USING CCD CAMERAS
Many of the most spectacular deep-sky images of galaxies, nebulae and star clusters are taken not with one-shot colour cameras such as DSLRs, but with specially cooled astronomical ‘CCD’ (charged-couple device) cameras with monochrome sensors. These produce greyscale RAW images, and in order to create vibrant full-colour images, astrophotographers need to use a set of filters on their lenses. For example, to capture a scene in the kind of palette most people see in – where trees are green, the sky is blue, strawberries are red – the imager must capture exposures through red, green and blue filters and later combine this data into a full-colour – sometimes called ‘true’ colour – ‘RGB’ image in image-editing software. Like all astro images, these pictures still need to be colour balanced carefully as sometimes filters can let in more or less light of certain colours, or the camera sensor itself might be more sensitive to some colours.
Many high-end astronomical imagestacking and processing programs contain automatic colour balancing features or tools to remove or neutralise background colour casts. These are usually excellent, though can sometimes produce an ‘off’ result, so do take time to look carefully at your final image’s colour; make manual adjustments if necessary and use any preview function available to move back and forth between iterations of the image as you tweak the colour.
CCD cameras capture in monochrome, but by using red, green and blue filters and combining the resulting shots, colour images are created