Post-processing astrophotography images is mandatory: you cannot avoid it. It can be a fairly long and technical process, but it is necessary to squeeze out the most you can from your images.
Everything begins with pre-processing your images, a step including image calibration and image stacking, which we have already covered in this article.
After that, it is time to post-process your stacked image with your software of choice. In this article, we will discuss the different options that are available to post-process your astrophotography images.
Note: Don’t miss the detailed video at the end of this article, It was created to help show you how to process your images with some of the software mentioned in this article.
Click here to skip to our Image Processing Demo Video.
What Does Post-Processing Mean In Astrophotography?
In astrophotography, the post-processing includes steps that are crucial to the quality of the final image. Those steps can be summarized as:
- Histogram stretching
- Gradients and light pollution removal
- Stars color calibration
- Stars reduction and Stars removal
- Sharpening and noise reduction
- Final tweaks
Of the steps mentioned above, it is worth to spend a few words on the Histogram Stretching, as it is of utmost importance in deep-sky astrophotography.
What Histogram Stretching Is And Why You Need It
With image stacking, you have combined all your light frames (the actual images of the sky) into a single image with an enhanced signal-to-noise ratio.
With deep sky astrophotography, this stacked image can be surprisingly dark, with only a few bright stars visible.
There is nothing wrong with it, as all the details and information are there, but hidden in the dark background. And this is why this process also goes under the name of background extraction.
Histogram stretching can be done manually using Adobe Photoshop or in automatic/semi-automatic way using astrophotography software such as Astro Pixel Processor, Star Tools, or His Majesty PixInsight.
A rigorous explanation on how digital data are recorded and how the histogram works can become fairly technical and is beyond the scope of this article.
To keep it simple, let’s say that when you perform the stretching of the histogram, you are broadening the histogram. Thus pushing details that were crammed in the blacks towards the middle tones.
And you do that slowly, in small steps, to ensure retaining the best possible image quality.
The process allows us to take full advantage of the image stacking process, and it results in a cleaner, brighter image with a lot of details that were not visible (or barely visible) in the single exposures.
Stars Reduction / Stars Removal
Star reduction is another process that is standard when editing deep-sky astrophotography.
While it seems odd that you want to shrink or remove stars from a photo about stars, this process aims to make the multitude of visible stars in the image less imposing and distracting.
By reducing enlarged stars due to the histogram stretching and by removing the smallest stars, you make the deep sky objects in the image more visible, as shown in the image below.
The procedure is particularly useful when shooting deep-sky objects, such as nebulae, that are in the Milky Way Band.
Software For Astrophotography Post-Processing
We can group the software for astrophotography post-processing in two categories:
- generic photo editors, such as Photoshop, Gimp, Affinity Photo, etc.
- Astrophotography editors, such as StarTools, Nebulosity, Astro Pixel Processor, Pixinsight, etc.
The main advantage of generic photo editors over specific astrophotography editors is versatility.
With a generic photo editor, it is easy to post-process all kinds of astrophotography, from deep-sky imaging to lunar and planetary shots, passing for star trails and starry landscapes.
In this article, for example, we discussed how to stack starry landscape images in Photoshop.
Not many astrophotography editors are this flexible.
Here is a list of software that are most commonly used to post-process astrophotography images.
Adobe Lightroom CC
Generic Photo Editor | Commercial From $9.99 Subscription Plan | Windows, Mac OS X, IOS
Pros
- Easy to use
- Powerful image development and image organizer
- Easy integration with Photoshop
- Can use photographic plugins
Cons
- Can’t do the complex editing needed for astrophotography (histogram stretching, Stars Reduction, etc)
- Limited to cosmetic tweaks
Adobe Lightroom is a popular, easy to use and fairly powerful RAW developer and image organizer.
Its usefulness in astrophotography is somewhat limited, as you cannot perform complex tasks such as histogram stretching, advanced light pollution, and gradient removal, star reduction, etc.
On the other hand, it is a terrific editor for the final cosmetic tweaks to your image and to organize them in collections, per tag, and location. Lightroom is also great for color proofing your images before printing them.
If you are subscribing to the Adobe Photography Plan, you also have Photoshop CC included for free. And here is where things get interesting.
To get the best from the two worlds, load your stacked images in Lightroom, organize them in collections, and call Photoshop from within Lightroom for the astro-specific editing (histogram stretching, etc.).
Then make the final tweaks in Lightroom.
