Dobsonian Telescope

Fork-Mounted Schmidt-Cassegrain, Altazimuth Mount

As we saw in the section on telescope mounts, there are basically two kinds of mounts: altazimuth and equatorial. A dobsonian telescope is an example of an altazimuth mount. Unfortunately, you won't be able to take pictures of much more than the Moon with a Dobsonian telescope, unless it is on a special platform that turns it into an equatorial mounting.

Many Schmidt-Cassegrain telescopes on computerized Go To mounts also use an altazimuth design. Some of these SCTs offer an auxiliary "wedge" that turns the altazimuth mount into an equatorial mount.

Unfortunately, the wedge adds cost and complexity to the use of the mounting. It can be more difficult to polar align a wedge-mounted telescope. Scope stability can also suffer on a wedge since most SCTs are of relatively large aperture and long focal length.

Telescopes on Go To altazimuth mounts are limited in the maximum length of exposures which they are capable of taking because of field rotation. Although they can track an object accurately as it moves across the sky, the field will appear to rotate around the center of the frame. This effect is more pronounced in certain areas of the sky where the longest acceptable exposure might be around 30 seconds. In other areas of the sky the longest acceptable exposure might be on the order of a couple of minutes.

It's not impossible to shoot long-exposure astrophotos with an altazimuth mount, but the slow focal ratio of most Schmidt-Cassegrain telescopes makes it much more difficult for a beginner in astrophotography.

Altazimuth Mounts for Astrophotography - The Bottom Line An Altazimuth mount is not a good choice for astrophotography because the image will rotate about the center of the field as the mount tracks the sky. You can turn some altazimuth mounts into equatorial mounts with the use of a wedge, but these add complexity and cost and the setup is usually not that stable.