What is Star Tracking?
A star tracker's job is to match the rotation of the Earth so that you can take long exposure images (at nearly any focal length) of the night sky. To properly track the stars that appear to move across the night sky each night, a star tracker must be polar aligned and balanced.
Photographing astro-landscapes is an exciting and challenging type of photography. It can be addicting and very rewarding. The standard camera settings I was taught, 20-seconds, at f/2.8 and ISO 6400, works well with nightscape photography: The image to the right was shot with these settings However 9 images were stacked to reduce the noise. These exposure settings may not be optimum, they will produce acceptable starry night skys during the darkest period of the night. These settings are good to use when setting up your composition. These shots will be used for the stationary part of the image
We know our earth rotates once every 23h56m4s. Its rotation axis (almost) goes through Polaris. Stars are not stationary relative to our earth. If we want to get an image without the stars trailing we use the 500 Rule. The 500 Rule refers to camera settings to get a good exposure of the stars and avoid star trails. If you set the shutter speed at any setting longer than dictated by the 500 rule, the movement of the earth will cause the stars in your image to show up as star trails.
The 500 Rule for Full Frame Camera
The 500 rule for a full frame camera requires you to set your camera to 21 seconds if you are shooting a 24 mm lens,500 / 24 = 21. This is calculated for a ISO setting of 400 and F/2.8.
The star tracker rotates at the same speed as the Earth, just in the opposite direction. This "Tracks' the stars and eliminates the motion of the stars relative to the camera. The exposure time can be up to minutes in duration, missing faint stars will show up in your pictures, star trails will disappear, and lower ISO helps to reduce the noise. This gives clear, pinpoint photos of stars and the night sky.
The image to the left was shot for 30 seconds at ISO 3200 using a star tracker. for the sky. You can see that there is sharper stars, better color and less noise.
This will blur the foreground with the camera movement. So the shooting sequence will be with the first shot taken while the tracker is off. This will get a clear foreground shot. Then the sky shot is taken for the sky. Then combine these two pictures to get the final image.
The star tracker will need to be Polar aligned and usually includes a polar scope. This is used to help find the north celestial pole and adjust the mount accordingly. A star tracker that has been properly polar aligned will match the exact rotation of the earth relevant to the night sky objects.
I was introduced to the Move shoot Move star tracker by Royce Bair. It is small and exceptionally easy to set up.
To set up the rotator for night sky photography, you need two ball heads: One below the tracker to adjust it to the axis of the earth and another on top of it to be able to set your camera to your composition. You will need to find Polaris in the night sky and aim the laser pointer at it. The incredible laser pointer is very powerful and easy to see it at night. It makes the adjustment as easy, quick, and accurate as possible. The adjustment takes less than a minute..
Because of the recent popularity of nightscape photography, newer mount designs, just for cameras, have appeared that are smaller and less expensive. Using these smaller and cheaper trackers, a new breed of astro-landscape photographers are taking a tracked exposure for the sky (referred to as "wide-field astrophotography"), a second exposure for the landscape (without tracking), and then blending the two in post-processing.