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Posted by Daniel Johnson on September 8, 2003 21:24:45 UTC

There are two questions regarding accuracy of tracking. One is for purely visual use, i.e. no long-exposure photographs. There's no reason a Dob couldn't track just as accurately as Autostar does in altazimuth mode--it's just that motorized & computerized Dobs are new. The Dob is, after all, just a very simple and stable altazimuth mount. The computerized part is actually not new--for several years, companies have made computers that will tell you when your Dob is pointed at the object you want--you just have to push it there by hand, using the computer's feedback. No one has yet written reviews of the new Orion Dobs, which sound as if they're motorized,if I read the ads correctly. So the jury's out there.
Then there's the question of photography. For anything other than snapshots of the Moon or planets, one must have a polar mount. Dobs, or altazimuth mounts of any sort, won't do--the camera's field slowly rotates around the star that's in the center of view, causing streaking. Professional observatories get around this by adding a field derotator, a third axis of rotation for the camera. Meade sells a derotator, but it's a bad idea for amateurs--it requires three motors working in perfect synchrony at continuously varying rates, rather than just one doing 99.9% of the work at a constant rate.
By the way, the ETX mount is not accurate enough for long-exposure photography. The LX-90 mount can be forced to serve. The LX200 mount is made with photography in mind. For examples, see my new web site (it is only a few days old) at
http://hostingprod.com/@dgjohnson.net/night_vision.html

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