Stack

Astrophotography
by Dennis Allen

Summer of 2019 members of our astronomy club, including myself, got hooked on stacking. Letting the camera take short unguided images, then stacking them with Deep Sky Stacker. By stacking, I no longer have to spend time acquiring a guide star with my off-axis guider. Images using my off-axis guider tended to look a bit out of focus, due to saturation. In stacked images, however, stars are nice and pinpoint sharp. And if I bump the scope, if an airplane or satellite goes by, it's easy enough just to toss out one bad image. I don't lose the whole run.

I built a 24" truss-tube Dobsonian with a truss-tube kit from AstroSystems (for complete construction details, click here). To do astrophotography, I purchased a third generation dual-axis equatorial platform. This platform gives me an hour of tracking, making photographic work possible and visual use quite enjoyable.

Long ago I replaced my old Nikon F with a Canon Rebel XT. What took me fifteen minutes with the Nikon F, I can now accomplish in five minutes with the Canon Rebel. I currently use a Canon Rebel XSi 450D because it has the very handy live preview feature.

In astrophotography, you’ll need an external trigger mechanism called an intervalometer. On amazon.com I found "Timer Remote Control RS-60E3 For Canon XS XSi T1i XT XTi" for $16.95. Does the same job as the those $150 units.

For prime focus photography, I use a 2" adapter, a t-ring, and a Canon bayonet ring to mount my camera to the telescope's 2" focuser. The camera needs to be closer to the main mirror than eyepieces, so my telescope's focus point has been adjusted for the camera. I use an extension tube for eyepieces.

Setting up for a shot takes a few steps. First, hook up your 2" adapter to your camera and plug in your intervalometer. Set your camera to manual mode, shutter speed bulb, ISO 1600, large jpeg. Rewind your platform. Insert your camera into the focuser. Find a bright star and focus it in the camera view finder. The live preview feature on my Canon XSi really helps me focus the camera.

Using the intervalometer, take a manual 30 second exposure of your focus stars. Is there any drift in the stars? If not, take longer and longer shots until you do start to see star drift. Then you will know how long to take your unguided shots. The polar alignment on my platform, for example, starts to show drift after 60-90 seconds. I can take 45 second shots comfortably, so I set my intervalometer to 0 second delay, 45 second duration, 47 second delay between shots (note that some intervalometers don't need duration time added to delay time), and an infinite number of shots. The battery on my Canon usually gives me about 4 hours of images, with display turned off.

The next step is to take dark frames. If you're doing 30 second shots, take a handful of 30 second shots with the camera aperture covered. Now you're ready to find your first target and take a set of images. In astrophotography, there's a lot of trial and error. Start off with a set of 30-40 images. Dimmer objects like M20 will need more images than brighter objects like M8.

Suppose you have an object too dim to see in the camera viewfinder, then what? You might want to look into a flip mirror, so you can flip from eyepiece to camera. I like to lock down my focuser. It allows me to swap the camera out/in for the eyepiece and still stay in focus.

After you do a night run, load your images in your computer (I prefer to separate my images into sub-folders). Start up Deep Sky Stacker. In DSS, open your dark frames, then open your first set of jpeg files. Have all files checked. Register checked pictures. In register settings, advanced, adjust the star detection threshold until 150-250 stars are detected. Start off with the preferred defaults. Might take a few minutes, but hopefully you'll get a good image. Afterwards, I like to move the RGB light bells until they overlap the main graph line (and each other). I believe this process is called adjusting color curves, something you may want to handle in your paint program. Anyway, perform a 'Save picture to file' as a 16 bit TIFF file. Then you can open this TIFF in your GIMP, PhotoShop or other paint program.

Note: I'm still new at this, but the two most basic paint program adjustments are light levels and color curves. In light levels, you move the black and white points as close to your bell curve as possible. I think this is called stretching. Color curves has to do with brightness, contrast and shadows. I can't tell you how to set your paint program adjustments, you'll just have to experiment. A lot of trial and error.

Conclusion

As I said, there's a lot of trial and error. You can try longer exposures. Instead of large jpeg, you might want to try stacking large RAW images (or both). With RAW images, you don't see as much detail in DSS, but you'll have more data to manipulate in your paint program. If your camera can handle it, try a higher ISO setting. There are a lot of settings you can adjust in DSS, a lot more in your paint program. I suggest you search youtube.com for tutorials on using DSS with GIMP or PhotoShop. Nice thing about stack images, you can always go back and rework them. Click here to see my results thus far.

Star-Trails

In the back of almost any Sky & Telescope, you can find an image of stars streaking around Polaris. You can create a star-trail photo with Deep Sky Stacker. Take a set of time-lapse photos, load them into DSS. Under stacking parameters: [Result] tab go from "Mosaic" to "Standard". [Light] tab go from "Medium" to "Maximum". [Alignment] tab go from "Automatic" to "No Alignment". I tried it on one of my old time-lapse sets. Once I adjusted the RGB bell curves, it looked good.

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This web page was last updated 06/19/24

Muskegon Astronomical Society