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Editing astrophotography is extremely important if you want to transform your raw data into high quality images. Learning how to edit astrophotography will improve your pictures and this skill may be more than half of the work that goes into astrophotography. Why is it so important? Editing will bring out the fine and often hidden faint details in the objects you photograph, it can remove the effects of light pollution and create a unique style in your final image.
So let’s take a closer look at the world of astro image editing and how to make astrophotography pictures look better.
Raw images are dull and need editing to bring them to life. When you look at an image out of your camera on your laptop you’ll notice that it is almost completely black with maybe a few stars peeping out here and there. This is your raw unedited image and this is totally normal. Your image at this stage is in linear form.
To get any kind of pleasing image from this we need to boost the signal in the image by editing it. This is known as post-processing astrophotography images. By following steps such as curve stretching and many others we will create an image that has colour and depth with many of the details you want to see in your image.
I’ll include Basic astrophotography post-processing steps and more advanced techniques for editing on this page and I’ll show you some real examples of an image I am working on as I write this. Please understand that techniques for editing astrophotography range from basic to more advanced. If you’re a beginner than you’ll need to start with the simple steps and with time and experience you can progress to the more complex techniques for editing astro images.
Here is an example of the power of editing:
On the left we have a linear image and on the right an edited image of the wonderful horsehead nebula:
How to get started with astrophotography editing?
The first thing you’ll need to know is which software you can use for your editing. There are many choices some better than others, some free and some paid for. I’ll explain what software I use and explain why. I’ve also made some astrophotography editing tutorial videos which I will link to so that you can learn more about how I edit my astroimages.
Keep the Stars in Your Pocket
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Get this designEssential Software for Astrophotography Editing
After trying most of the astrophotography software available, especially the free options and some of the paid ones, I will give you my opinions on which are the best for beginners and more advanced astrophotographers.
What is the best free software for astrophotography editing?
What is best for me may not be best for you as we all have different preferences. But, I think I can say that of all the astro software I’ve tried for editing my astrophotos I prefer the following:
- Siril (Free)
- Photoshop (Monthly subscription)
- 12-month discount subscription
- For Mac or PC
- New Generative AI
- Excellent tool which I use every day
Dedicated software:
- Pixinsight (about 300 Euros)
- Seti Astro Suite (Free)
- Startools (Paid)
General software:
- Photoshop (Paid)
- Gimp (Free)
- Affinity Photo (Paid)
How to improve astrophotography images without Photoshop?
Understanding the Astrophotography Editing Workflow
Initial editing adjustments:
My first editing adjustments include the following: colour calibration, background extraction. I usually do these in Siril after I have my stacked linear image.
Exporting to other image editing software
Once I have done the above adjustments to my image I export to editing software such as Photoshop. I have been using Photoshop for many years and like the way I can stretch my images and work on colour balance and more. I am beginning to look at alternatives and many suggest that I should switch to the astro dedicated software Pixinsight. For now, I am using Photoshop in conjunction with other tools to edit my astrophotos.
How to Edit Astrophotography using Core Techniques
Once you’ve captured your raw astrophotography data, the real magic begins: image processing! Think of it like developing film in a darkroom, but instead of chemicals, we’re using software to transform that raw data into a stunning final image. Here are some of the core editing techniques we use to bring out the beauty of the cosmos:
Histogram Stretching: Bringing Light to the Darkness
Raw astrophotography images often look surprisingly dark, sometimes almost completely black. Don’t worry, your precious photons are still there! Histogram stretching is usually the first step, and it’s like turning up the lights in a dimly lit room. We’re essentially telling the software, “Hey, that faint glow? That’s actually important data! Let’s make it visible.” We do this by adjusting the black, white, and mid-points on a graph called a histogram, which represents the brightness levels in your image. It’s a fundamental step in revealing those faint nebulae and galaxies hidden in the darkness.
