-degrees per second because the station rotates to keep the same side facing the Earth. If the ISS had instead stayed fixed relative to the stars, like the Hubble telescope, you get the same problem with Earth observations. You can not simply do both.
That is what makes it hard to focus on one part of the sky. Given the limited view from one of the windows, you can only observe a region of the sky for about 10 minutes.
That does not mean that The ISS does not have equipment to observe the sky, it does for instance have the Alpha Magnetic Spectrometer.
answered Jan 29, 2016 at 15:25
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$\begingroup$ Should it be arc-minutes - not arc-degrees? $\endgroup$– uhohCommented Jan 29, 2016 at 22:07
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$\begingroup$ If they look straight down, the surface will appear to be moving at 7 km/sec, which is about 80 arc-minute / sec. The photographic speed of the telescope is slow (f/10) so they may not be able to completely de-blur with a fast shutter speed. They might be limited to taking photos of things from the side (oblique angles) which would be 1000's of km away and move more slowly. Or they could use the slewing from the scope mount (the link says the scope is controlled by the ground). But in that case they'd also be able to slew at 4 arc-minutes / sec (near orbit plane) for celestial objects. $\endgroup$– uhohCommented Jan 29, 2016 at 22:16