COLOURS OF THE COSMOS
Astrophotographer Will Gater explores the use of colour in astro imaging and offers advice on how to strike the right balance when shooting the stars
Advice to help you keep a balance – and some scientific veracity – when it comes to colour in your astrophotography.
A strop ho tog rap hers face many challenges when capturing views of the night sky, but one element of the craft presents a particular collection of hurdles all its own: colour. Capturing, processing and interpretation of colour in astronomical imagery is arguably the most complex, and sometimes controversial, facet of the subject.
Colour deep-sky imaging using filters requires a skilful blend of art and science to produce spectacular results. But even when taking ‘simpler’ wide-field and nightscape images, a strop ho tog rap hers must tackle issues such as light pollution that can dramatically affect what’s shown.
Combine all this with the fact that we don’t all see colours the same, that we will view images via different mediums and – perhaps most crucially – that everyone has their own opinions on what they like to see, and you can begin to see why colour in astrophotography is often a source of passionate debate. And that’s before you even consider that we’re talking about pictures your eyes could never see!
In this article we’ll look at some of the practical ways you can approach the treatment of colour in your own astrophotography. While there should always be room for artistic licence, our advice is based on the premise that we want to represent celestial phenomena as accurately as possible. The challenge here is to produce not just a captivating image but also one where we’ve objectively assessed what the scene should actually look like.
Nowadays, with powerful editing software that can drastically manipulate a digital image file, it is in the post-processing of an astrophoto where many colour choices are made. But the moment of capture – particularly in nightscape and star-trail photography – will always be a crucial moment that hugely influences the tone and the interplay of dark and light in a shot.
Contrast versus detail
Imagine you’re shooting a twilight nightscape, including a bright planet, using a DSLR. With a short exposure you might capture a darker, highercontrast scene, with rich twilight colours but little foreground detail; perhaps just black silhouettes. Using a long exposure the same view would look markedly different: the sky would be brighter while the overall contrast would likely be lower, but you could pick up more detail in the shadows. You could even aim to blend two such exposures
“Use some basic astrophysics to guide how you tweak colours in post processing”
in post-processing. Whatever the approach, the point is that in the act of selecting the exposure for an image, you’re already making an important decision that will affect the colours in the final picture and – even if inadvertently – the feelings it’ll convey and how it will direct the viewer’s eye. Of course, such aesthetic considerations are only one side of the coin. Since you’re aiming for a faithful rendition of the view depicted, you also need to think about the science of what’s shown; and it’s this that should ultimately provide the guide for the processing and enhancement of colour in any kind of astrophotography, from planetary imaging to long-exposure, deep-sky work.
Take, for example, the bright band of the Milky Way, which is a central element in countless astro images. Our Galaxy’s core is composed of older, more red and yellow stars, while the spiral arms in its disc are inhabited by hot, young, bright, bluewhite stars. So in long-exposure images showing the Milky Way stretching across the sky, we would expect the star fields of the Galaxy’s central bulge, in and around the constellations of Sagittarius and Ophiuchus, to have more of a yellowish-white colour to them; while further away from the core – where the disc of the Galaxy traverses Cygnus and Cassiopeia – the Milky Way’s star fields should show more blueish-white tones. Often these rich star fields also show a hint of ochre and brown in places;
this is where their light is reddened by interstellar dust and gas clouds, so don’t be put off if your Milky Way images have some muddy brown colours – they are real!
The challenge of achieving pleasing and accurate star colours crops up wherever there are stars depicted in an image, but it’s perhaps most keenly felt in long-exposure, deep-sky work. Here again, though, you can use some basic astrophysics to guide how you tweak colours during post-processing.
