So you are having fun under the stars taking tons of images of the night sky, and now you need to process them?
Whether you are a beginner or a seasoned astrophotographer, Deep Sky Stacker is a great starting point.
Here is an in-depth tutorial on how to use DSS to calibrate and stack your best deep sky images.
A note of caution though: we didn’t feel like writing a detailed recipe to be followed blindly, because in astrophotography, every image is different. So instead, we will explain in detail the workflow and let you experiment with the setting.
But the best thing about DSS is that the default settings are almost always great.
Note: If you’d like to see a live video tutorial of Deep Sky Stacker in action, then don’t miss the detailed video at the end of this article.
Click here to skip to our walkthrough on using DSS.
What Is Deep Sky Stacker?
Deep Sky Stacker, DSS for short, is a commonly used software for doing image calibration and image stacking of astrophotography images.
Despite being a free software, DSS offers a complete set of advanced settings and methods and is also one of the fastest calibration and stacking software for deep sky imaging.
It can, in fact, make use of multiple processors to speed up the calculations.
And this is why DSS is a must-have.
Operating System Required
Two versions of DSS are available: the 32-bits version, ideal for old computers and Windows tablets, and the 64-bits version, which is your best option when working with modern computers.
Unfortunately, as often the case with freeware astrophotography software, DSS is Windows only.
If you are a Mac or Linux user, your best bet is to use Virtual Box (or VMware or Parallel) and create a Windows virtual machine. The downside is that you need a Windows license.
If you are ok using the 32-bit version, you can use WINE to “compile” a DSS version for Linux and Mac OS X.
Note that Apple has dropped the 32-bit support with the newer 10.15.0 Mac OS X version. If you use Catalina (or a newer version), you will have to wait for the people behind WINE to develop a version to wrap 64-bit Windows software.
What Type Of Astrophotography Is DSS Used For?
As mentioned before, DSS is most used for deep-sky astrophotography, such as star fields, nebulae, and galaxies.
If you want to stack starry landscapes or planetary images, DSS is not the right software for you.
With the No Alignment mode, you can also use DSS to create star trails, both with and without landscape.
Deep Sky Stacker’s Main Features
The main features of DSS are:
- Full image calibration
- Image stacking for deep sky astrophotography images
- Image stacking for star trails (images are not registered, only stacked)
- Image groups
In doing those tasks, DSS offers a plethora of different settings and algorithms to tweak the way images are calibrated and stacked.
With groups, it is easy to stack together images taken on different sessions while calibrating them with the proper set of calibration files.
By being able to photograph a target over multiple nights and to stack together all the images from the different sessions properly is a great way to increase the total integration time and the quality of the final image.
One “feature” I personally like about DSS is that the default and recommended settings work very well 99% of the time, and this makes the software extremely easy to use for beginners while remaining powerful enough to satisfy the needs of the most advanced astrophotographers.
Notable features are the possibility to drizzle 2x or 3x an image (or a part of it) and three different stacking modes for dealing with images of comets.
How To Use Deep Sky Stacker
Deep Sky Stacker File Formats
DSS can use all the most common file formats for images: .bmp, .jpg, .jpeg, .tiff, .raw and .fits.
Interesting is the possibility to directly use RAW and FITS files, which are the most common file formats for still astrophotography images.
The interface is clean and easy to navigate. It consists of 3 main sections:
- The left “shoulder” is a menu bar with options grouped in 3 groups called Registering and Stacking, Processing and Options.
- The main section is displaying the image (either the light frame or the stacked image). This section includes a tools bar and visual stretching and zoom controls.
- The bottom section shows the list and groups of the images loaded in DSS, along with useful information such as settings used to record the images, quality score, etc.
If the stacked image is loaded, from this section, you have access to DSS editing tools.
The “Registering and Stacking” Menu
This menu consists of 3 groups of options:
- The first group allows you to load your light and calibration frames into DSS. Alternatively, you can load a list of images you have saved from a previous use of DSS. You can also save a new list or clear the current list.
- The second group is to batch select the images you want to use among those loaded in DSS. You can select all, or only the images with a quality score above a certain threshold. And you can, of course, uncheck all images.
- In the third and final group, the most useful options are those for registering (aligning) and stacking the images.
Red items in the menu indicate the steps that are required in order to successfully begin registering and stacking the images.
The Processing Menu
This menu allows you to:
- Load a single image in DSS, most likely a stacked one.
- Copy the current image to the clipboard.
- Create a star mask for the current image (useful for further editing in, say, Photoshop).
- Save the current image to file.
The Options Menu
The first option of the menu allows you to tweak the settings for registering and stacking the images.
The Register Parameters allows you to set a threshold for star detection. Since you cannot test the settings in this window, I suggest you to modify the threshold from the Register checked picture… menu.
