<p>For taking long-exposure photographs, the telescope is rotated to keep the stars motionless. This isn't easy, and is called &quot;tracking&quot;.</p>
<p>For small telescopes, the best way to achieve good tracking is to use an <a href="https://en.wikipedia.org/wiki/Equatorial_mount" rel="noreferrer">equatorial mount</a>. This allows one to rotate one axis of the telescope with a <a href="https://www.rothervalleyoptics.co.uk/telescope-motor-drives.html" rel="noreferrer">small motor</a>, turning it once in 24 hours (actually 23 hr 56 minutes - a sidereal day) to keep the stars stationary.</p>
<p>Of course some objects like planets, comets, asteroids actually move relative to the stars. To get images of such &quot;wandering&quot; objects, you can use a computer to track the object in the sky. In photos of fast moving asteroids, the background stars turn into <a href="https://earthsky.org/space/near-earth-asteroid-2020-oy4-close-pass-july-28/" rel="noreferrer">streaks</a>, as the telescope has been programmed to track the asteroid, not the stars.</p>

CC BY-SA 4.0
2022-01-19T08:28:20.157Z
The term "equatorial mount" is perhaps more common, and the wikipedia page has several pictures.
Dr Chuck
CC BY-SA 4.0
2022-01-19T08:51:06.717Z
Quite right, edited
James K
CC BY-SA 4.0
2022-01-19T07:58:36.013Z
2022-01-19T08:50:15.087Z
2022-01-19T08:50:15.087Z
James K
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2022-01-19T13:01:53.803Z
2022-01-19T13:01:53.803Z
Carl Witthoft
<p>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.</p>

CC BY-SA 4.0
2022-01-20T17:40:43.697Z
also see https://www.scantips.com/lights/stars.html
Aaron F
<telescope>
How do telescopes avoid blurry images?
Yevgeny Simkin
# How do telescopes avoid blurry images?

(Asked by: username_0 on 2022-01-19)

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.

**Tags:** telescope

> **Comments:**
> **username_1**: also see https://www.scantips.com/lights/stars.html  

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## 2 answers

### Answer by username_2 on 2022-01-19. Score: 7

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](https://en.wikipedia.org/wiki/Equatorial_mount). This allows one to rotate one axis of the telescope with a [small motor](https://www.rothervalleyoptics.co.uk/telescope-motor-drives.html), 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](https://earthsky.org/space/near-earth-asteroid-2020-oy4-close-pass-july-28/), as the telescope has been programmed to track the asteroid, not the stars.

> **Comments:**
> **username_3**: The term "equatorial mount" is perhaps more common, and the wikipedia page has several pictures.  
> **username_2**: Quite right, edited  

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### Answer by username_4 on 2022-01-19. Score: 1

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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