Processing
By adjusting RGB channels a featureless raw image of M31 can be transformed
Most astrophotographers start out using a DSLR camera. These versatile cameras produce detailed deep-sky images, but while narrowband data from mono cameras creates clear contrasting colours, DSLR data can be complex to process. This is because of the way light is distributed onto the camera sensor. Narrowband filters allow separate processing of Red, Green and Blue (RGB) channels, whereas DSLRs and one-shot colour cameras pick up the wavelengths of light simultaneously. The sensor distributes this light via its own RGB filter arrangement, the Bayer matrix. If RGB is managed carefully, you can bring out additional details from DSLR images while maintaining a natural appearance.
We used an image of the Andromeda Galaxy, M31, to demonstrate RGB processing functions in Adobe Photoshop. RGB control is done via three main settings: ‘Levels’, ‘Selective Colour’ and ‘Hues/Saturation’. Each of these are in the ‘Adjustments’ menu, click ‘Image > Adjustments’. With these functions, you can align and alter colour channels. If not done in gradual stages, you risk making your process too severe. We recommend performing more than one colour ‘stretch’ at stages throughout processing, in order to add gradual colour developments.
After stacking files in Sequator, we open the TIFF file in Photoshop: click ‘File > Open’ and crop the image by clicking ‘ Image > Crop’. This is an important step before the main process. When stacking images over several hours – or from two different nights – your image-alignment can shift, causing uneven colours around the edges. By cropping these out beforehand, we prevent anomalies affecting the colour balance.
Fine adjustments
Our image is ready for processing but at this stage it looks dull (see image, below, left). We suggest starting with ‘Levels’ – click ‘Image > Adjustment > Levels’. This enables a manual colour balance to ensure you haven’t got one dominant colour overtaking the process. This can vary between cameras: for example, a modified DSLR will have a dominant red channel.
The ‘Properties’ tab in ‘ Levels’ (‘Red’ box) allows you to select each of the RGB channels separately, starting with ‘Red’ (see Screenshot 1). Make sure the ‘Histogram’ channel on the right-hand side is set to ‘Colours’ rather than ‘ RGB’ so that you can monitor changes. Move the sliders so that the left-most dark tab is touching the edge of the ‘Red’ histogram. Next, drag the right-most white tab (which will also move the inner grey tab) until the grey tab is near the right-hand side of the histogram (see Screenshot 2).
Repeat for ‘Green’ and ‘Blue’ channels then rightclick the layer and select ‘Merge Down’. We are aiming for a ‘Histogram’ which shows all colours contained evenly within the data. Hold down ‘Shift, Ctrl, Alt, N and E’ to create a new layer that retains these changes.
We repeat this process again. For this second attempt the histogram for each channel is wider, allowing us to make finer adjustments. The M31 image (see opposite, below right) is showing greater colour detail, and the background looks natural. Again, use ‘ Merge Down’ and create a new layer. As the central area looks a little green, we can adjust this in ‘ Selective Colour’ and ‘ Hue/Saturation’.
Both settings are also in the ‘ Adjustments’ menu (click ‘Image > Adjustments’). When performing tonal or saturation adjustments, it’s worth changing the blending mode (second drop- down menu in the ‘Layers’ window) from ‘ Normal’ to ‘Colour’. This allows colour adjustment without altering image brightness.
‘Selective Colour’ allows you to modify the tones of each colour by altering ‘ Cyan’, ‘ Magenta’, ‘ Yellow’ and ‘Black’ (CMYB) levels. We use the hand-tool in the ‘Hue/ Saturation’ properties window to click on areas we think need a colour boost (see Screenshot 3). For M31 we concentrate on red, yellow saturations round the core, and blues and cyans round the outer edges. Create a new layer for each adjustment. You can use the ‘Opacity’ slider (situated above ‘Layers’) to control the strength of saturation adjustments for each layer.
Our final image (see right) has undergone the above colour adjustments. In order to extract its full potential, we can use other provisions from the ‘Adjustments’ menu, including ‘Brightness/Contrast’ and ‘Curves’. We also suggest applying masks to protect or highlight various regions and using noise reduction techniques to remove unwanted artefacts.
One of the key ingredients for astrophotography, however, is good colours, and the above Photoshop settings will help you achieve this.