PROCESSING

Improving sky backgrounds in deep-sky astro images

How a Photoshop plug-in can reduce light pollution and vignetting around a deep-sky target

Astrophotography processing is about more than just stretching data and reducing noise, those unwanted artefacts. It is also important to create an even, realistic-looking background for your deep-sky objects. A flat, dark background will enhance the appearance of the target, but depending on the conditions you are imaging in, this can be a challenging thing to achieve.

Generally, you don’t want to focus on making the background sky completely black, as this can look artificial. Thanks to light pollution and imagevignetting (where an image appears darker around the edges), captured images will often consist of uneven gradients – different patches of brightness or colour – that become more obvious as we stretch the data. These gradients can be difficult to correct evenly, which is where a software plug-in to help with gradient control can be particularly useful.

In Adobe Photoshop, gradient control can be carried out via basic functions such as ‘Curves’, but a Photoshop plug-in that can swiftly flatten gradients and restore an even background colour saves a lot of time. In this article we will show you how to use Russell Croman’s useful GradientXTerminator Photoshop plug-in to adjust the gradients in a wide-field image of the Jellyfish Nebula, IC443. The plug-in is downloaded from his website, bit.ly/3tsJNHr, and a 30-day free trial is available before purchase. Once the plug-in is downloaded, you need to locate it – for Windows computers it is found in the ‘C:/drive > Downloads’.

The file needs to be transferred to your Photoshop plug-ins directory, the location of which depends on the version of the software you are using. For Photoshop 2022 on a Windows computer, click and drag the file to ‘C:\Program Files\Common Files\ Adobe\Plug-Ins\CC’.

Stand-out nebule

It’s best to apply GradientXTerminator after a ‘stretch’ of the data using the ‘Levels’ function (click ‘Image > Levels’), as we’ve done in our ‘Before’ image (see above, left). So in the ‘Levels’ window, drag the anchor points closer in towards the histogram, allowing it to widen and details to emerge. Although this step was repeated several times to make the nebulae stand out in our ‘Before’ image, there is still a vignette disrupting the image, so it’s now time to remove these gradients.

Before you use the Gradient XTerminator plug-in, remember that you only want to apply it to the background, rather than the target. If you allow the plug-in to change the target appearance too, it can remove some colour or nebulous detail. To avoid this happening, use the ‘Lasso’ tool in Photoshop: select the icon from the Toolbar on the left-hand side of the screen and then drag the ‘Lasso’ tool around the objects. Although this action automatically ‘selects’ regions within the ‘Lasso’, you want to ‘deselect’ these regions to ensure GradientXTerminator ignores them. To do this click ‘Select > Inverse’ to focus on the regions outside the ‘Lasso’ (see Screenshot 1).

Now it’s time to apply the plug-in, which is accessed via the ‘Filters’ menu, by clicking ‘Filters > RC Astro > GradientXTerminator’. For the first gradient adjustment, in the plug-in window, select ‘Detail > Medium’ and ‘Aggressiveness > Low’ (see Screenshot 2). Although this applies a broad correction to the background, some gradients are still present.

To make a finer adjustment to complete the gradient removal, without the risk of over-flattening our background, select the ‘Magic Wand’ tool from Photoshop’s Toolbar. A menu appears on the top of the screen, allowing you to set the strength. Set the ‘Tolerance’ to ‘7’ on this occasion, but the best value will depend on the image. Generally, the best value will range between ‘5’ and ‘10’.

Background checks

Now click on an area in the background, which depending on the ‘Tolerance’ setting chosen should select most of, or all of the background (but not the object). It is important to ensure the target is left untouched by GradientXTerminator, so this process takes a little trial and error. Make sure you experiment with the ‘Tolerance’ setting until you have most of the background selected minus the target(s) of your image.

Then click on GradientXTerminator again, and this time set the ‘Detail’ to ‘Fine’, and the ‘Aggressiveness’ to ‘High’ (see Screenshot 3). This will ensure finer gradients are dealt with, giving a smoother result. In the ‘After ‘ image, the background now looks more even, with colour gradients and vignetting removed. All that is left is to make some final Photoshop adjustments by using ‘Selective Color’ and ‘Brightness’ (click ‘Image > Adjustments’) and ‘Dust and Scratches’ (‘Filters > Noise’), before saving it.

3 QUICK TIPS

1. The best ‘Tolerance’ settings depend on the deep-sky object – ensure dusty details are not highlighted.

2. Apply GradientXTerminator in stages: use the ‘eye’ icon for that layer to view each change.

3. For Magic Wand, the ‘Alt’ key allows you to add or subtract regions. A plus or minus will appear next to the icon.

Photos: CHARLOTTE DANIELS