This Photoshop plug-in can bring out the natural deep-sky details by suppressing noise

Removing image noise, or unwanted artefacts, is one of the main aims of deep-sky astrophotography processing. The more we can reduce noise, the more freedom it allows us to process our images of nebulae and galaxies. This is particularly true for nebulae, where delicate gaseous details are easily overwhelmed or hidden There are various noise reduction tools to help overcome the issue, but care needs to be taken applying them; use them too heavily and the target will look soft or blurry. In this article we will show you how to apply just the right amount of adjustment to the stars in a deep-sky image of the North America Nebula by using a plug-in.

Stars are often one of the main causes of unwanted artefacts in astrophotos, and this complicates noise reduction processes because there are often hundreds, if not thousands of them in an image. One way to manage this is to remove stars from the image so that they can be processed separately. While software to remove stars exists (including StarNet++ in PixInsight) these work best for CCD cameras and can struggle with DSLR camera data. Also, if you are using Adobe Photoshop or GIMP image-editing software, it is often the case that both the nebula and stars must be processed at the same time. This is where a plug-in (software add-on) that specifically targets the appearance of the stars is very useful.

Getting ready

Here, we are going to run through how to use Russell Croman’s ‘StarShrink’ plug-in to enhance an image of the North America Nebula in Photoshop. StarShrink is purchased and downloaded from his website at www.rc-astro.com/resources. It has a 30-day free trial and works with both Windows and MacOS operating systems. Once the file is downloaded, you need to locate it – for Windows computers it is found in ‘C: drive > Downloads’. This file needs to be transferred to your Photoshop Plug-ins directory, the location of which is dependent on the version of Photoshop 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.

We recommend applying StarShrink at the end of your processing workflow, once adjustments to the main objects in a deep-sky image have been made.

The reason is because the appearance of the stars will change throughout most of the image amendments you make in Photoshop, often increasing their size. This is the case for our initial processing of the North America Nebula. In our example (see the ‘Before’ image, opposite, left) we have already made all adjustments required to maximise the appearance of the nebula itself, including ‘Brightness/Contrast’, ‘Levels’, ‘Colour Balance’ and ‘Hue/Saturation’; all of which are accessed ‘by clicking Image > Adjustments’. A median filter has also been applied, by selecting ‘Filter > Noise > Median’, to reduce some of the overall image noise, but the stars are still looking bloated in our image.

We can manage these stars to an extent, by applying ‘Dust & Scratches’, which is achieved by clicking ‘Filter > Noise > Dust & Scratches’, but it is easy to overdo this function and soften the appearance of the target (see Screenshot 1). So, in order to prevent this, we will use StarShrink to target and ‘shrink’ the stars.

Once the StarShrink plug-in is open, by clicking ‘Filter > StarShrink’, the preview window will focus on a region of your image (Screenshot 2). To move to another region, just click and drag within the window.

Starry adjustments

At the bottom of the preview window are three adjustment sliders. By moving the top one, ‘Radius’, you can adjust which stars will have a shrink applied; a smaller radius will target smaller stars. The middle slider, ‘Strength’, allows us to determine how strong the shrink we apply is, and the bottom slider option, ‘Sharpness’, helps to maintain a natural star profile. If the ‘Sharpness’ value is too high, stars will appear as tight white pinpricks with hard edges, but a lower value will retain colour around the stars’ edges, allowing them to subtly complement the image.

Be careful not to shrink every star to the same size, as this can make your image look over-processed. While stars are responsible for noise, we don’t want them to look artificial as they are still an important part of the overall image. We can use a layer mask to select any stars we’d like to leave unamended. Once we have performed a shrink, we can do this by clicking the mask icon with the star shrink layer we’ve highlighted (see Screenshot 3). Next, a white mask will appear next to the layer. After we have altered our brush size (see top left window in Screenshot 3) we paint over selected stars to undo the change and then choose which ones we want to stand out in our image. Be sure not to overdo the shrink, as this will create unwanted warping around the larger stars.

In our final image, you can see that the North America Nebula’s starry regions are now far less noisy. The attractive, natural looking stars add to the depth and beauty of the deep-sky picture.

3 QUICK TIPS

1. Click and drag the image screen within StarShrink to see the effect of your adjustment.

2. If there are varying sizes of stars, apply StarShrink more than once and change the radius each time.

3. If using a mask, you can alter the brush size by using your bracket keys.

ALL PICTURES: CHARLOTTE DANIELS