Deep Sky Stacker Settings for Astrophotography
I have been using Deep Sky Stacker to get the most out of my astrophotography images since I began shooting through a telescope in 2011. This useful and easy-to-use freeware tool simplifies the pre-processing steps of creating a beautiful deep sky image.
The concept of stacking in astrophotography is simple, by combining multiple images together, the signal to noise ratio can improve.
With so much time and effort going into your capturing sessions outside, the least you can do is give yourself the best results possible when stacking. In this post, I will explain the Deep Sky Stacker settings I use to stack and register all of my astrophotography images.
If you haven’t already done so, download Deep Sky Stacker for free.
Deep Sky Stacker
Over the past 5 years, I’ve stacked images created by both a DSLR and a CCD Camera. Whether you are stacking .RAW image files from my Canoa DSLR, or .FIT files from a CCD camera, the right settings can be the difference between a good image, and a great one.
For many amateur astrophotographers, Deep Sky Stacker is an integral part of their image processing workflow. For myself, I find that Deep Sky Stacker does an exceptional job of registering astrophotography images taken using a variety of methods. This includes everything from untracked DSLR and camera lens shots to deep sky astrophotography through a telescope.
DSS (DeepSkyStacker) can register images of everything from a wide angle Milky Way panorama to a deep sky emission nebula. Most of my experience with this software has been on a Windows 10 PC, stacking Canon RAW files from a DSLR. To run Deep Sky Stacker on a Mac computer, a workaround such as using a virtual machine is necessary.
Let’s take a look at the main features of this software:
- Registration of picture sets
- Creation and use of offsets, flats and darks frames
- Native use of RAW files from most DSLR
- Mulitple Stacking methods including average, median, kappa-sigma clipping and more
- Preview of all pictures including RAW and FIT file types
- Simple and intuitive user interface
DSS offers some advanced features I have not yet put into practice myself, such as comet stacking. The steps outlined on this page are most useful for beginners using a DSLR camera to capture their images. The official website offers some great resources for understanding how the process works.
All of the images I register and stack using DSS are then brought into Adobe Photoshop for final image processing. The image below shows a stacked image before and after processing in Photoshop.
Tutorial (Deep Sky Images)
I’ve had most of my success using trial and error. For example, I was able to produce an image with the correct color balance using a CCD camera with an RGGB Bayer pattern. I discovered this during my Markarian’s Chain imaging session, by using a specific color adjustment setting.
Here’s the good news, generally the default settings work best.
I have experimented with many different combinations of options for stacking DSLR raw files, and have found that most of the default settings work best. There are a few things to keep in mind, but I have introduced more problems to my images than enhancements by changing the settings.
Stacking images from multiple nights
I regularly capture images on the same deep-sky object over multiple nights to increase the signal-to-noise ratio. I shoot through heavy light pollution in my backyard, which means I need to capture up to 4x or more the amount of exposure time someone living under dark skies would. Jerry Lodriguss has explained the process of compensating for brighter skies with added exposure time on Sky and Telescope.
All of this exposure time is great, but how do you properly combine it all?
Below: The Rosette Nebula processed using Deep Sky Stacker and Adobe Photoshop
File Preparation Before Stacking
If you follow my astrophotography tutorials, you will have captured light frames, dark frames, flat frames and offset/bias frames during each of your imaging sessions. These support files will go a long way towards improving your final image. I recommend capturing new support files for each night, rather than using previously captured data.
Only stack your best images
Before opening the files in Deep Sky Stacker, I pre-qualify the images I want to stack. I use a RAW image preview application called Adobe Bridge to review and organize my images. Any photos with football-shaped stars from hiccups in autoguiding are tossed in the recycling bin. The same goes for frames with airplanes, satellites or passing clouds.
What about .FIT files created be a CCD camera? These files can be hard to preview, due to the fact that they need to be debayered first. For this file type, I inspect and remove poor quality frames within DeepSkyStacker itself. This method can be a bit tedious, but a necessary step to ensure your final image only includes the best data.
In the DeepSkyStacker 3.3.4 settings, you are able to choose “Select the best 80% pictures and stack them“. I do not rely on this system to separate my good image frames from the bad. I manually pre-approve all of the images (light frames) using a RAW image previewer before loading them in.
Keep Your Image Sets Organized
Organize your images into 4 folders. Lights, Darks, Flats and Offset/Bias.
In the Main Group:
Open Picture Files
Select all of your light frames from your first night of imaging. Since you have already reviewed and approved all of the images in this folder, this is simply a matter of selecting every RAW file in your light frame folder.
Select the dark frames you captured from the same imaging session. The images need to be the same exposure length, ISO and temperature as your light frames. These can be easily captured with the lens cap on your camera. I recommend using a minimum of 15 dark files or more.
Here’s an example of the Andromeda Galaxy, stacked using only Dark frames. This image would have been much easier to process had I taken the time to collect flat and bias frames.
Flats require a little more effort than dark frames but can be collected in a very short amount of time. Stretch a white t-shirt over the objective of your telescope, and smooth out all of the folds. Shine an evenly lit bright light at the telescope objective, and capture a number of shots with your DSLR set to AV mode. 15 flat files can make a significant improvement to your final image.
Offset/bias files are quick and simple to capture. Just take 15 exposures with the lens cap on your DLSR. These exposures need to be the fastest possible shutter speed using the same ISO as your light frames. (On the Canon 450D, that’s a 1/4000 second exposure)
How to combine images from Multiple Nights
Use the tabs to group your image sets
Once you’ve got your picture files (lights) and all of your support files loaded into the main group, it’s time to load up your files from night 2. Click on the small Group 1 tab at the bottom left of the screen, and repeat the process for opening files from imaging night 2.
