Expert processing tips to enhance your astrophotos
ASTROPHOTOGRAPHY
Processing: Removing stars
Subtract stars to get more from deep-sky data
Avoid distortion while processing by taking the stars out of the picture with free software
BEFORE
AFTER
Astrophotography processing is all about the art of data control and getting the right balance between stretching data to tease out details, without going too far and ‘clipping’ it. Once the data is overstretched, delicate details are lost, or the image becomes noisy. Stars are one major obstacle during processing. They limit the extent to which data can be enhanced, as they distort or become bloated. This reduces the desired ‘natural’ appearance for deep-sky images. Software that can remove stars therefore allows the user to work on all the delicate details, without being hindered by stars. One example is the free software StarNet++. Originally developed as a PixInsight module, it’s now available as a standalone program via www.starnetastro.com/download. StarNet++ is compatible with both CCD and DSLR data, and so a popular choice for most astrophotographer toolkits.
Version 2 of the software was released early 2022 and includes a handy graphical user interface (GUI) that makes it more intuitive. This is the version we used to process our image of the Flaming Star Nebula (IC 405). Here we will go through the steps we took to remove the stars using StarNet++ and then use Photoshop to recover a ‘stars-only’ file which we could reapply to our fully processed nebula at the end.
Takeaway time
Our initial image (above) was a stacked TIFF file we labelled ‘Start image’. First, we opened this ‘Start image’ in Photoshop by clicking File > Open and navigating to our file location. We then minimised our Photoshop screen and clicked on the StarNet++ icon on our desktop. A StarNet++ GUI menu popped up (see screenshot 1). By clicking on ‘Browse’, we navigated to the location of our start image and selected it.
Both the top and bottom boxes of the StarNet++ menu then filled with the file’s path location. We amended the bottom box slightly: adding ‘starless’ next to ‘Start image’ (screenshot 1). This means that StarNet++ saved a separate ‘starless’ version of our file. We then clicked ‘Run’.
A command window called ‘StarNet console’ appeared, showing the progress of the star removal. ‘Done!’ appeared on the command window once StarNet++ had finished running. A starless version of our ‘Start image’, ‘Start image starless’ now appeared in the same file location as our ‘Start image’. We opened this in Photoshop (see screenshot 2) and found the image free of all stars. This is the image we would be processing. However, before we moved on to do that, it was important that we develop a stars-only file containing just the star data minus the nebula, as we would need this at the end to reapply stars to our processed image.
To do this, we clicked back onto the original ‘Start image’ which was the first file we opened in Photoshop. We clicked Image > Apply Image. We then selected the starless version of our image as the ‘Source’ file and selected ‘Subtract’ from the Blending dropdown menu (see screenshot 3). This subtracted our starless file from the complete data, leaving just the stars. An image containing just the stars appeared. It is important to do this step before you process the image further.
We then clicked back onto our ‘Start image starless’ file to process our nebula without the stars. We did this by applying Photoshop adjustments including levels, colour balance, and hue/saturation (clicking Image > Adjustments for each of these) and then adding the ‘Dust and scratches’ and ‘Median’ noise reduction filters (Filters > Noise).
Once satisfied with our nebula, we could reapply our stars. To do this, we clicked on our ‘star-only’ file created earlier (see screenshot 3). By clicking Edit > Select All or using Ctrl + A, the star-only image is selected. We then pressed Ctrl + C to copy the file. Clicking back onto our processed, starless image, we then pressed Ctrl + V to paste our stars as an opaque layer on top. Finally, we selected ‘Screen’ from the layers blending menu (highlighted in screenshot 3) to blend the starless image with the stars, creating the final image you can see above.
3 QUICK TIPS
1. StarNet++ sometimes mistakenly removes galaxy cores. Photoshop masks can be used to recover these.
2. Don’t crop before reapplying stars, because this messes up star positions. Crop either before stars are removed or as the final step.
3. Add and remove the star layer at various stages, to check in on how your image is looking.
Charlotte Daniels is an amateur astronomer, astrophotographer and journalist