The dark art of stacking
A guide to one of astrophotography’s powerful and mysterious techniques
One of the main reasons astrophotographers buy a star tracker is to enable them to ‘stack’ images. Many beginners think this is too complicated to get to grips with, but all stacking means is taking lots of short exposures of the same object, then combining them to make a single image – the equivalent of a much longer single exposure. This can then be processed further to remove noise (unwanted artefacts), and bring out details and colours that can’t be captured in a single image.
Stacking is primarily used for high-resolution images of deep-sky objects such as galaxies and nebulae, or to capture faint details hiding within the feathery tails of comets, but it can also be used to make stunning images of the Milky Way’s frothy star clouds and constellation portraits too.
Getting started is easy. With your tracker aligned accurately, select your target, focus on it and set your camera to take multiple exposures, keeping them short to reduce trailing. Start with 20 or 30 frames. Most cameras can be set to take a sequence of 10 images, so you’ll probably have to take three sets. Some stackers use an ‘intervalometer’ to take runs of dozens or even hundreds of images in one go, but you don’t need one right away.
When taking multiple exposures you must keep checking your lens hasn’t misted over; taking a hundred photos only to find out that your lens was blinded after the first 10 is not fun!
Images taken, you load them into a ‘stacking program’, software like RegiStax, DeepSkyStacker or Sequator. Follow the instructions; your computer will produce a single, high-resolution image. Then the hard work begins; with image-processing software you can adjust your stacked image’s levels, saturation and contrast until it shows what you want it to.