Enrich a nebula’s colours and add depth to the background with photo-editing software
When you take an image with a camera, in a perfect world you would get every exposure spot-on, meaning there would be no need to make any further adjustments. But the reality is that we nearly always need to make adjustments after taking photographs, usually using our preferred photo-editing software. In most cases it’s a matter of studying a histogram and using the ‘Levels’ and ‘Curves’ features or, in the case of PaintShop Pro, an actual ‘Histogram adjustment’ tool.
The histogram itself is simply a way of showing the varying light levels captured in your image from the dark tones on the left (the ‘Blacks’) to the white highlights on the right (the ‘Whites’). A bump in the middle, representing the middle tones, can also be adjusted. The histogram can also be used for adjusting the individual ‘Red’, ‘Green’ and ‘Blue’ (RGB) colour levels in the same way, making it a very flexible and powerful tool in your photo-editing armoury.
In this month’s article we will look at how to use PaintShop Pro’s ‘Histogram adjustment’ tool to improve the colours in a nebulae image. As an example, we are using an already stacked image of the North America and Pelican Nebulae (above).
Start on green
The stack of 78 images was taken with a modified Canon 300D DSLR camera through a telescope. As the resulting stack is a little green and lacklustre, the first action in PaintShop Pro is to select and click the following from the top menu: ‘Adjust > Brightness and Contrast > Histogram adjustment’. This brings up the ‘Histogram adjustment’ box (as shown on the right-hand side in Screenshot 1). There are plenty of options here, so first ensure you tick the ‘Preview on image’ so you can see any changes taking place live on the screen. For now, the rest of the options can be left as they are, but the first job is to deal with the greenish tinge.
To do this select ‘Colours’ and then ‘Green’ from the drop-down list in ‘Histogram adjustment’ box (See Screenshot 2). This shows the histogram for the green part of the image, so now use your mouse to click and hold the left-hand black adjustment handle at the base of the histogram and carefully slide it to the right until it’s close to the start of the green section. The green is now darkened, removing the cast, but it’s showing up some blue in the image that you might not want. To deal with this, keep the ‘Histogram adjustment’ box open and repeat the steps for the ‘Green’ adjustments, but instead choose ‘Blue’ from the drop-down list and adjust it slightly (See Screenshot 3). In both the ‘Green’ and ‘Blue’ sections, make subtle adjustments until you’re happy with the result and then click ‘OK’ at the bottom of the box to apply them to the image.
Remember to reset
If the image still looks a little dim, you can adjust it by re-opening the ‘Histogram adjustment’ box. If the ‘Settings’ menu says ‘Last used’ then press the ‘Reset to default’ to the far right of the box, otherwise the last set of adjustments will be applied again, and you don’t want that. The ‘Reset to default’ sets the ‘Histogram adjustment’ function to the default of ‘Luminance’, which allows you to adjust the black adjustment handle at the left and slide it slightly to the right. Again, do this in small increments and watch carefully how the image changes until you’re happy with the result. Although this has darkened the image slightly, you can amend this by moving the middle ‘Gamma’ adjuster a little to the right to change the curve of the histogram. Again, do this in small adjustments until you’re happy with the result.
At this stage you haven’t needed to move the white adjustment handle for this particular image, but it may benefit from a slight adjustment. Remember, it is down to personal taste how much you push the adjustments. When you’re happy, click ‘OK’ to finish the ‘Luminance’ adjustment.
The image has been improved, but if you’d like the nebulae to be a little redder, you can now open the ‘Histogram adjustment’ box again and click ‘Select > Colours > Red’ (See Screenshot 4). This time, move the white adjustment handle inwards a little at a time, watching how the image changes. Once you’re satisfied, click ‘OK’ to close the box.
These simple steps have removed the green and slight blue cast, darkened the background to a more natural view of space and brought out the hydrogenalpha (Ha) red emission of the nebulae, all by using one tool to make multiple small adjustments. The image can be further processed, if necessary, using the ‘Saturation’ tool but, for now you can enjoy the final, processed image of the North America and Pelican Nebulae (see main picture).
3 QUICK TIPS
1. Always make a series of small adjustments rather than one big one.
2. Make sure the settings tab is either empty or press the ‘Reset to default’ button.
3. Don’t be afraid to explore the other options in the ‘Histogram adjustments’ box to further enhance your image.
Paul Money is an astronomy writer and broadcaster, and is reviews editor for BBC Sky at Night Magazine