For those interested in night sky photography with a DSLR and tripod:
I was going through my pictures from my Kruger trip last May and I found a series of images of the night sky. In this series I shot a series of six images at f/4, 6400 ISO with different exposure times. This series demonstrates how varying the shutter time affects the number of stars you can see.
In the first picture the Southern Cross is highlighted top give you an idea of what you're looking at 2 second exposure5 second exposure8 second exposure13 second exposure20 second exposure30 second exposure
From exposures of 20 seconds and onwards stars will start to form lines due to the rotation of the earth. The image below shows a small portion at 100% of the above pictures. It clearly shows the stars forming lines in the last two images which were 20 and 30 second exposures. It also shows that my travel tripod is not sturdy enough as the images shift upwards
To get images in which the stars form trails you have to at least use an exposure time of 10 minutes. Exposures of one hour and more are ideal. To use these kinds of exposures you need to have a moonless night and no ambient light as with these long exposures any object lit by the moon or ambient light will be horribly over exposed. Here's an example star trail image shot on the same night as the above images. It's only a 672 second exposure (f/4 @ 100 ISO) as there was too much ambient light in the camp (Satara, A-circle) for a longer exposure.
If you're interested in star trail photography then I have a detailed explanation on my website which you can find here