The Canon EOS and Photography Forums

Typically how long can you leave the shutter open before the stars look streaked or blurred. I know it depends on several things, but typically standing in mid-america.

Besides a few other factors (such as where in the sky you are looking), focal length plays a very significant role. Whatever that time is for a 50mm lens, it will only be 1/10th of that time for a 500mm lens.

For 25 micron star trails (i.e. stars that pretty much look like points), you need to keep the exposure under about 2 seconds with a 500mm lens. This scales with focal length, so you get only 1 second at 1000mm, or 20 seconds at 50mm. These are approximations of course. Whether a star looks like a point or a short trail depends on how much you enlarge the image.

As you move closer to the celestial poles, the stars appear to move slower, so, for example, the north star appears to move very slowly. It if was actually at the north celestial pole rather than just close it it, it would not appear to move at all. Stars on the clestial equator (90 degrees from the pole in any direction) appear to move fastest.

ahhh...very good information. I had not thought about focal length, but it make perfect sense. thank you very much.

 While on the subject, would it hurt the image sensor to leave the shutter open for (of course depends on batt life) more than, say, an hour.  I took a 15 min exposure and really liked the "streaking" of the stars.
thanks again...great info.

Depending on where you live you will also find your shots corrupted with aircraft andsatellite trails across the sky.  Your really need a tracking mount to counter the effects of the earth's rotation.  using something like the Skywatcher EQ6 you can look at exposures of up to two minutes.  Longer than this and you need to guide the telescope.  One appraoch is to take a series of shots (possibly a couple of hundred) using short exposures and stack them using software such as Registax.

Ken Lindup