In astrophotography, we take multiple long exposures to capture faint details in nebulae and galaxies. DeepSkyStacker can take those long exposures and stack them on top of each other, improving our signal to noise ratio.
It will also take our calibration frames and subtract them from our light frames, giving us a cleaner image, and again improving our signal to noise ratio, all at the press of a few buttons. In this tutorial I will walk you through the process of using DeepSkyStacker, so that you can reap the benefits of a stacked image.
Video Guide
Below is my Youtube video that will show you the process of how to stack deep sky astrophotography images using DeepSkyStacker. I also explain what each type of calibration frame is and why they’re so important for the stacking process.
Full Step-by-Step Guide
Step 1 – First you will need to download DeepSkyStacker. It’s a completely free download for Windows and you can download it here.
Step 2 – Locate your light frames (image files) by either clicking on Open picture files… and browsing to them on your PC. Or you can simply drag and drop the files into DeepSkyStacker if you already have the folder open. Note: it’s a good idea to check your image files before adding them in to DeepSkyStacker so that you can check that none of them were ruined, for example by a wind gust.
Step 3 – Now add your calibration frames by clicking on the respective wording for each type. Note: if you’re using bias frames then you do not need to use dark flats. See my video explaining calibration frames for more detail here.
Step 4 – Ensure all of your image files are selected by clicking on the Check all button.
Step 5 – Before registering your images, it’s a good idea to ensure that all your image files are the same settings (ISO, exposure length etc). If you’re using images from multiple night sessions, it’s a good idea to add them into separate groups. You can do that by adding your first night’s files, and then clicking on the Group 1 tab at the bottom of the screen.
Step 6 – Now we’re ready to register our images. Registering them before stacking will give them a score, meaning that DeepSkyStacker will only stack the best frames, based on the % that you select to stack. 85% is the default but I tend to use 90-95%. Select Register checked pictures.
Step 7 – Now we’re going to amend some settings to get the best output from DeepSkyStacker. First, click on the Stacking Parameters button. There are lots of settings here that can be dependent upon what you’re trying to achieve. For this tutorial I am going to assume you’re trying to stack images of one deep sky object and not trying to create a big mosaic.
Settings for Results tab:
- Standard mode
- Use all available processors (this will speed up the stacking process)
Settings for Light tab:
- Kappa-sigma clipping (Kappa 2.00, number of iterations 5). I find this option works best for me but your results may vary. Kappa-sigma clipping will remove satellite/plane trails from your light images as it stacks them, meaning you can still use that image data.
Settings for Alignment tab:
- Automatic – This will align your images automatically. If you carried out a meridian flip during your imaging session and have images with a different rotation, DeepSkyStacker will flip these for you as it aligns them.
Settings for Intermediate Files tab:
- TIFF files – TIFFs can be opened by any post-processing software such as Adobe Photoshop
Settings for Cosmetic tab:
- Untick all boxes
Settings for Output tab:
- Create output file
- Create output file in the folder of the reference frame (unless you want to save it somewhere else)
Now select OK
Step 8 – Click on the Advanced tab and ensure the box is ticked called Reduce the noise by using a Median filter. We also want to detect the number of stars here so click on Compute the number of detected stars. This will use one of your light frames to count the number of stars. You may need to adjust the slider to change the % based on the number it detects. The higher the number, the more processor intensive it’s going to be. Anywhere between 200-450 is fine.
Step 9 – Click on the Actions tab. Tick the boxes Automatic detection of hot pixels and Stack after registering. Now select the percentage of images you want to stack and click OK.
Step 10 – Now we will be shown the Stacking Steps window. Click on the Recommended Settings button. DeepSkyStacker will make recommendations to us based on the files we’ve added. These settings will vary between each user, and is dependent upon the images you have added. Therefore my recommendation is to select all the options that DeepSkyStacker is recommending. For reference, my settings are:
- As I am using long exposure and good SNR images: Use AHD debayering
- As I am using a modified DSLR: Reset all white balance settings
- As I am using bias frames: Set the black point to 0 to improve calibration
- Light frames stacking: Sigma-clipping combination method
- Creating a Bias frames master: Sigma-clipping combination method
- Creating a Flat frames master: Sigma-clipping combination method
- If the resulting images look to grey: Use per-channel background calibration
Now select OK twice to begin registering and stacking.
Step 11 – Once DeepSkyStacker has stacked all your images, it will display a stretched image on your screen. Ignore the quality of this image, DeepSkyStacker makes a very rough adjustment and it often results in an ugly image. At this point you can now close DeepSkyStacker, ensuring you select No to it saving the changes it has made to the Autosave file. This ensures that you’re saving the stacked TIFF file as the output without the extra adjustments DeepSkyStacker made when it displayed it on the screen.
Now the fun part of post-processing begins. I have a number of tutorials on YouTube to get you started, but if you’re a complete beginner, why not check out my Astrophotography processing in 10 minutes video here.
