My Gear - Dobsonian Astrophotography

Astrophotography with a Dobsonian?

My deep sky astrophotography methods are somewhat different than most who participate in this hobby. Traditionally, astrophotography is done with a small refractor telescope mounted on a German equatorial mount (GEM). The overwhelming advantage of using a GEM is the ability to precisely track the sky for long periods of time as the Earth rotates. This is important because astrophotography requires long exposures on the camera. The shutter will remain open and collecting light for much longer periods of time compared to regular photography. It isn't uncommon for astrophotographers to use exposures of several minutes for each photo. These individual photos are then stacked on top of each other using special software in order to increase the signal-to-noise ratio of the final image and bring out the really faint details in the night sky. Often, several hours of camera exposure time will be used to create a single image. But if your mount can't track the sky accurately enough as it moves through the frame, then the resulting photos will be smeared and show stars as bright streaks instead of points.

Images: Two astrophotography rigs featuring refractor telescopes on German equatorial mounts

Photo Credit: Alan Del Ponte

The downside of using a GEM is that they are somewhat limited by how large of a telescope they can accurately carry. Most mounts affordable to the backyard astrophotographer can only support refractor telescopes or small reflector telescopes up to about 8 inches in diameter. Larger telescopes can be (and are) used with German equatorial mounts, but the mounts designed to hold them are much larger and start to become prohibitively expensive.

For visual astronomy, where you look through the telescope with an eyepiece, larger diameter telescopes are more common. The larger the diameter of your telescope, the fainter the objects you will be able to see and the more detail you will be able to make out in them. No telescope design embraces this idea more strongly than the Dobsonian scope. Named after its creator, John Dobson, and structurally designed sort of like a cannon, the Dobsonian scope can support a very large optical tube and make it easy to maneuver for visual use. They are also the most cost effective when it comes to aperture diameter bang for your buck. That's why I started out with an 8 inch diameter Dobsonian when I first picked up astronomy and then upgraded to a 17.5 inch one shortly after.

Images: My 8" inch f6 and 17.5" inch f4.5 Dobsonian scopes

While they are excellent instruments for visual use, Dobsonians aren't exactly well-suited for astrophotography. Most of them don't track at all, instead relying on the user to nudge the scope now and again as the object they are looking at slowly drifts out of view as the Earth rotates. The ones that do track the sky are also not the best for astrophotography, because they can't track accurately enough for the long camera exposures that are usually required in deep space imaging. One reason for this is that a Dobsonian has to track along two axes instead of one (like a GEM). The Dobsonian has to track in the up/down axis and the side-to-side axis, whereas the GEM tracks in one axis along the exact path of the target.

Motorized Dobsonians like this can track well enough for visual use, often keeping the target in the view of the eyepiece for 30-60 minutes, but the target will still drift around ever so slightly, even when it appears to be staying in the middle. This is likely because of slop in the drive systems and the slight friction of traditional Dobsonian bearings, which are usually teflon on a formica surface. Additionally, the optical path of most commercially-produced Dobsonians is such that a camera won't reach the point of focus in them. The draw tube of the focuser will be all the way in and still not be far enough in for the image to be in focus. This can be fixed by moving the primary mirror of the scope closer towards the sky end of the tube (which is something that some people may not feel comfortable doing on their own).

Many people buy Dobsonians when they get started in astronomy, and then later wonder if they can do deep sky astrophotography with them. Most will either fail on their own or be told by others that their Dobsonian is only good for images of the moon or possibly the brighter planets.

Challenges aside, I do all of my deep sky astrophotography with my 17.5 inch Dobsonian. It's a highly unconventional method and I only know a couple others that do it successfully with results similar to what can be achieved with a GEM. My success so far can be attributed to luck, some crafty engineering on my part, and the sheer light-gathering ability of such a large scope. While a small refractor telescope in the 60-120mm range might need to take exposures of several minutes to properly expose an image, I've found that a fast scope in the 17.5 inch range can do it in about 10-20 seconds. I've made some modifications to my scope's bearings, which enable low-friction, relatively accurate tracking for periods in this 10-20 second range. Then I stack many of these short exposures together the way regular astrophotographers do, and end up with respectable images containing many faint details and structures in deep sky objects.

Though it isn't perfect, there are several advantages to doing things this way. The short exposure times allow me to capture enough data to produce a nice image in a much shorter time period than would otherwise be possible. Many astrophotographers will spend multiple nights imaging the same target. Some even automate their rigs to image through the night after they've gone to bed. Meanwhile, I can go out for a couple hours after sunset, be in bed at my normal time, and still have collected enough exposure time to create a quality image. I can sometimes even image multiple targets on the same night.

Short exposures also help prevent bright backgrounds and gradients from light pollution, along with bloated stars, from happening in the processing stage of astrophotography (where you take the final stacked file of all your exposures and process it using photo editing software to bring out dim details and present the image the way you think it looks best). I've also been told by other astrophotographers (who have processed my images their own way) that my scope and camera setup produces images of good quality that are easy to process.

Additionally, the wide 17.5 inch aperture and long 2000mm focal length of my telescope allow for very high resolution images that show structure and fine details in objects not often captured in other astrophotos taken with traditional smaller scopes. It's definitely not a mainstream method of astrophotography, but sometimes I think I've stumbled upon a unique and interesting way of doing this. Some will scoff at the idea of using a Dobsonian for imaging, but I think that my images speak for themselves. I hope you agree and get the same enjoyment from them that I do!

-TJ