Curves Adjustment: Fine-Tuning the Details
While histogram stretching is like using a dimmer switch for the whole room, curves adjustments are like having individual spotlights for each part of your image. Curves give you much finer control. You can selectively brighten or darken specific tonal ranges – maybe you want to bring out the faint outer arms of a galaxy without blowing out its bright core. It’s all about carefully shaping the light and contrast to reveal the intricate details.
Watch my video about curve stretches to find out more about the different kinds of stretches you can use to enhance your images:
I am now going to show you the most important steps that should be front and centre in your astrophotography processing workflow. I’ll show you what a huge impact they will have and some ways that you can easily put them into practise today.
These steps will improve the colour, detail, clarity, and general appearance of your images.
Colour Calibration
The colour coming out of your camera is not calibrated, and therefore your object will appear very strange. All cameras have different sensors, so colours vary greatly. Filters used also affect the colour in your stacked image. Therefore, my first step in processing any astrophotography image is colour calibration.
How do we calibrate the colour in our images? By adjusting the balance between the red, green, and blue channels – think of it like adjusting the color knobs on an old TV. We also control the saturation, which is the intensity of the colours. Do you want those nebulae to really pop, or do you prefer a more subtle, natural look? It’s all up to you!
When I see the stacked image data from my camera, a ZWO ASI533 MC PRO, which is a one-shot colour camera, I usually see a bright green background because of the camera’s green bias in its sensor. The whole image is distorted.
The aim of colour calibration is to standardise the colour of the deep sky object you are imaging so that it matches generally accepted colours for that object. I’d certainly like to start by removing the green background. I don’t know about you, but I don’t see much grass in space!
There are two main ways to do colour calibration, and these are:
- manual colour calibration (white balance on a star and blacks on a dark sky background area)
- photometric colour calibration
In most image astrophotography processing software that is used for astrophotography, such as Pixinsight, Siril, etc. we can do both very easily. My chosen software lately is Siril, which is excellent and quick to stack and has many image-processing features that I use before moving to Photoshop and my major edits.
I normally choose the photometric calibration because it is already set to modify colours to a palette that matches a particular object in the night sky. I just do a search for the object in Siril, and it finds the object. Then I simply apply the colour calibration.
Manual calibration requires first setting a box clear of any stars in the background and setting that black point, then choosing an area of detail that is quite bright and clicking apply. What exactly the software does, I couldn’t tell you, but I suppose it analyses the image colours and then resets them to more suitable values. Instantly, the reds pop out, and the background darkens if done correctly.
It’s that simple in Siril, and other programs have scripts that you can run to do this, so do play around with these.
Noise reduction
Because we’re often pushing our cameras to their limits with long exposures and high ISO settings, astrophotography images tend to be noisy. Noise looks like a grainy texture, and it can obscure fine details. Noise reduction is like gently blurring away that graininess, revealing the smoother, cleaner signal underneath. It’s a balancing act, though – too much noise reduction, and you’ll lose those precious details! Check more about noise reduction as well as other processing techniques here.
Noise is such a big problem in night photography because the signal is so low and the objects we are photographing are so dim and far away. Many galaxies are millions of light years away from us. No, we are not going to get a strong signal!
We should focus especially on the relationship between the level of signal we want to capture from our object and the level of noise that any camera produces and that is present in each image. This relationship is called the signal-to-noise ratio.
A higher signal-to-noise ratio will improve the quality of the image, and the opposite is also true. Anything we can do to boost the signal or decrease the noise will help. It is difficult to boost the signal unless we use larger and better telescope designs or better lenses and optics, this is why we work on reducing the noise. It’s definitely the cheaper alternative!
Reducing noise in images is possible in several ways; here are a few of the most important:
- using a high-quality camera that produces less noise (especially a cooled camera).
- taking shots with higher exposure times to improve the signal to noise ratio. The signal captured from a faint object increases more than the level of noise which actually does not increase much when lengthening exposure time of each image. i’ve tested this out myself as it doesn’t seem intuitive but it is true, longer exposure time means less problems with noise and better images.
