Stacking comets with DeepSkyStacker.
DeepSkyStacker is a fabulous piece of software for stacking images together. It works very well for static deep-sky targets, but when it comes to comets, you need to do a bit more work. That’s because comets do one thing other deep-sky objects don’t: they appear to move against the background stars. Always consider that when planning on imaging comets. If the movement of the comet is greater than the resolution of your optical setup, then unless you are guiding on the comet itself you will need to use shorter sub-exposures to prevent blurring. Comet 45P/Honda-MrkosPajdusakova, which came very close to Earth earlier this year, had a rapid apparent movement forcing many imagers to use sub-exposures of less than 30 seconds. Subsequently, each sub-exposure showed the comet in a slightly different position. After registering your images, the stacking process must take the position of the comet into account and stack on the comet itself to avoid blurring details in the coma and tail.
Another thing to note is that the details within a very active comet can change over the course of a few minutes. Try to avoid taking your subs over an extended period, as when stacked, they will once again blur if the comet’s features have changed during the period the exposures were taken.
Once you have used DeepSkyStacker to register your subs (don’t forget to capture them in RAW in the first place), the stacking process outlined here needs to be followed so the program ‘knows’ where your comet lies on each exposure.
If the comet is fairly faint you may need to increase the star detection threshold when registering to include the comet in the detected stars. If the comet cannot be detected, don’t worry, we’ll show you later how we can give DeepSkyStacker this information before stacking.
Now find the comet again
Once your subs are registered, click on the first image in the window below the preview panel. At the side of the image preview, four buttons will be visible. Click the third one down, a green comet shape.
When pressed, tiny green rings will appear around the position of the stars the registering process has mapped. Hover your mouse over a star and you will get a prompt to set the comet here. In other
words, DeepSkyStacker is asking you: ‘Is this the comet?’ The point you can click will jump from star to star as you move your mouse across the image. Hover the mouse over the brightest spot within the comet’s coma. A ‘set comet here notification’ will appear.
If the comet in your image is too faint to see, look just above the top-right of the preview image and move the middle grey marker closer to the black point on the left. This will brighten the view without affecting the image. Find the comet’s position and hover the mouse over it. Zoom in on the image by rolling the mouse scroll wheel forward. It takes a while to get to grips with this. Target the mouse pointer directly over the bright spot; a magnified view (top left) also helps
with this. Hold down the shift key and press the left-hand mouse button. The position of the comet will now have been selected in this sub.
It’s worth getting the position spot on as it improves the quality of the final image, resulting in straight star trails. Click on the next image and repeat the process, working your way through all your subs. Note that the #Stars column total will now have a (C) beside it, indicating that a comet’s position has been added. Before moving onto the stacking process, check that all your subs show this.
Everything is now ready for stacking. Click Stack Checked Pictures, then
Stacking Parameters. The Comet tab will now be visible. Several stacking options are available: Standard Stacking ignores
the comet and stacks on stars, while Comet Stacking ignores stars and stacks on the comet position marked in each sub. Stars + Comet Stacking attempts to stack on both. Under the Light tab, select the best ‘stacking mode’ settings for your images. Simplest to use is the Entropy Weighted Average
Stacking Mode, though be careful if using this mode with full frame RAW DSLR images. These frequently crash the software due to their sheer size.
Once you are happy, click OK to close the stacking parameters window, then OK again. Your images will now start stacking on the comet’s position and automatically save the stacked TIFF image.
Comet C/2014 Q2 Lovejoy in January 2015, stacked on the stars – resulting in a trailed and blurred comet
If the comet is too dim to identify in a given frame, you can increase the brightness using the slider at the top-right of the preview pane
Hover your mouse over a star and you will be able to set it as the comet; you’ll need to do this for every frame you want to stack
An alternate view of C/2014 Q2 Lovejoy in January 2015 from the same data; this time stacked on the comet with trailing stars
The Comet tab shows the three stacking options available; switch to the adjacent Light tab to pick the stacking mode