Image stacking is the starting point in the editing workflow of any of your deep space images. One photo stacking software you can use is the Deep Sky Stacker, also known as DSS.
With this Deep Sky Stacker tutorial, you will be using photo stacking to obtain a final image. This will have more details and less noise than the initial photos, which is what we’re after.
Deep Sky Stacker is a free software that makes it easy to pre-process deep sky astrophotography. You can download it here.
DSS is available for Windows. The most recent version is 64-bit only. But you can still download older 32-bit versions from the official website.
DSS is easy to use and the interface is simple and intuitive.
In the first section you can find the controls to Register and Stack your light frames. This means you can load all your images and calibration files.
Then you tell DSS to calibrate, align and stack your images. And you will end up with a single image.
After the process is complete, DSS will save the output image as a 32-bit .tif file called Autosave.tif. You can find it in your image directory.
The resulting image is automatically loaded in the Processing section.
The Processing section allows you to load an image or to work with the current one. Here you can balance the channel, change luminosity and increase saturation.
You can also create a star mask and save your work as a .tif file.
In the Option section you can tweak the register and staking settings. You can see the different menus for these settings in the images below.
The icon splits your image view in four panels. These show parts of the image zoomed in.
The slider will brighten the image so that details hidden in the dark will become visible.
When you select an image from the images list, Deep Sky Stacker will show it in this section. A small menu will also appear. You can further manipulate the image here. The most important option is comet mode.
Comet mode allows you to select the position of a comet in the image. More about this later.
The image list shows all the loaded images, as well as calibration frames. If the images have already been registered and graded by quality, this info appears next to the names.
Here you can also check the files you want to work with.
The group tab allows you to load images from different sessions, e.g. if you are imaging on several nights. This allows you to calibrate each set with the proper calibration files. Remember, for example, that darks are taken in the field and are temperature dependent.
DSS will then combine all the sets into a final image.
As said before, DSS is Windows only. But there are two ways to use it on a computer running Mac OS or Linux:
Let’s go through a practical example. I’ll use the data I took to photograph comet 21P Giacobini Zinner. This way you’ll see how all the DSS functions work together. And what the classic workflow for this program looks like.
Before loading your RAW files into DSS, go to RAW/Fits DPP settings… and be sure to check the option “Camera White Balance” and uncheck “Set the black point”.
Most of the stars out there are yellow, like our Sun. Your camera white balance should be set to Sun.
In the Registering and Stacking section, you should click on Open picture files… and select all your light frames. If you have any, this is the time to load the calibration frames. These include Darks, Flats, Dark Flats and Offset/Bias files.
If this terminology is confusing and you don’t know what calibration frames are, check our Useful Glossary Of Astrophotography Terms.
Once you load your images and calibration frames into DSS, their file name and EXIF data will be in the previously empty image list.
You might have data from different sessions that you want to combine. Load them in different groups, one for each data set.
If you want you can save the file list, in case you should start over. Click on the Check all… link: this will select all loaded images.
This step is only if you have a comet in your image. If this is not the case, you can jump to the next step.
This is the most tedious part. You have to manually specify the position of the comet in all your images by clicking on it. To do this, start by selecting the first image in your list.
Then click on the green comet icon in the small menu that appears on the right side of DSS windows.
When you run the mouse on the image, you’ll see a small zoomed in view. This will appear on the left of the image. Use that to place your cursor on the comet nucleus.
If you cannot see the comet, move the middle cursor in the Visual Stretch bar to the left. This will brighten the image.
It is now time to register, aka align, and stack your images. To do so, click on Register checked pictures… link in the left menu. The window shown below will pop out.
After the stacking is complete, Deep Sky Stacker will save the resulting image. After that DSS will enter in Processing mode, showing you the final image and a number of ways to tweak it.
Most people don’t like to process the stacked image in DSS. They prefer to do everything using more powerful software such as Pixinsight or even Photoshop.
You may be struck by the darkness of the final image and lack of color, but this is rather normal. Play with the parameters and try to tweak the image.
Below the image you have a set of tabs and a chart showing a global luminance curve and the histograms for the different channels.
Next, I jump to the first tab. You can move the histograms of the individual channel left or right on the graph.
Try to align the histogram peaks somewhere near the point where the S curve starts rising steeply, as in the image below.
This should brighten the image quite a bit and details will start to be visible.
Finally I add some saturation (third tab) and apply these changes to the image.
When happy, click on the link Save picture to file… in the Processing section of the left menu and save the image as .tif 16-bit.
Be sure to check the box to apply the modifications to the image, as opposed to embedding but not applying them.
Deep Sky Stacker is a rather easy to use but powerful free software. You can use it to pre-process your astronomy images. This short tutorial showed you how to start working with DSS. But it is far from a complete description of what Deep Sky Stacker can do.
The manual is a good place to go looking for specific info, and don’t be afraid to experiment. Try different settings and see how your photos turn out!
We have a great post on astrophotography software resources to check out next!
