While shooting astronomical objects lies at the heart of astrophotography, it is only half the job done. Post-processing is the step where the real magic happens. The faint details captured in your RAW image come to life during post-processing. While there are certain technical and scientific aspects to post-processing astro-images, one can’t ignore the artistic eye required to transform your images.
There are many misconceptions about post-processing in astrophotography. People, more often than you can imagine, will tag these images as “edited” or “photoshopped”. While there is no denying that many of these images are processed in Photoshop, it is not what people would have meant. During post-processing, generally speaking, nothing is “added” to the captured data. Your raw images undergo “changes” and “enhancement”. In this guide, we cover the basics of astrophotography post-processing.
Why post-processing is required
Digital cameras see the night sky differently than human eyes. While the naked eye does not perceive many details, digital camera sensors are sensitive to those lights. Many details remain faintly recorded during the shooting process and/or hidden behind digital noise. Post-processing is essential to bring out those faint details. Here are a few important aspects of post-processing in astrophotography:
- Enhance details: By stacking multiple images and by digitally enhancing the image, the faint details of the Milky Way or deep-sky objects are brought out.
- Reduce noise: Due to the low-light situations at night, a lot of digital noise is introduced to your images, even at low ISO. By stacking images and applying noise reduction, images are made “cleaner”.
- Colour correction: Colour tones are balanced for a natural or artistic rendition of the image.
- Contrast boost: In order to separate landscape elements from the sky or to focus on certain areas of an image, contrast is added during post-processing.
- Image sharpening: To make the stars, nebulae, and galaxies pop out, they are sharpened during post-processing.
Pre-processing: Steps before processing your image
The term pre-processing refers to the stage where you prepare your RAW images for the post-processing step. Here are some of the steps you can follow to prepare your images.
- Import all the images to your PC/laptop from the camera’s memory card.
- Create separate folders for each target. Doing it on the morning next to the session helps a lot. Rename those folders with the name of the target and date of capture (e.g. Andromeda galaxy 30.10.2024).
- Create separate folders for any calibration frames you have captured. Within the main target folder, you can create separate folders such as Light Frames, Dark Frames, Bias Frames, etc.
- Go through your images. Remove any test shots or blurry shots. You do not want to include these during stacking.
- Create a copy of the RAW data if you have the option.
- For solar and lunar images captured with telephoto lenses, you will need to crop and align the images before stacking. Cropping is required to decrease the file size (and get rid of unnecessary areas), hence decreasing the load on your computer. Aligning the images helps the stacking software to do a better job. Run your images through PIPP, a software that does this cropping and alignment job for you. Remember to save the files in an uncompressed format (e.g. TIFF).
Stacking your images
During shooting the night sky, if you have captured multiple images of the same object, the first step of post-processing will be to stack these images into a single image. However, this does not apply to single images. There are a number of software available to stack astronomical images. Here is an overview of some of them:
Wide-field Astrophotography
If you are into wide-field astrophotography, including nightscapes and deepscapes, Sequator will be your choice for stacking the images. This free software allows you to stack multiple Milky Way (and other wide-field) images and produce a single stacked image. The software has many inbuilt features, including Reduce Distortion, Reduce Light Pollution, Enhance Star Light, etc. It also allows you to mask the foreground area (if your image has both sky and foreground region) and keep the foreground sharp while aligning and stacking the sky portion. It can also help you create a star-trail image. Unfortunately, this software is available only on Windows computers. For Mac users, Starry Landscape Stacker is a paid alternative. StarStax is an alternative for creating star trail images.
Deep Sky Astrophotography
This is the most basic and useful software for stacking deep sky images. Apart from stacking your images, you can also load your calibration frames and DSS will calibrate your final stacked image. The interface is easy to look at and to use. If you have photographed a comet, DSS allows you to align and stack the comet images. You can save your images in TIFF format. ASTAP is a stacking software similar to DeepSkyStacker.
Planetary, Solar & Lunar Astrophotography
For stacking planetary, solar and lunar images, Autostakkert is considered to be one of the best software. After running your images through PIPP (for preprocessing, an optional step), Autostakkert aligns and stacks your solar and lunar images. You can also load your calibration frames here, and it will calibrate your images. Astrosurface is an alternative.
While there are many advanced stacking software, including Pixinsight and Siril, the ones listed above are more recommended for beginners.
Post-processing software for astrophotography
Post-processing your astro-images is a mix of science and art. It is during post-processing that you add your artistic touch to the image, making the image “yours”. While there are a few options available in terms of the available software to post-process your images, Pixinsight and Photoshop are two of the most versatile and widely used ones.
