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Telescopes rotate with the Earth. If a telescope isn't fixed on something like the North Star then everything in its field of view is moving pretty rapidly, yet photos from these devices are crystal clear. I'm very curious how these are taken. The light from distant galaxies is extremely faint - and they must be whizzing by given their relatively minuscule size in the sky. I've been googling this but either I'm asking a very dumb question or it's not easy to find an explanation online.

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For taking long-exposure photographs, the telescope is rotated to keep the stars motionless. This isn't easy, and is called "tracking".

For small telescopes, the best way to achieve good tracking is to use an equatorial mount. This allows one to rotate one axis of the telescope with a small motor, turning it once in 24 hours (actually 23 hr 56 minutes - a sidereal day) to keep the stars stationary.

Of course some objects like planets, comets, asteroids actually move relative to the stars. To get images of such "wandering" objects, you can use a computer to track the object in the sky. In photos of fast moving asteroids, the background stars turn into streaks, as the telescope has been programmed to track the asteroid, not the stars.

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  • $\begingroup$ The term "equatorial mount" is perhaps more common, and the wikipedia page has several pictures. $\endgroup$
    – Dr Chuck
    Jan 19 at 8:28
  • $\begingroup$ Quite right, edited $\endgroup$
    – James K
    Jan 19 at 8:51
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If you calculate the lateral motion of a star in the sky in degrees per second, and compare that with the shutter time of your camera (and the field of view per pixel in degrees), you'll see that no blur is recorded until the shutter time gets reasonably long. As JamesK wrote, use either an equatorial mount or a programmable system to keep a specified target centered in your image plane.

Now, there are other causes of blurring. If you have a nice big primary mirror, atmospheric non-uniformities due to wind or thermal variations cause phase front errors. These can only be corrected with an adaptive optics system of some type.

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