Adobe Photoshop CC
Generic Photo Editor | Commercial From $9.99 Subscription Plan | Windows, Mac OS X, IOS
Pros
- Versatile and Powerful Photo Editor / Image Manipulation Software
- Suitable for deep sky and planetary astrophotography as well as star trails and starry landscapes
- Astrophotography Action Sets and Plugins Available
- Subscription Plan with Photography Bundle
Cons
- Lacks Some Advanced Features for Astrophotography
Photoshop is one of the most commonly used software in the field of photography editing and image manipulation, and it can be used to post-process astrophotography work.
If you are a beginner astrophotographer, you are on a tight budget or you already own Photoshop, you should give it a try as all the basic post-processing steps can be performed in this software.
If you need more advanced features, you can also expand Photoshop capabilities thanks to many astrophotography related Action Sets, Plugins, and Panels.
Finally, with Camera Raw filter and other photographic plugins (like for smart sharpening and advance noise reduction), you can perform with ease all the final tweaks an image may need.
As a Photoshop user, I tried many plugins and action sets for astrophotography, and here is my must-have extensions list.
Astronomy Tools by ProDigital
Actions Pack For Deep Sky Astrophotography| Commercial $21.95 | Windows, Mac OS X
A rich set of actions suitable for post-processing astrophotography images. The set includes actions such as star reduction, enhanced DSO, light pollution and color gradient removal, sharpening, and noise reduction.
Photokemi’s Star Tools by Ken Mitchel
Actions Pack For Deep Sky Astrophotography | Commercial $14.95 | Windows, Mac OSX
Similarly to Astronomy Tools, this action set is most useful for deep space astrophotography.
It offers advanced star removal and star reducing actions, semi-automatic histogram stretching, different sharpening and noise reduction actions, as well as actions such as nebula filters and star color enhancement.
There is also a set of extra actions, available for $6.95.
GradientXterminator by Russell Croman
Plugin For Deep Sky Astrophotography | Commercial $49.95 | Windows, Mac OS X
This plugin is a gradient removal tool that is easy to use and extremely effective. Despite a rather steep price (a trial is available for you to test the plugin), this is a terrific add-on for Photoshop, if you are serious about deep-sky astrophotography.
Hasta La Vista Green! (HLVG) by Regelio Bernard Andreo
Plugin For Deep Sky Astrophotography | Donationware | Windows
Despite its old age, this plugin is still useful, and it does an excellent job of removing green noise and the green casts such noise may cause in some images.
Astro Panel By Angelo Perrone
Panel For Starry Landscape And Deep Sky Astrophotography | Commercial | Windows, Mac OS X
Astro Panel consists of a rich set of functions and methods that produce high quality starry landscapes and Milky Way images.
It is also easy to process Deep Sky Photos thanks to advanced functions for reducing digital noise and hot-pixels, eliminating the gradient, managing artificial flat, and much more …
Furthermore, astronomical images aside, you can use the Astro Panel to edit classic landscape images too.
Affinity Photo
Generic Photo Editor | Commercial $49.99 | Windows, Mac OS X, IOS ($19.99)
Pros
- Affordable
- Powerful
- The interface and commands are similar to Photoshop for an easy switch
- Suitable for deep sky and planetary astrophotography as well as star trails and starry landscapes
Cons
- Lacks third-party actions sets, plugins and panels
Affinity Photo from Serif Lab is a great, affordable alternative to Photoshop, and you do not need to pay for a subscription plan.
With Affinity Photo, you can carry out with ease all of the basic astrophotography post-processing.
But since there are no plugins, action sets, and panels to help you out, you have to learn to do things manually, even the more advanced tasks such as star reduction.
Gimp
Photo Editor | Freeware | Windows, Mac OS X, Linux
Pros
- Freeware
- Great community and lot of info available
- Powerful
- Suitable for deep sky and planetary astrophotography as well as star trails and starry landscapes
Cons
- Interface a bit confused
- Lacks third-party actions sets, plugins and panels
Gimp is the historical freeware alternative to Photoshop. Since it is freeware and on the market for many years, there is a big community of users, so it is easy to find relevant tutorials and guides to help you out.
The software has a slightly confusing interface, particularly if you are trying to switch from Photoshop, but it is powerful enough to let you edit your astrophotography images with ease.
Unfortunately, there are no third-party action sets, plugins, or panels to help you automate some tasks. As with Affinity Photo, you have to learn how to do everything manually.
Star Tools
Astrophotography Post-Processing Tools | Commercial $45 | Windows, Mac OS X, Linux
Pros
- Affordable
- Multiplatform
- Offers many advanced tools
- Trial without time limit
Cons
- Interface bit confusing
- Convoluted workflow
- Slower than other software
StarTools is a deep-sky post-process editor that does everything you need except the initial light frame calibration and stacking.