As per the Stacking Parameters, those are grouped in several tabs.
In the first tab, you can decide which mode to use to combine the images among Standard, Mosaic, and Intersection modes. You can enable 2X or 3X Drizzle and decide whether to align the RGB channels in the final images.
The other tabs are shown in the image below.
As I said earlier, the nice thing with DSS is that default parameters are almost always adequate, and most of the time, you will be changing settings only in the Result and Comet tabs.
Next, you find the settings for Raw/FITS Digital Development Process (DDP)
You may be tempted to play with these settings. For example, I found that using No White Balance Processing gives more natural colors, and so I use that and begin stacking the images.
But then, DSS insisted telling me only one frame was to be stacked. I switched back to Use Camera White Balance and, despite the pink-ish images, I was able to make DSS run till the end.
My advice is to leave everything as it is:
- Use Camera White Balance
- Use Bilinear Interpolation in the Bayer Matrix Transformation
- Check Set the black point to 0
Finally, you can Load and Save your settings, as well as access the Recommended settings for the loaded images.
Since I don’t see how to reset DSS to the original settings and that most of the changes will be remembered even after closing the program, I suggest you to Save the original settings after installing DSS.
This way, if you like to play with the many options, you can revert back to the default settings with ease.
The Visual Stretching
This slider on the top right of the interface performs a visual stretching of the image, to allow you to quickly check what kind of information is hidden in your images.
These changes are only for visualization purposes, and your image remains untouched.
The Tools Bar
The last part of the interface I would like to present is the toolbar that will appear when clicking on an image loaded in DSS.
Here you can draw a custom rectangle by selecting the Custom Rectangle Mode and click-drag with the mouse on a part of the image.
While you draw your custom rectangle, the size in pixels of the selected area is also shown on screen.
Interestingly, the other two rectangles appear on screen, labeled as 3x and 2x.
By matching the size of the custom rectangle with that of one of those two rectangles, you are sure that after drizzling 3x or 2x the custom rectangle, the final image has the same size of the original, uncropped, one.
Another interesting tool is the Edit Comet Mode: use that to select the position of the comet in your light.
DSS cannot detect the comet in your light frames, so you have to mark the comet position in all your images.
You can do it by hand and, if you find you cannot select the comet on an image, hold the shift key while clicking to force DSS to record the position. Also, before you move to the next image, you have to save the comet position using the Floppy Disk icon in the toolbar.
This can be a very tedious task, but, if the timestamp of your images is accurate, you can mark the comet position on the first and last image, as well as the image with the highest quality score (the reference frame).
DSS will then use this info to guesstimate the position of the comet in all the remaining images.
A Basic Step By Step Tutorial
Let’s now see in detail a typical DSS workflow for deep-sky image calibration and stacking.
Step 1 – Organize Your Images
Once you are at home, it is time to organize your images.
During the years, I have refined this process and found that a good practice to order all your data is to create a parent directory whose name is something like date_location_ target_used-gear.
Inside this directory, organize your data and calibration frames in subdirectories named something like Lights, Darks, Flats, Dark_Flats, and Bias.
This way, it will be easy for you to load the images in DSS using the proper image type.
Step 2 – Cull Your Images
Before you load your images in DSS, it is good practice to review them to eliminate those that have obvious problems, such as trailing and jumping stars, planes, etc.
This also allows you to discover any meteor you may have recorded during your session: sometimes, you will be gifted by a spectacular sight.
The process of going through your images is known as culling, and you can use software like Adobe Bridge or any fast image viewer able to read RAW files.
Step 3 – Load Your Images And Calibration files
It is now time to load all your data in DSS. For this, find the Registering and Stacking menu and click on Open Picture Files… to load all your light frames.
To load the calibration files, simply click on the respective type in the menu.
All loaded images will appear in the window at the bottom of the interface, and the different kinds of images will have a different icon next to the name and also a different type.
If you need to load data from a different imaging session, click on the Group tab at the bottom of the interface and load the data in that tab, as just explained.
This way, the lights frames in each group will be calibrated with their own calibration files, before being stacked all together.
If you are loading images that were already analyzed once with DSS, you may see that a quality score is also available.
If you have already created some master calibration files, such as a masterbias (one of the few calibration files you can reuse), you can load them, and DSS will use them to calibrate your light frames.
Once you have loaded all your files, you can click on Check All to select all the files loaded into DSS. You can now move to Registering your images.
Step 4 – Register (And Calibrate) Your Images
To register your image, you have to click on the Menu item Register checkedpictures…
The Register Settings window appears on screen.
Here you can check different options:
- Re-register previously registered images (useful if are experimenting)
- Let DSS automatically detect hot pixels (leave it checked)
- Stack after registering
Before doing anything here, go to the Advanced tab and click on Compute the number of detected stars. DSS will calculate how many stars it can detect in your images.
