How Long Can You Expose A Night Sky Before Getting Star Trails?
When taking photographs of the night skies there is a simple way to avoid smearing the stars and getting them sharp. (As opposed to star trails which are awesome, but different).
It's called the rule of 600, which is astrophotography's equivalent to the 1/focal length of shooting hand held. Basically the rule says that you cannot shoot with a shutter speed of over 600/<focal length> in seconds. So when using a 600mm lens for example, you can only keep the shutter for 1 second before star trails start showing up. (300mm lens can do 2 seconds, 10mm lens can do 60 seconds and so on).
This was once a very simple rule with 35mm cameras. It got complicated when different sensor crop factors were introduced. Actually, there is quite a bit of trigo-math involved concerning the angle of view. (you can check out Wikipedia if you want the hard math).
Kamil Tamiola made things simple by providing a tool that takes in the camera model and focal length, and provides the number of seconds you can leave the shutter open to eliminate star trails. It is simply called... Well Kamil did not name it, so I am gonna call it The Awesome Calculator To Eliminate Star Trails When Shooting The Night Skies.
Here is How Kamil Describes the issue:
Earth, just like any other celestial body, is a subject to constant motion with respect to other celestial bodies. If you expose your photograph long enough you shall start observing the aforementioned motion in a form of star-trail effect.
An obvious question arrises, how long can one expose a photograph in order to acquire maximum amount of light, yet with no visible star-trail effect?
If you want to experiment with night skies photography, this is a great little resource to remember.