- image stacking reduces noise levels. The square root of the number of stacked images increases the signal-to-noise ratio. Therefore, stacking 100 images improves the signal-to-noise ratio approximately ten times, which is mostly why we stack images in astrophotography. You stack only raw data prior to processing.
- We can reduce noise in astrophotography processing software such as Photoshop, Pixinsight, Gimp, and so on. The only problem with this is that decreasing noise usually makes images less sharp, but this is a balance, and some noise reduction in the early stages of astrophotography image processing is very helpful.
- Deepsky imaging in colder temperatures reduces one kind of noise, i.e., thermal noise. This has caused me all kinds of problems, especially when imaging with a DSLR camera in the summer. Local temperatures climb to 32 °C and above, and the temperature in the camera gets quite a bit warmer than this. One solution is to use camera cooling, and many dedicated astronomy cameras, such as the ZWO 533 MC Pro I now use, have a cooling feature. No matter the time of year, I keep this set to 0°C, and this eliminates a lot of noise I had when using my old DSLR camera.
- Test different camera settings and also consult the manual for the model of camera you use. For example, my DSLR requires an ISO setting of 800-1600 for minimum noise. I tested this and found that it is best to use 800 ISO. With an astronomy camera, gain setting is important so always be sure to check the best camera settings to minimize noise and improve the quality of your image. This will help you enormously when it comes to processing later.
If you want great pictures in astrophotography, you must consider noise and try to reduce it as much as you can. If you have too much noise, you will have to image for much longer and stack many more images to try to remove it. I bought a better camera, and the results are stunning! It was almost at the stage where I couldn’t use my DSLR all nigh
Star Removal and Reduction
Now I will explain how I edit my astrophotography images. Before you read this watch my video to see my latest astrophotography processing workflow:
Subscribe to my YouTube channel for more videos about astrophotography. The following are links to the tools I use in the video:
- Arcsine stretch tool. Get it here: Markshelley.co.uk
- Carboni’s astrophotography tools info here: Prodigitalsoftware.com
To process astrophotography images I have found that star removal and reduction is a great way to enhance the image of the target without bloating the stars. The reduction part can also create an image that focuses more on the intended target and reveals more details by toning down the background star field.
In some cases, such as when photographing the Veil Nebula, the background stars are very overwhelming and drown the image in a sea of light, which hides the nebula from the eye. Removing some of these stars or reducing their brightness can bring out the details in the galaxy in your image or nebula.
Below, I have tried to show how much difference this can make and how it can improve your astrophotography images by conducting a test. I’d love to hear what you think.
Test: Comparing Image Created with and without Star Removal and Reduction
This is my final image of M81 created without using any star reduction or removal technique except for “make stars smaller” applied to the image when processing in the Astronomy Tools set, an excellent Photoshop plugin.
The imaging details are Optolong L-Pro Max broadband RGB filter.
150 x 120 seconds = 300 minutes, or 5 hours of integration time.
Celestron 130slt reflector OTA. Ioptron CEM 26 EQ mount with auto-guiding scope.
The one downside to this method of astrophotography processing is that while stretching the image we also stretch the stars, and they can become bloated and overpowering. This image does have reasonably unobtrusive stars, though, and is not bad. Let’s take a look at a second image using the same data but processed with star removal and reduction and see how they compare.
Astrophotography Processing Methodology
There are many ways to carry out star reduction, but the method I am using is as follows:
- I begin by loading my stacked image in.tif format, giving it a few small stretches, and then using levels adjustment. When the stars appear but are not too bright, I copy the layer, do a levels adjustment to darken the image a little so that only the stars show in the image, and then name this layer “stars”.
- I then again copy the background layer and perform star removal using a tool by Russel Croman called StarXterminator. I recommend this tool, and it works much better for me than Starnett++. I then have an image without stars, as in the picture below:
Now that I have a starless and star-only image, I can proceed to stretch and enhance the starless image aggressively in order to bring out the details, and I don’t need to worry about blown-out and distracting stars as I will add those in later to suit my tastes.
