Colour handling: how to produce full-colour planetary images

Correctly assembling a full-colour image from separate red, green and blue images requires a degree of skill – and practice

 
Pictures: Pete Lawrence 

 

Of all the stages in image processing, colour balancing is the one that causes the most heated debates. It’s often said that if you give 10 planetary imagers the same red-, green- and blue-filtered images of a planet and ask them to assemble them as a full-colour image, you’ll end up with 10 completely different images.

This happens because we all have our own individual idea of what a planet’s colour should be. As we process, we tend to drag the colours towards this subjective reference until, when it matches our mind’s-eye view, we reveal it to the world. The mind is also quite good at bending the reference and it’s all too easy to convince yourself that the image that you’ve been working on for the last few hours is perfect, even though in reality it’s way off!

The only way that you can get colour right is to identify a set of external colour benchmarks and work towards these, forgetting the voice in your head that’s trying to tell you otherwise. Although this sounds simple, it’s not, and even the most seasoned imagers sometimes get it wrong.

The Sun and the Moon are easy to deal with: the Moon’s colour is very subtle to start with and the Sun’s colour is fashionably subjective. By that, we mean that the many colourful images of the Sun taken through white light and specialist filters are artificially coloured and controlled by current convention rather than reality.

The planets, however, are a different matter – they need careful nurturing to get them right. Wherever possible, colour cameras should be allowed to achieve an auto-white balance for the best results. If the planet’s image is dim, turn the gain up using the control software so that the camera can achieve this.

For accurate images made up of filtered red, green and blue (RGB) components it’s important to capture them all at the same saturation level. If not, the end result will show a colour cast. Check your control software documentation to see whether it provides a level meter during capture.

 


STEP BY STEP

 

How to assemble a full-colour planetary image from red, green and blue components and handle the colours correctly

 

1. Monitor Calibration
Before trying to get a good colour rendition of a planet, it’s important to check that your monitor is set to display colour properly. There are various online sites to help you with monitor calibration and many monitors come with their own software utilities to help you set them up properly.

 

2. Image build
Build the RGB image from its components: the filtered red, green and blue images. If the end result looks grainy, a Despeckle or even a weak Gaussian blur can help to reduce the effect of noise in the end result. Be careful not to overdo the blur and remove detail, though.

 

3. Curves
Duplicate the RGB layer, keeping the original as a reference backup. Open the curves tool and create points at quarter intervals along the adjustment line. Adjust overall brightness with the central point and then, using small movements only, tweak the appearance using the first and third quarter points.

 

4. Sharpen
Sharpen using a high pass filter. Initially set the radius for the high-pass filter to its lowest setting and drag it slowly to higher values, previewing the effect as you go. Settle on the value that gives the most natural result. If colours look muted, increase the lower image’s saturation slightly.

 

5. Selective colour
Merge the high pass/overlay image with the layer below and duplicate the result as a further layer. For images of Jupiter and Mars, this is a good time to experiment with the selective colour adjustment described below. Select yellows and subtly adjust the black slider to reveal hidden detail.

 

6. Reference
To ensure you haven’t got a colour cast, on Mars the bright poles should appear a shade of white with no colour tone. The equatorial belts on Jupiter have a subtle grey-orange tone, while the equatorial zone is slightly off-white. The rings of Saturn should look grey, with the equatorial region a dark straw yellow.

 

This article is an extract from Part 1 of our new series of in-depth guides to image processing techniques. To read the rest of the article, see the January 2012 issue of Sky at Night Magazine

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