Create a stacked image of our Galaxy
Use image-stacking software to enhance detail in different areas of a Milky Way image
An essential tool for astrophotography, image stacking – or using software to effectively lay multiple exposures on top of one another – allows you to build up a much longer total exposure time and to reveal more detail in your image, something that is particularly helpful if you don't have a tracking mount and need to take short exposures. Stacking also increases the ‘signal to noise ratio’ – the ratio between the light you want to capture (the signal) and unwanted artefacts (noise) – which produces a smoother image.
To do the stacking, we'll be using a piece of free Windows software called Sequator (https://sites. google.com/view/sequator) that can stack your night-sky images quickly and includes clearly explained instructions. It can be used to stack any images that contain stars, but is ideal for wide-field images as well as for creating star trails. Here we're going to look at how to use Sequator to stack images of the Milky Way.
Before we get started, however, there's one key feature of Sequator that should be flagged up: it allows you to select a specific area to stack from an image, as well as any areas you want to leave untouched. For example, if you are stacking images with an area of interest in the foreground, you may find that the stars are moving relative to the foreground from one exposure to the other. But you want to avoid the foreground becoming blurry when Sequator stacks the images, which will happen when it rotates each frame slightly to fix the stars in the image.
To deal with this, normally you would need to blend one of the individual exposures in Photoshop or similar, but doing this seamlessly can be difficult. Sequator will do this for you, and the result is a stacked image that doesn’t have a blurry foreground.
Enhancing different sections
Let's see how Sequator does this on our Milky Way images. We started with 20 exposures of our Galaxy taken on a Canon EOS 1100D DSLR camera and a 10-18mm lens on a tripod, each 25 seconds at ISO 3200 with an aperture of f/4.5 (see above, left). We shot in RAW format and included 30 dark frames taken with the same settings and the lens cap on.
To begin the stacking process, open Sequator and drag and drop your images into the Sequator window. Once this is done, the ‘File Category’ box will pop up. Next, select ‘Star images’. If you have taken dark frames, drag and drop these too and from the ‘File Category’ box select ‘Noise images’. You can also include flat frames, but we didn’t use them here.
Sequator requires you to name your output file before you stack. To do this, double-click the ‘Output’ button at the bottom of the top left window. It will default to the folder that your original images came from, so just type your chosen filename into the 'File Save' window. Next, we need to set the stacking parameters. To get the software to stack on the stars, you need to click on the word ‘Composition’ and in the box below select the ‘Align stars’ option. Below that, you also need to select the ‘Freeze ground’ tab (See Screenshot 1). In order for this ‘Freeze ground’ feature to effectively freeze the foreground of the image, it’s important that the camera doesn't move between taking each of the exposures.
Now we need to select the area that we want to stack and the area we want to remain static (see Screenshot 2). To do this, click on the ‘Sky region’ tab and in the bottom window click on the ‘Irregular mask’ button. When you move your mouse over the image you will see a circular brush appear. Left click and hold down the mouse button to paint out the sky region that you wish to stack. By using the mouse tracking-wheel to adjust the brush size, it will make it easier to get around the foreground shape. If you happen to make an error while painting with the brush, you can undo it with a right click of the mouse.
If your image has a lot of light pollution, you can double-click on the ‘Reduce light pollution’ tab. We didn’t need to use this feature in our example, however, because our images were taken from a dark-sky location. The next stage is to increase the brightness of the stars. Do this by double-clicking on the ‘Enhance star light tab’ and use the slider in the bottom window to select the strength of this enhancement. In our example we kept the slider in the middle. Next, click the ‘Start’ button to begin the stacking process (see the stacked TIFF image, left).
Once done, you can complete the processing in your favourite image-processing software. Our final image of the Milky Way (opposite, top, right) has enhanced the detail of the foreground and brought out the wonderful starry features of our Galaxy.