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What is exactly a Dobsonian telescope, and what are the differences between this technical choice over a Schmidt-Cassegrain or a Newtonian configurations?

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Dobsonian is a type of mount, whereas Schmidt-Cassegrain and Newtonian are different configurations of the optics inside the telescope. Dobsonian mounted telescopes are almost always Newtonian due to the Newt's ideal placement of the eyepiece.

This is a Dobsonian mounted Newtonian scope:

Dobsonian telescope

A Dobsonian mount does not track the stars as they move in the sky, so it's not suitable for some applications but it's a great "looking around" mount and is really easy to use. I've set people at my Dob before and they intuitively get it within a few minutes. My German equatorial on the other hand takes a good deal of expertise to use.

Dobs are great for large or heavy scopes because they effectively balance the scope so that moving the scope requires very little effort.

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did you build it yourself ? –  Stefano Borini Jun 1 '11 at 19:58
    
I did not, no. Maybe one of these days I'll build myself a 24" or something nice and big. Right now I have an 8" Orion SkyQuest. It's just the right size to be a big hit at star parties! –  Jeff Snider Jun 1 '11 at 20:01
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The idea of a Dobsonian is to spend all the money on the mirror and build the simplest possible mount - so giving you the most photons/$ –  Martin Beckett Jun 24 '11 at 2:24
    
While tracking with a dobsonian isn't as 'natural' as doing so with other types of mounts: Orion sells dobs with tracking/goto systems suitable for visual observation but not long exposure imaging. Aftermarket kits are available for some other brands as well. –  Dan Neely Apr 9 '12 at 12:44
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Technically, a Dobsonian is a type of mount: an altazimuth (up/down, left/right) mount with large bearings and no slow motion controls or locks. The large bearings make its operation smooth and, as a result, it has no need for slow motion controls or locks.

Any type of telescope can be mounted on a Dobsonian mount, but most commonly it supports large Newtonian reflectors, with apertures from 6- to 40-inches. Because it doesn't need counterweights, it is very light in weight for its size, so that even the largest Dobsonian reflectors are portable.

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