I went through a series of stretching and other post-processing techniques until I got an image result I was pleased with and then combined it with the star layer from before. The way I did this was to blend the layers together using the “lighten” setting in Photoshop and adjust the opacity slider as I saw fit to about 60%.
This image shows the final result I got for the same data:
Test Results
Which of the two images you prefer is a matter of your choice and opinion. I personally think they are both good images, and there is a lot to be said about the first image without star reduction. The second image, in my opinion, shows more detail in the galaxy, as I was able to stretch aggressively and apply all enhancements to the galaxy without worrying about the stars.
I think the two images could be more closely matched in terms of brightness, but the differences, despite this, are clear, I think. In this case, there is not a heavy star field such as exists for other targets, so the star reduction was perhaps not so important in the overall final image.
In the future, I think I should give an example with a sky background filled with more stars that will highlight more clearly how effective this astrophotography processing technique can be in improving your astrophot
Sharpening: Bringing Out the Crispness
Sharpening is like focusing a blurry lens. It enhances the fine details in your image, making stars appear crisper and nebulae more defined. But be careful! Over-sharpening can create ugly halos around stars and make the image look artificial. It’s all about finding that sweet spot where the details are enhanced without introducing unwanted artifacts.
Gradient Removal: Dealing with Uneven Lighting
Gradients are those annoying, uneven brightness variations that often creep into our astrophotos. They’re usually caused by light pollution, moonlight, or even reflections inside the telescope. Think of it like a spotlight shining unevenly across your image. Gradient removal tools help us even out that background, creating a more uniform and pleasing canvas for our celestial objects.
Removing Artifacts: Cleaning Up the Mess
Astrophotos can be messy! We often encounter things like hot pixels (stuck-on bright pixels), satellite trails (streaks of light from passing satellites), and dust shadows (dark spots caused by dust on the sensor or optics). Removing these imperfections is like cleaning up a messy room – it’s an important part of making the final image look its best. It’s the final polish that makes everything shine.
Advanced Techniques:
Masking: Using masks to selectively apply adjustments to specific areas of the image.
Deconvolution: This improves details by increasing resolution and clarity of the image. Deconvolution tries to reduce blurring in an image and reconstruct what the sharp image looks like.
HDR Composition: Combining images of different exposure lengths to capture a wider dynamic range. I’ve used this process with the Orion Nebula where the core of the nebula gets blown out with only about 30 seconds of exposure, but the finer details require longer exposures of several minutes.
I took a series of images with a 30 second exposure time and others at 3 minutes. After stacking the longer and shorter exposures separately, I made initial adjustments to the two stacked images and then combined them into one image using the shorter exposures for the core and the longer exposures to bring out the details in the dimmer areas of the image.
Starless Processing: Techniques for processing nebulae and galaxies separately from the stars. I use StarXterminator from Russel Croman as a plugin added to Photoshop to remove the stars. I also create a star only image in a separate layer. After editing the starless image I blend the two images together adding the stars as I see fit.
Specific Software Workflows: Detailed steps for using particular software packages (PixInsight, Photoshop, etc.)
Combining Different Data Sets: (Ha, OIII, SII, LRGB, etc.)
Pixel Math:
This tool which exists in software such as Siril or Pixinsight, enables you to compose an image from different mathematical combinations of RGB channels or channels containing signals made up of HA or OIII or Sulphur obtained during narrowband imaging with special filters.
Results and Examples
I’d like to show you a few examples of the huge difference astrophotography processing can make to your images. Below are several of my own examples where I transformed images from black nothingness to a visible stretched image with contrast that can then be further improved. The final results gave me a lot of satisfaction.
As you look at the images below, posted in the order that I processed them, I’m sure you’ll be inspired by how my skills in astrophotography Photoshop editing have improved. You can go through the same journey!
Attempting to Process Someone Else’s Image
Before
After
The above image was processed from the tiff file posted in Cloudy Nights forum1. What I did to see if I could get a good result from this stacked image was to first do color calibration and reduce green noise in Siril and then process the image in Photoshop CC.