The colours of stars
Professional astronomers group stars into ‘spectral types’, which tell us about the appearance of key chemical ‘fingerprints’ in their light as well as their temperature. It’s the differing temperatures between stars that cause the colour variations we see. For example, a hot ‘O-type’ star like Mintaka in Orion’s Belt appears blueish, while a cooler ‘K-type’ star, such as Aldebaran, will have an orange tint. Among the other spectral types there are also ‘F-type’ stars, which tend to appear closer to white. These are of particular use to astrophotographers as you can use a planetarium programme, like Stellarium or Starry Night, to find such a star in your image. Then during editing and processing, check that you have it showing a neutral white-ish colour; this way you can be confident that the overall colour balance of your image is accurate.
When examining and processing colour in astro images it’s often easy to concentrate on the main features, while forgetting about the background; do so at your peril. Backgrounds present their own complications. The night sky is not perfectly black so, in most cases, when tweaking the colour of an image you should aim for a neutral, extremely dark
grey backdrop. However, certain deep-sky images can be the exception, as they often feature regions of colourful nebulosity across the whole frame.
Sometimes, though, there will be strong colour casts or gradients within an image that affect the background. Occasionally these will come from natural sources, so we’d argue for preserving them and making sure they’re faithfully portrayed. For example, you’d expect colour images taken in deep twilight, or when a Full Moon is high up, to show a washed-out background with a blue cast – and note that when a full Moon is rising it initially washes the sky out with a yellow-grey light that changes to a whiter, bluer hue as the lunar disc gains altitude.
Similarly, nightscape photographers imaging from dark-sky locations often capture the atmospheric phenomenon known as ‘airglow’, which can create greeny-blue swathes of colour across an image. Airglow can really throw off the processing of an image if it’s not recognised as being present in a shot, as the photographer desperately tries to correct the strong, green background gradients.
Learn to love levels
However, the most common cause of background gradients or colour casts is usually light pollution. Specialist filters can reduce the effects of this unwanted intrusion to a certain extent, but some – in doing so – impart their own colour cast to a photo, which has to be dealt with in processing later.
How, then, do you go about correcting and tweaking the colour of an astro image in image- editing software? One of the ways to begin to correct or ‘balance’ the colour within an astrophoto is to use the ‘levels’ tool commonly found in software like Photoshop and GIMP. Usually this feature is used to increase the brightness and contrast in an image, to pull out detail. However by choosing to adjust individual colour channels, one at a time, you can also vary the colour balance within a shot. To get a feel for how this alters the colours in an image, first
select a colour channel (either red, green or blue) from the drop down ‘channel’ menu and then move the middle ‘slider’ icon in the ‘input levels’ window; you’ll see the colour of your image in the main window change, and by further experimenting with the different channels and other sliders hopefully you’ll get close to an image tone you’re happy with.
Both Photoshop and GIMP also have an advanced ‘Colour balance’ tool that enables you to alter the balance of colour in an image between three pairs of colours: cyan and red; magenta and green; and yellow and blue. Adjustments made with this feature can be applied – independently – to the shadows, midtones and highlights of an image, meaning it’s incredibly useful for precision colour correction.
Some image-editing programs also have a feature that can be used to adjust a picture’s ‘temperature’; in other words, how yellow or blue the tones in the final image will be. This tool is frequently used by aurora photographers – for aesthetic reasons – to give an image a ‘cooler’ feel, with the natural, green colour of the auroral light becoming a more greenycyan hue. How far you are willing to push this is a matter of personal taste.
And that is, of course, true of everything we’ve discussed in this article: how far, and in which chromatic direction, you want to take your colour balancing and correcting is ultimately down to you, and will likely vary from image to image depending on a multitude of factors. But, hopefully, armed with the ideas we’ve looked at here you’ll have some fresh ideas and new perspectives to consider the next time you’re shooting and processing a celestial shot of your own.
The different star colours in Auriga: bright nearly-white Capella (top right), orangey K-type Hassaleh (bottom right) and the blueish B-type Elnath (bottom left)
In Photoshop you can adjust the levels for each of the red, green and blue channels individually
Astrophotographers tend to ‘cool’ aurorae shots by adding more cyan to their natural green using the temperature controls