Chapter 4: Shooting star trails (1)
Tags: Astronomy, Sky, Astrophotography, India, Photography

One of the most surreal looking images formed in astrophotography are usually star trails. Star trails are formed in an image due to apparent motion of stars as the earth rotates around its axis. This causes giant arcs to form in an image as we shoot for longer and longer durations. As we get farther from the poles in an image, the arcs seem to be larger, until we reach the celestial equator, then the arcs start to move in the opposite direction.

Star trails may appear very different based on how they are shot and finally produced in post processing. Factors such as exposure time, shooting location, shooting angle etc. affect the overall look and appearance of the trail.

Shooting celestial poles and circumpolar stars:

Polaris and a few circumpolar stars shot and processed by Ken Christison

The image above depicts the north celestial pole star(closest to north pole and visible to naked eye), Polaris along with a few circumpolar stars that form the surrounding arcs. Circumpolar stars can be simply defined as the stars for a particular latitude that never seem to rise or set. They seem to stay above the horizon all day long and all year round. For the people living in India, the little dipper, or the Ursa minor is a constellation in which nearly all the stars are circumpolar. They just rotate around Polaris and never seem to set below horizon.

To shoot such an image, one needs to find a dark site which is away from the earth’s equator and near the northern or southern pole. The next task would be to locate the northern or southern pole in the night sky. Even a rough idea of the same could work. For example, for someone in the northern hemisphere, finding Polaris isn’t a tough task. One can make use of various apps made for locating stars and other celestial bodies. These apps work very similarly to a phone compass. They make use of the mobile device’s geolocation in terms of latitude and longitude, and the accelerometer sensor to exactly locate the night sky objects using Augmented reality. Another way to locate Polaris is to find the big dipper i.e. the Ursa major in the night sky, the last two stars of the bowl of the big dipper point directly towards Polaris. Polaris also marks the end of the handle or tail of Ursa minor.

A guide to locate Polaris in the night sky.

After locating the north celestial pole or Polaris, compose the frame correctly and set the aperture value to as low as possible on a fully manual camera. For shooting star trails the shutter speed can vary anywhere from 20 seconds to 60 seconds up to personal preference. Longer shutter speeds per shot means less total number of shots to work with in post processing. ISO is generally kept as low as possible to reduce the amount of digital and thermal noise in the final image. After getting the white balancing as per requirement, set up an intervalometer that would reduce the camera shake as we do not press the shutter release button each time to capture an image. The more total number of shots we take, the longer arcs will be obtained in the final image. Most modern day digital cameras have intervalometer built in, if not, one may make use of a third party/aftermarket intervalometer. Set up the intervalometer’s duration per shot and the total number of shots required. These individual shots are then stacked and combined in post to create trails up to a few hours long. Noticeable trails can be produced in total 10 minutes of shooting overall, however, impressive results take at least 100 minutes of data combined to be produced. The total amount of time can be easily calculated by the duration per shot and total number of shots.

The total shooting time(in minutes)= duration per shot(in minutes) x total number of shots

In the upcoming chapters, other types of trails and how to edit them will be explained.