Remember, you can stack different variations of exposures together in Deep Sky Stacker. This means a range of ISO sensitivity and exposure length.
Some imaging sessions may include all 3 supports files to compliment the light frames, some may not. This is fine. After all of the image files have been loaded into their respective categories, it is time to register and stack the frames into a single file. Finally, make sure to click “‘check all“, to make sure that all of the frames you have loaded are selected.
Before we click Register and Stack images, let’s take a look at the current default settings.
Accessing the Register and Stacking settings is accessible by clicking “Settings…” under the options tab.
The default settings for registering is set to a 10% star detection threshold. In my experience, the default value of 10% has worked very well for stacking images captured using my 12MP Canon 450D. If you decrease the star detection threshold, deep sky stacker will detect fainter stars. The number of stars in a given light frame is displayed in the lower half of the screen. With a light frame selected, look for the #Stars category.
The following checkboxes should be checked before moving hitting “OK”, and letting DSS begin its process.
- Register already registered pictures
- Automatic detection of hot pixels
- Stack after registering
The Deep Sky Stacker website states that the automatic detection of hot pixels only works if using Super-pixel, Bayer Drizzle, bilinear and AHD interpolation modes. However, I leave this box checked regardless and hot-pixels and stacking errors have never been an issue.
Unless you are experiencing errors in the stacking process, leave all of the values in the stacking parameter dialogue box unchanged. Yes, this sounds like a conveniently simple option, but default values are usually set for a reason! If you want, go ahead and click on the different modes in the “Result” tab. The program will show you a preview of the final composure created using Mosiac and Intersection modes.
I prefer to use Adobe Photoshop for the final framing and cropping of the image.
As for the stacking parameters of the light and dark frames, Kappa-Sigma clipping and Median work well in the Light, Dark, Flat and Bias/Offset categories. I do not use any additional features such as the detection and cleaning of hot pixels in the Cosmetic tab.
One setting I do change, however, is the output location folder of the Autosave.tif file. I prefer that these images populate in a specific folder of my choice rather than mixed in with a folder of light frames.
Depending on the quality of and amount of light frames available, I usually select the best 80-90% of pictures and stack them.
Ready to Stack?
You’ve got all of your lights, darks, flats and offset/bias frames loaded. The default settings are currently selected, and the ever-comforting green bar is displayed (confirming your use of all support files) But wait, if only there was a way to confirm all of the files are as they should be.
The Stacking Steps Window
Before you run DSS, be sure to check and see if there are any warnings in the dialogue window. In the case above, there was a single Flat frame with a miss-matched ISO speed. These warnings are useful for catching little mistakes in your file organization that can potentially make a big impact on your image.
At this point, you can remove or add any frames based on the information that DSS has provided.
If all looks well, and there are no more warning messages in the Stacking Steps window, you can proceed to run the register and stacking process. I enjoy the information preview about the estimated total exposure time.
Deep Sky Stacker Tutorial (Video)
In the video tutorial below, I walk through some of the basic settings used in Deep Sky Stacker. I then bring the image into Adobe Photoshop for further image processing.
When DSS has completed its process of registering and stacking all of the image frames together, a preview of the constructed Autosave.tif file is displayed onscreen. Based on the design of this software, you would think that the next logical step would be to make adjustments in the RGB/K Levels, Luminance, and Saturation area.
If you plan on processing your image in Adobe Photoshop, I recommend leaving these settings as they are.
Balancing levels, curve adjustments, and boosting saturation are all staples of an Astrophotography processing workflow in Adobe Photoshop. Photoshop offers many more options and a higher level of control than Deep Sky Stacker for such edits.
What about the Recommended Settings option?
Deep Sky Stacker has a “Recommended Settings” option that offers suggestions based on the image files submitted. Some of the recommendations include changing the stacking mode used such as “Use Median Combination Method”.
I have tested both the recommended settings and the default settings and found the default to produce better results.
If you are determined to see the subtle differences in the final stacked image, you can go through the entire process using the default Deep Sky Stacker settings vs. the recommended settings. I found that the recommended settings had varying results, with fuzzier more washed-out stars than the original stack. I prefer to try both stacking methods and compare the results on a per-image basis. You may find that the stacking modes suggested by DSS improve your image.
Below: The Omega Nebula stacked in Deep Sky Stacker. Final processing in Photoshop.
Stacking wide-angle camera lens images
Although I mostly use DSS for deep sky images, it is also very useful to stack wide angle astrophotos through a camera lens as well. The same signal-to-noise benefits can be achieved by stacking multiple images together.
Here is an example of a wide angle shot through a Canon 17-40mm F/4L Lens:
Recommended Settings and Tips
For my wide-angle shots, I use a modified Canon DLSR with a light pollution filter. The settings I recommend below will work well for a modified DSLR shooting through moderate to heavy pollution. Those shooting with a stock DSLR may have to experiment with these settings to produce a pleasing result.
White Balance Settings
If you are using a modded DSLR, make sure to leave the white balance checkboxes unchecked. Using an auto white balance or the camera white balance with a modified camera will produce odd results. I would also suggest checking off the “set the black point to 0″ option.
This should provide you with a final image with a background sky that is much easier to correct in post processing. Gradient Xterminator does a great job at correcting gradients in wide-angle shots of the night sky.
As for DeepSkyStacker’s recommended settings, the graphic below shows you which ones I like to use on a wide-angle starry sky photo. One of the important settings is to use Per Channel background calibration – as the RGB background calibration does a poor job of producing correct colors in my experience.
Have some tips to share about DeepSkyStacker? Let me know on Twitter – and I will get back to you!
I hope you were able to learn something new about Deep Sky Stacker following my tutorial. It’s one of the few applications that hasn’t changed since I began using it in 2011, and it continues to deliver consistent results.
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