Last updated: April 9th, 2023
Astrophotography can be a challenging hobby. That’s why good image processing is vital to take your astro images to the next level. There are several pieces of software that you can use to take your images from linear (raw data with no changes) to a mind-blowing finished product. Some software are free (like GIMP), while others cost a onetime fee (PixInsight), or a subscription (like Photoshop). You can find a listing of other software options on our “Software & Apps” page. However, for now we will focus on PixInsight and Photoshop. Below you will find a basic breakdown on processing or if you have experience jump straight to our “Processing with PixInsight” or “Processing with Photoshop” pages.”
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Quick Jump
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Coming Soon!
Processing Basics
Processing is a challenging but rewarding part of astrophotography. As the hobby evolves, we constantly learn new skills and methods. To give you a simple overview of a processing workflow, we’ve included some of the basic but key steps below. Please note that steps within a workflow will vary depending on the needs of your image, and the software you use. Each piece of software has its own way of accomplishing the same tasks. For each astro image, there are many possible ways to process it and unlimited final results.
Stacking
Stacking refers to taking multiple exposures and using software to calibrate, align, and then stack them into a single image. Once you have your single image, it can be processed further. The more exposures on a target reduces noise and increases signal. Programs like DeepSkyStacker, PixInsight, Siril, or Sequator are just a few that can be used to stack.
Channel Combination (if applicable)
If you are working with a monochrome camera, you will need combine each of your channels. Whether you are shooting SII, Ha, OIII or luminance, red, green, and blue (to name a few combinations) into one single image for further processing. You can do this before or after stretching depending on what works best for you. Moreover, if you are using PixInsight, you need to decide whether to apply DBE (Dynamic Background Extraction) or ABE (Automatic Background Extraction) on each channel individually, to the combined image only, or to both. This decision will depend on your data and how much time you want to spend processing. For more details on how to combine channels, check out our Processing with PixInsight page or our Processing with Photoshop page (coming soon).
Stretching
Before we go into what stretching is, it is important to know the basics of a histogram. A histogram is a graph that shows pixels values of an image from black to white (brightness values). A pixel is a tiny dot that makes up an image, and its value determines how bright or dark it is. On a histogram, these values range from 0 (black) to 255 (white).
Stretching refers to taking a RAW image (linear) and manipulating the histogram to expose the signal (or increase the contrast). When you first take a look at an image in RAW format on your computer, it will typical not look like much is there. By stretching your image will begin to lighten your picture and show all the details that you’ve picked up. There are various tools such as “levels”, “curves”, “auto-stretch”, etc. depending on the software you are using that can accomplish a stretch on your image.
Cropping
When stacking multiple exposures, it is normal to have “edge” artifacts on your image as each exposure you have taken does not match up perfectly. To fix these artifacts, you need to crop around the edges to trim them off. Additionally, cropping is useful to change the composition of your image. Cropping tools can be found in most image editing software and apps.
Background Correcting
It is completely normal in this hobby to face challenges such as light pollution, camera imperfections, chromatic aberrations, flat-field errors, etc. When processing your images, you will need to recognize and correct any gradients, vignetting, or color imbalances in your image background sky. Before moving on to color balancing the rest of your image, you will need to ensure that your background sky has a neutral and flat color. You can use a process like DynamicBackgroundExtraction (PixInsight), Lens Correction (Photoshop), or the Gradient tool (Photoshop) to remove gradients or vignetting. Background Neutralization (PixInsight) and using masks and layers in Photoshop can be helpful correcting the color balance of your background. There are also many scripts, manual methods, add-ons, and tools within various programs that can help you with background neutralization
*It should be noted that quality calibration frames (darks, flats, dark flats, and bias) can significantly improve the quality of your data.
Color Balancing
When you process your astrophotos, you may need to adjust the color balance of your images. Color balance refers to the relative amounts of red, green, and blue colors in your image, which affect how natural or realistic it looks. Sometimes, your image may have a dominant color that makes it look too red or green, for example. This can happen for various reasons, such as: filters, light pollution, and the type of camera you are using. You can correct any imbalances of color by manually adjusting the curves or levels to individually modify the red, green, and blue channels in Photoshop. Additionally, you can play with the white balance tool to adjust your colors with the “Color Balance” icon in Photoshop. For PixInsight, there are several tools for color calibration, such as: PhotometricColorCalibration, PixelMath, ColorCalibration, AssistedColorCalibration, and SpectrophotometricColorCalibration. These are only a few of many examples of how to color balance.
Noise Reduction
Noise in photography can come from several things and a lot of what you do when imaging can increase or decrease noise. For example, Increasing the amount of exposures you take will reduce noise while strengthening the signal and dark calibration frames can be helpful. For the purpose of this intro to processing, we will focus on reducing the noise in processing. When taking images in the dark, you will encounter noise in your astro images. As you stretch and push your data, you will likely increase or highlight that “noise” in your photo. Noise typically will create a “grainy” look in your image and while reducing noise is an important step in any workflow. There are many programs that have noise reduction features like Photoshop, GIMP, Siril, PixInsight, Star Tools, Affinity Photo, and Sequator.
Final Touches
This is where you can add saturation, vibrance, sharpening, etc. to taste. These adjustments can bring out mineral colors in your lunar images or make the colors pop in your nebulae photos.
As you practice processing more, you will improve your skills and develop your own workflow. Usually, it is a blend of various techniques that you learn over time and from other astrophotographers in the community. Different techniques are required for different targets and dependent on the equipment you used. Experimenting is encouraged because there are multiple paths to choose from to create an image you are happy with.
