It was suggested that I should put together a quick tutorial on how I use DeepSkyStacker to stack/process images, so this is my attempt to do that.
DeepSkyStacker is free software designed for stacking deep sky and comet astrophotography. The software author specifically disclaims its use for planetary or lunar imaging and I’ve never tried it so I don’t know how it works on those. Since my main focus is deep sky stuff (particularly Messier Objects) it’s a valuable (yet FREE! did I mention free?) part of my astrophotography toolkit.
The first step of astrophotography is to set up your equipment, get your telescope aligned, camera set up and so on. Let’s assume I’ve already done that. Including good drift alignment, that normally takes me between 30 minutes and an hour, the biggest part of that being the alignment. Once I’m ready to start imaging, I find the target, center it and attach the camera. For the purposes of this tutorial, let’s say I’m imaging M51, the Whirlpool Galaxy. My next step is to get the target into focus. While I have a parfocal ring on some of my eyepieces, it’s not so perfect that I don’t need to refocus when I put the camera on. Once the camera is attached, I focus it using my electronic focuser and, starting tonight, my Bahtinov Focus Mask. Once it’s as focused as I’m able to get it, I’m ready to start imaging. Sometimes, I use guiding like PHD Guiding, sometimes I don’t. It depends on how long of exposures I want to take. If I want to take longer exposures, then PHD Guiding becomes necessary (though I have a real problem with it when guiding on objects in the south part of the sky…). I generally capture my images using Autostar Envisage, which is software from Meade (the maker of my camera and scope). There are two things I really like about Envisage. First, it’s simple and quick to use and second, it automatically correlates the dark frames with the temperature off of the camera. Therefore, the images I save (using Fits3P) have already had the dark frame subtracted. While Envisage captures images, it saves a copy of each sub frame to the hard drive while stacking the best of the images and displaying the result onscreen. It has some issues, but gives me some idea if I’m getting a decent image or not, but I essentially never wind up using the stacked images from Envisage. Once I’ve captured the images I want to capture, I start up DeepSkyStacker and start processing, oftentimes while Envisage is capturing another target.
Selecting Your Images
Now that we’ve captured a bunch of images, we’re ready to stack, so I open up DeepSkyStacker.
To add my images to the list so they will be stacked, I click on “Open Picture Files” from the menu on the left. I select the images I want (you can add as many as you like from different folders if you choose) and they are added to the list at the bottom of the main pane. Once I’ve got all of the images I’m wanting to stack, I click on “Check all”. This will check all of the images so that they will be included when you stack. But wait! We’re not ready to stack yet. First, I normally click “Save the File list…” and save the list so I can reload it later. Next, I go through image by image in the list, clicking on them and unchecking the ones I’m not happy with. I generally do this visually. What I’m looking for are round stars. If I notice streaking (on all the stars) or ovals or lines or two stars everywhere there should be one, then I uncheck the image. Those images will just add noise to the final result. If I don’t have a ton of images, I may choose to keep some that are slight ovals, but generally I’m pretty ruthless. You can see in the image below that the stars are streaky. This is a reject.
Once I’ve gone through all of the images, I generally right click on the rejects and choose “Erase from disk…” I get rid of these images, because I’m never going to use them – they are just clogging up my hard drive. In the end, I’m left with a list that only contains images I want to stack. I then re-save the list of images by clicking “Save the File list…” so I can reuse it later.
Stacking Your Images
It’s now time to align and stack! I click “Register checked pictures…”. When you click on this it brings up the Register Settings dialog. On the Actions tab, I check all three boxes and set the “Select the best…” box to 100%. Then I click on the Advanced Tab. This is where the automatic alignment is set up. Generally I’ll start at 5% and click “Compute the number of selected stars” and look at the result. If the number is above 50, I raise the percentage until I get it below 50. If there are below 10, I lower the percentage and reclick the compute button until I’m happy. You can’t go lower than 2% though, so if it can’t detect stars at 2%, you’re hosed. Of course, that probably means the images aren’t very good.
Since this one came back at 14, I’m going to keep that there. Next, I click on the ”Stacking parameters…” button.
I generally use Mosaic and 2X Drizzle because I can always size it down and crop it later. The effect of the settings there are explained pretty well on the DeepSkyStacker web site. On the Light tab, you can select whatever method you’d like to use. These also are explained on the DSS web site, but I generally go with Median Kappa-Sigma clipping with a Kappa of 2.00 and Number of iterations of 5. (I don’t know what those numbers mean and I haven’t played with them – they are the defaults.) On the Alignment tab, I leave it on “Automatic”. On the Cosmetic tab, I have “Detect and Clean remaining Hot Pixels” checked. They are detected at a threshhold of 72.8% which works pretty well for my DSI-II and are replaced with the median value. This detection threshhold can create long render times and weird things to happen if it’s way off. Once time mine got moved and I had bizarro results until I changed it back. It took me a while to figure it out. Okay, once everything is set, I click OK on the Stacking Parameters dialog box and then OK on the Stacking Steps dialog box. The first thing it does, is determine star positions on all of the images.
Once that’s done, it stacks the images.
Once it’s done, I’m presented with this:
Beautiful! Right? Um, did something go wrong? That doesn’t look right at all. What’s wrong? Nothing – it’s just time to adjust the curves on the image.
Now it’s time to make the image look its best. First, adjust the RGB/K levels. I generally adjust the top and bottom levels (with Linked settings) to get a histogram similar to what’s seen here:
When I click apply, it will re-calibrate the high and low points on the histogram and redraw the image using the new settings. This is what I get:
This isn’t quite as crazy as the one above, but it’s still not right. That’s okay, we’re not done yet. Next, on color images, I generally go to the Saturation tab and set the saturation to 18. This works best for my camera and settings, but your results may differ. Experiment with it after you do the next steps if you’re not happy with the end results.
Now it’s time for me to adjust the curves, so I click on the Luminance tab. To really understand how to get the most out of this, you’ll need to eventually understand what each of the six sliders actually adjusts, but I’m not going to get into that here. It’s better to just experiment later with them and see the results. (Just a note: I never move the 0° slider from zero – it just raises the darkest regions of the image up, and I never want or need that. ) What I do is adjust the sliders until the luminance curve (the black line) looks like this in relation to the image histogram and click Apply (you have to click Apply to see any changes you make):
Okay! Now we’re getting somewhere. The Whirlpool Galaxy is emerging from the image. It’s not perfect (it’s really red for example), but we’re getting much closer to something I can use. I next go back to the RGB/K Levels tab, uncheck “Linked settings” and move the high mark for the red up a bunch. This actually cuts the amount of red in the image. Note that I could also fix this in Photoshop later if I wanted to, but I might as well make quick changes like this here. After I hit Apply, I get:
Okay, I’m ready to save this image now. There are still some issues I’ll need to correct, some of those will be fixed when I use a Luminance layer and others will be fixed when I adjust levels and curves in Photoshop, but that’s a topic for another time. I’ve got a good image that I could either just go ahead and use or do further processing. To create the Luminance layer, I load the L images saved by Envisage and get rid of the bad frames as described above and combine the same way this tutorial describes with the exception that I don’t change saturation or color since it’s grayscale.
After saving both the color and luminance images as 16-bit TIFs I pull them into Photoshop, convert to 8-bit (I have an older version of Photoshop), copy the lum layer over to the color image and set it as a luminance layer. After messing with levels and combining, here’s what I wind up with:
I hope this has been helpful. If you’ve got questions, post them in the comments and I’ll try to answer them.
Update: There is now a Turkish version of this tutorial!