Once you have the stacked image from, say, Deep Sky Stacker, you can post-process it in StarTools, taking advantage of the many tools the software has to offer.
The interface is a bit confusing, and it may take a while to get used to the convoluted editing workflow.
Fortunately, the trial version never expires, so you can take all the time you need to experiment with StarTools before deciding if it is for you or not. The only limitation of the trial is that you cannot save your results.
SiriL
Multipurpose Astrophotography Editor | Freeware | Windows, Mac OS X, Linux
Pros
- Freeware
- Multiplatform
- Active Development
- Suitable for different kinds of astrophotography
- Fairly easy to use
- Powerful full-grown astrophotography software
Cons
- Develop the image is a lengthy process
- Interface a bit confused
I’m no expert with SiriL, but it is probably the only full-grown astrophotography editor that is freeware and multiplatform.
Siril will allow you to perform all the essential steps in your astrophotography editing workflow, from image calibration and stacking to (manual or auto) histogram stretching and post-processing.
Since it is free, if you are looking for an astrophotography package, SirilL is worth downloading and having a go with it.
Nebulosity
Deep Sky Astrophotography Editor | Commercial $95 | Windows, Mac OS X
Pros
- Capable full astrophotography editor
- Can calibrate and stack your images
- It offers many advanced tools
Cons
- Not abandonware, but development is somehow slow
- The interface feels old and not very user friendly
Nebulosity 4 was my first software specific to astrophotography. It is intended for deep sky astrophotography and is fairly easy to use.
It offers a good way to calibrate and stack your images, and you can use it for stretching the histogram, tighten the stars, calibrate the background colors, and perform sharpening and noise reduction.
But the interface is not as intuitive, it looks “old,” and while development is there, it is not as quick compared with other software.
Astro Pixel Processor
Deep Sky Astrophotography Editor | Commercial €60/Yr (Renter’s License) Or €150 (Owner’s Renter) | Windows, Mac OS X, Linux
Pro
- Great deep sky astrophotography package
- Powerful
- Easy to use
- Batch processing
- 30-days free trial available
- Suitable for creating stunning mosaic with ease
- Active development
- Rental license available
Cons
- Vignetting removal tool could be better
- No Stars Reduction methods available
Astro Pixel Processor is my goto software for my deep sky astrophotography and I decided to go with the renter’s license to always work with the latest version of the software.
The interface is easy to navigate, options are explained by text messages that appear when you hover on the options with the mouse, and the different tabs are numbered.
This means that there is no guessing in establishing the best workflow: just follow the numbers from 1 to 6 and jump at the tab number 9 for post-processing the stacked image.
You can run all the steps once at a time or set them up and run all with a batch processing: this way, you can do other stuff while the software calibrates and stacks your images.
If you are looking for a way to edit your deep-sky images and create mosaics, I vouch for Astro Pixel Processor.
PixInsight
Multipurpose Astrophotography Editor | Commercial €230+VAT | Windows, Mac OS X, Linux
Pros
- The best and most complete astrophotography editor on the market
- Multiplatform
- Suitable for Planetary and Deep-Sky astrophotography
- 45-days free trial available
Cons
- Expensive
- Extremely steep learning curve
- Requires a powerful computer to run smoothly and conveniently fast
I will be honest with you: I requested a trial (and it was granted twice), but both times I ran away from PixInsight screaming in despair.
Not that PixInsight is bad or lacks crucial functions, but because it is very complicated to use for beginners and the learning curve is very steep.
Granted, PixInsight, being the software of refinement for the category, there are tons of tutorials and guides online (Light Vortex Astronomy has some of the best ones and are free). But you need to spend a lot of time in front of your computer, particularly if you have an old one.
But if you can master it, you will be rewarded with Pro-grade deep sky astrophotography images.
A Comprehensive Video About Post-Processing
In this video, I show you how to post-process a deep sky image using some of the software discussed in this article.
While it is not a complete tutorial in post-processing deep sky images, it gives you a feeling of how easy (or not) is to use those software and where they differ.
Conclusions
Stacking astrophotography images is only the first step in the lengthy astrophotography editing process. In this article, we have discussed the different software that is available to post-process the stacked image to obtain a compelling image of the night sky.
Some are free, some are commercial, some are specific to deep sky astrophotography while others are generic photography editors, and they all have their pros and cons.
This guide will help you to decide which software is best for you.
Personally, I am a fan of Astro Pixel Processor for deep sky astrophotography, as it is powerful and easy to use, and of Photoshop for its flexibility.