Pixinsight
Pixinsight is a dedicated astrophotography post-processing software. While you can’t process other daytime images here, for astrophotographers, it is the Holy Grail. It offers you all the required tools to transform your images into a masterpiece. Starting from stacking your images, you can process and enhance your images with Pixinsight in many ways. Here are some of the most common tasks carried out in Pixinsight:
- Image stacking (via Star Alignment and Image Integration or via WBPP)
- Gradient Removal (via ABE, DBE or Multiscale Gradient Correction)
- Multi-channel combination (via Channel Combination)
- Colour calibration (via Spectrophotometric Colour Calibration and more)
- Image Stretching (via Histogram Transformation, Masked Stretch and many more)
- Luminance noise reduction (via ACDNR and more)
- Chrominance noise reduction (via SCNR, TGV Denoise and more)
- Sharpening (via Deconvolution and many more)
- Image and colour enhancement (via Curves Transformation, Colour Saturation and many more)
- Annotate image
Pixinsight offers you an all-in-one solution for processing your astrophotography images. While the software is most suited for processing deep-sky images, it is also very popular among wide-field astrophotographers for processing the sky portion of the image. Like Photoshop, it has a huge selection of plug-ins (both free and paid) that will make your life easier.
The interface of Pixinsight with its long list of tools may look intimidating at first; you will soon get used to with it once you develop a workflow.
Siril and Astro Pixel Processor are two alternatives to Pixinsight.
Photoshop
Photoshop is another versatile software that is used widely to process astro-images. Although the tools inside Photoshop are not made specifically for astrophotography, they can be used to process and enhance your images. Here are some of the tasks that can be carried out in Photoshop:
- Image stacking (by manually aligning the images, converting them to Smart Objects and stacking them in Mean/Median mode)
- Channel combination (via channels)
- Colour calibration (via levels, curves, and many other options)
- Image stretching (via Levels and Curves adjustment)
- Noise reduction (via many noise reduction tools available)
- Sharpening (via Unsharp Mask, High Pass etc.)
- Colour adjustment and image enhancement
Photoshop is mostly suited for wide-field astrophotography, but it can be used in combination with Pixinsight to add the final touches to deep-sky images. It can also be used as a standalone software to process deep-sky, lunar and solar images (after stacking). There are many plug-ins available for Photoshop that are dedicated to astrophotography, including AstroPanel, StarSpikes Pro, GradientXterminator, and many more.
Alternatives to Photoshop are GIMP, Affinity Photo, Luminar etc.
Other software and tools
There are a few other astrophotography software (or tools) available that can be used to enhance your images further.
GraXpert: Graxpert is free software that can be used either as a standalone software or it can be added within Pixinsight as a tool. In essence, it is a one-click solution to remove gradients from your images. The AI-model helps you get rid of the colour cast from your astro-image and provides a clean output. The tool can also be used for noise reduction.
Starnet: Starnet is another extremely useful tool that removes stars from your image. This star removal process is necessary as it gives you the option to enhance the background nebulae or galaxies without enlarging the stars. Astrophotographers employ this method to keep the stars smaller while making the background details brighter. The stars can be added back to your image at a later stage. Starnet is available for free as standalone software, or it can be integrated into Pixinsight.
Tips for processing your image
Post-processing your astro-images can get challenging and tricky at times. However, with some experiments and practice, you will gradually develop a workflow to suit your taste. Here are some tips that will help you achieve a better output:
- Do not over-process your images. In the beginning, you might feel the urge to boost the colours or details of your image, but these often result in an unnatural-looking image.
- Sharpen your images carefully. Over- or under-sharpening is common and should be avoided.
- Carefully look at images from other astrophotographers. Take note of what you like about the image and what you don’t. While processing your own image, implement those.
- Do not rush through your processing. It is okay to take your time (unless you have a deadline). Once you are done with the processing, let the image sit for a day. Processing for hours will inevitably make your eyes tired, and it is easy to overlook any mistake. Come back to your image later.
- Throughout the processing period, save your files in uncompressed formats like TIFF, FITS, PSD (for Photoshop), XISF (for Pixinisght), etc.
- When you are done with your processing, you can export the file as a JPG or PNG for sharing across different platforms or for printing. However, it is always a good idea to save a copy of the complete project. In case you need to go back to your image and reprocess it, these projects will give you a starting point, and you won’t have to re-do the whole process.
- Do not crop your images at the beginning unless you are completely sure about it. Cropping should be done at a later stage.
- As the processing software develops every now and then and new tools are made available, post-processing will remain a life-long learning process.
- Be flexible and experimental with your processing methods and workflow.
- The learning curve might seem steep at the beginning. With a little bit of patience, you will get there.
In astrophotography, post-processing is as important as the capturing part. This is where the image becomes subjective. Each photographer will have their own taste and method of processing images, and this is what makes astrophotography so interesting. You will come across thousands of photographers photographing the same object, e.g. Andromeda galaxy. But each image will be different from the other one. Post-processing can make or break an image. All you need is patience, practice and creativity to transform the RAW images into a creative masterpiece.
Clear skies!