The Rule of 600 In Astrophotography

The Rule of 600 In Astrophotography

Ever wondered how to stop stars from appearing to trail across your picture of the night sky? The "rule of 600" is a method to try to calculate how long the maximum exposure time is before unacceptable star trails become evident in your pictures.

rule of 600
Star trails over the ESO 3.6-metre Telescope, which hosts HARPS, the High Accuracy Radial velocity Planet Searcher, the world's foremost exoplanet hunter. This image comes from Your ESO Pictures Flickr Group, where anyone can submit photos connected with ESO.

Why do we see star trailing?

Star trailing occurs because the Earth is spinning. The stars seem to move across the night sky over time. If a camera has too long an exposure the movement of the stars will be seen in the final image. You can overcome this by using a sidereal tracking tripod, but what if you don't have one. How long can you expose an image for before the star trails are clearly seen?

How to use the "Rule of 600"

The rule if 600 is simple. You just divide 600 by the focal length of the lens you are using. So if you have an 18-55mm lens, using the rule of 600, you set it to 18mm then 600/18 equals 33 seconds. One thing to be careful of though is that this only works for full frame DSLR cameras. Many DSLR cameras have less than full frame sensors. For example, the Nikon D3200 has a 1.5x crop sensor (and many Canon cameras are 1.6x). So to calculate this camera's maximum exposure time I would have to first multiply 18mm by 1.5x which equals 27. Then divide 600 by 27 to make a maximum exposure time of 22 seconds. Here is a link to a chart to help you work out your maximum exposure time.

But stars move at different speeds across the sky

This is a rule of thumb and assumes you are taking pictures in the worst case scenario, where the stars move the quickest, at the celestial equator. If you take a picture of the star Polaris, which is over the celestial north pole the star barely seems to move all night. This is in stark contrast to stars along the celestial equator which move the furthest for a given length of time. 

Related Posts: Full length, Astrophotography

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