Basically, in Photoshop I worked on some curves and levels adjustment and used the Astronomy Tools actions to take out space noise, reduce stars and increase some saturation. I am not happy with the end result but the nebula is there.
I believe the problem with the data is the high level of light pollution and the Gradient Xtractor tool was used to remove this (applying it two times). The image appears rather dark on the web and once converted to jpeg. Perhaps I should have lightened it further?
The original data was from 15 x 300-second exposures at ISO 800. I will try to capture my own version and see what I can get.
Astrophotography Photoshop Processing – Some of My Images Before and After
When I process my images I follow the following workflow generally:
- Sort through photos of the astrophotography target and reject those photos with star trailing, bad focus or tracking errors. I normally do this in Adobe Bridge.
- Stack the images in Sequator.
- Set colour calibration, reduce green noise, and do autostretch in Siril.
- Post-process images in Photoshop adjusting curves, levels, noise, saturation, etc.
On one or two occasions I have had some problems with images processed in Siril so I also save the raw stacked image so if there are any issues I can stretch and adjust these myself in Photoshop.
M101 The Whirlpool Galaxy
Before
After
This image is the first I’ve taken of this target M101 and is just 1hr. of 45-second exposures at ISO 1600. I used darks when stacking. the difference between the RAW stacked image and the initially processed image in Photoshop is like night and day, don’t you think?
With time the processing skills you have as an astrophotographer will improve. I will add some of my latest images to this post to show you what I have now managed to do with Photoshop and a couple of clicks after more than a year and a half of experience in this journey.
M81 and M82
Before
After
It can clearly be seen in the above pictures that Photoshop enables us to remove light pollution, (my skies are Bortle 5), any gradients caused by street lights, and so on, enhance contrast, reduce stars, and remove noise in the image. I do use darks in the stacking process and the more images that are stacked the lower the noise in the images. The photos of the Bodes Galaxy and the Cigar Galaxy are approx. 2hrs 15 mins of 45-second individual exposures at ISO 1600.
During the course of my amazing journey into astrophotography, I have experimented with a number of different programs that improve the stacking and editing of images. At first, as mentioned above I used Siril after stacking in Sequator for some processing but now I tend to go directly to Photoshop as my skills with using it have improved a lot.
So, let’s take a look at what can be achieved with better astrophotography Photoshop skills that someone with 18 months of experience might have. All of these are my own personal work imaged with my Celestron 130slt Newtonian telescope and using a Canon 600D modified for astrophotography.
The above two images show the results of two nights of exposures I took of the Bubble Nebula (NGC 7635) from my back garden. The image was taken using a clip in Hydrogen Alpha filter during an almost full moon, which is the main reason I used the filter instead of taking normal colour images. The image consists of the following:
59 x 3-minute exposures at ISO 800
12 x 4-minute exposures at ISO 800
17 x dark frames of 3 minutes each
Total exposure time was 3 hours and 45 minutes.
Stacked in Sequator
Processed in Photoshop as shown in this helpful video2 by Trevor Jones of Astrobackyard.com. Basically two curve stretches and levels adjustment plus local contrast adjustment and enhance DSO and reduce stars in Astronomy Tools action set. Gradient extractor was also applied at a low level.
Conclusions
I am really happy, particularly with the last two images shown taken in hydrogen alpha. Of course, any image could do with more exposure, and in the future, I may add to this data. I also plan, as soon as the Moon wanes again, to add RGB colour data to this image and process a final image. This should be quite spectacular! I will add those images and write about how to process HA images and combine them with RGB data to produce a final image that should really pop! At the moment I am new to this process and still learning, so much more to learn…
References
[1] Cloudy Nights Forum. Retrieved from https://www.cloudynights.com/topic/756842-need-help-first-time-shooting-the-heart-nebula-with-a-modified-dslr/#entry10896583
[2] Astrobackyard. DSLR Astrophotography Tutorial – Processing Narrowband H-Alpha (Ha) – YouTube. Retrieved from https://www.youtube.com/watch?v=S08fBXo2vsw
