![]() ![]() ![]() 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. 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. ![]() One reason for this is that a Dobsonian has to track along two axes instead of one (like a GEM). 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. 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. While they are excellent instruments for visual use, Dobsonians aren't exactly well-suited for astrophotography. 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. They are also the most cost effective when it comes to aperture diameter bang for your buck. 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. No telescope design embraces this idea more strongly than the Dobsonian scope. 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. 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.įor visual astronomy, where you look through the telescope with an eyepiece, larger diameter telescopes are more common. Most mounts affordable to the backyard astrophotographer can only support refractor telescopes or small reflector telescopes up to about 8 inches in diameter. The downside of using a GEM is that they are somewhat limited by how large of a telescope they can accurately carry. 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. Often, several hours of camera exposure time will be used to create a single image. 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. It isn't uncommon for astrophotographers to use exposures of several minutes for each photo. The shutter will remain open and collecting light for much longer periods of time compared to regular photography. This is important because astrophotography requires long exposures on the camera. The overwhelming advantage of using a GEM is the ability to precisely track the sky for long periods of time as the Earth rotates. Traditionally, astrophotography is done with a small refractor telescope mounted on a German equatorial mount (GEM). My deep sky astrophotography methods are somewhat different than most who participate in this hobby. ![]()
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