Next, Move the Star detection threshold slider to the left, to reduce the threshold and detect more stars, or to the right, to increase the detection threshold, thus reducing the number of detected stars.
If you have very few stars, do not hesitate to set a threshold as low as 2%, else, aim to detect a number of stars in between 200 and 300.
Go back to the Actions tab and click OK if you only want to register your images.
IMPORTANT: if you only want to register the images, only the light frames should be selected from the list of loaded images. Selecting calibration files will mess things up later.
If you have already culled your images, you can check the option Stack after registering.
This will automatically stack the registered images. If you are sure you have kept only the best images during culling, you can set Select the best X% percent and stack them to 100%. If you have not culled the images use a lower percentage, say 80%-70%.
Note that images with obvious problems are discarded and will not be considered as images to be stacked.
Before hitting OK, whether you will only register the image or automatically stack them, have a look at the Recommended Settings to see if there are problems with the settings that need to be fixed.
Also, from this window, you can tweak the Stacking Parameters, like for drizzling or use the comet mode.
The Register settings will also show a warning if calibrations files are missed, but you can ignore that and continue.
Step 5 – Stacking Your Images (if not done from the previous step)
If you have only registered your frames, you can stack your images clicking on Stack selected pictures… in the Registering and Stacking Menu.
Before doing so, though, select all your calibration frames.
Also, mind that when you do stack selected images, you will not have to decide the percentage of best images to stack: all the selected images will be stacked.
Since the registering process has graded the images by quality, you can use the option Check above threshold… to select only the images with quality higher than the one you’ll specify.
Review the Stacking Parameters, and when ready, click OK.
Step 6 – Stacked Image Post-Procession And Saving
When DSS has finished stacking the images, it will save in the original directory a file called Autosave.tiff
This is a 32-bit tiff image with no post-processing applied. You can load it in Photoshop, convert it to a 16-bits image using the Exposure and Gamma method.
DSS though, has some post-processing abilities that are meant to quickly check the image: you can align the channels, modify the Luminance curve, and add saturation.
Once you are happy, you can save the image by clicking Save picture to file… in the Processing Menu.
Here chose the TIFF 16-bit format and to embed the adjustments you made to the image. If you reopen the image with DSS, those adjustments will be applied to the image, but any other editor will simply ignore them.
If you are sure you are happy with the adjustments you made in DSS, then you can also choose to apply the adjustments to the saved image.
Finally, before quitting DSS, you can create a star mask to later use it to help editing the stacked image in Photoshop or similar software.
Before quitting the program, you can save your settings or the list of files for future uses.
Quick DSS Troubleshooting Tips
Only One Frame Will Be Stacked
If you see this happen, DSS is probably not able to detect enough stars. By decreasing the Star Detection Threshold, you will probably resolve the problem.
If, despite detecting a suitable number of stars, DSS still complains that only one frame will be used, make sure you didn’t check No White Balance Processing in the Raw/FITS Digital Development Process (DDP). If this is the case, check Use Camera White Balance instead.
Calibration Files Warning
If some calibration files are missing, or their parameters do not match those of the light frames, you will get a warning. These warnings are not fatal errors: you can ignore them and continue as usual.
Remember that not calibrating your light frames (or trying to calibrate them with poor calibration files) can produce images of lower quality.
Sometimes you can get the message that there is not enough space on disk or memory. This is usually the case when you activate the 2x and 3x Drizzle option on the full image.
If the problem is running out of memory, either avoid drizzling your images or use a smaller custom rectangle to drizzle only a smaller part of the original images.
If the problem is not enough space on disk, you can set a different working directory on a disk with more space available.
This can be the case if you are using DSS on a small virtual machine on mac. I usually get that error, so I activate the sharing directory in VirtualBox and use a working directory that is on my mac and not on the virtual machine hard drive.
You can change the working directory in the Stacking Parameters.
Deep Sky Stacker Software Alternatives
DSS is one of the fastest image calibration stackers out there, plus it is free and easy to use.
Nonetheless, you may want to have a look around.
Sequator is another beginner-friendly stacker that, while more limited than DSS, is still worth considering: it is free and can be used to stack both starry landscape images and deep-sky images. Plus star trails.
Starry Sky Stacker is a simple deep sky image stacker for us Mac users. While it is user friendly, it is not free and is quite basic.
If you want to discover more alternatives to DSS, you can read our review about the most common software for image stacking for astrophotography.
If you prefer watching tutorials rather than reading long articles, I made this video for you.
In this article, we have got to know in detail how to use DSS for calibrating and stacking deep sky images.
It is well worth the effort to get to learn it as it is free and one of the easiest software for astrophotography that is suitable for beginners and seasoned astrophotographers.
Here is the finished image from the data used to illustrate the tutorial: the California Nebula wide field.