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Dob Vs. Refractor

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Posted by Daniel Johnson on October 27, 2002 14:14:42 UTC

Decades ago, before Dobsonian scopes hit the scene, I made a couple of small Newtonian reflectors. It was rewarding (and back then, 6" wasn't considered small).
It will take you at least three times as long to make a refractor as it will to make a Dob, since you have four surfaces to grind (you'll be slightly faster after the first one, so it won't take four times as long. However, using successively smaller grit sizes requires a minimum amount of time with each one or you end up with pits in your glass that may not be big enough to see, but that will make your scope useless--I learned that one the hard way, and had to go back and regrind the last three grit sizes). So making your refractor will be a much, much longer process. How much is your time worth? How many weekends are you willing to spend NOT USING the scope, because it doesn't exist yet? (Most ATMs don't make their own diagonal, which would be more time consuming and lessen the difference some if you do it.)
I wouldn't ordinarily advise someone to make his first mirror larger than 8 inches, but if you're crazy enough to make your first refractor 8 inches instead of 4 or 5, you probably have the fortitude to make a 12-inch or 14-inch Dob. A 14 inch Dob will show you things that are simply invisible in any 8-inch scope.
As for the worry about turned down edge (having the outer 1/8 or 1/4 inch of your lens or mirror worn down too far), yeah, you can hide that in your lens holder. However, you can hide it (if it happens at all) with a Dob, too. And losing 1/8 inch from a 12 or 14 inch scope is nothing compared to losing that much from a smaller scope.
Not only do you have to figure four surfaces instead of one, you also must have the two curves on each side of each lens perfectly coaxial, which you can probably do but it takes more time.
Mounting an 8-inch refractor is much harder than mounting a Dob. Much harder. Here I don't mean the optical tube, but the mounting that holds the tube. A good, solid mount is as important as a good optical surface. The Dob is as simple and solid as they come. All of the new, giant telescopes in the world sit on altazimuth mounts (basically Dobs!) because of the stability (granted, they have to rotate their instruments to make up for the lack of polar alignment, but that's another story).
You've heard stories that a good refractor outperforms Newts or Schmidt-Cassegrains for planetary resolution, because of the lack of central obstruction. This is true for instruments of the same size, IF the refractor is either an apochromat (which you aren't making) or long enough in focal length for a doublet lens to counteract chromatic aberration. So your refractor will have to have a tube as long as a 14-inch Dob's if you want to optimize your performance. And the advantage of a refractor applies only to scopes of equal size. Sky and Telescope has published the statement that an 8-inch Schmidt-Cassegrain matches a 5-inch refractor for planetary viewing. So, if both are perfectly made, a 14-inch Dob will easily match an 8-inch refractor for planets, and crush it for everything else. (Well, for wide-angle photography a refractor may have a wider field, but the cost of a photography-capable mount is not low.)
Now a word about parabolizing. It adds labor, but it was NOT the biggest time-consumer in making my mirrors. However, I suspect that the difficulty is inversely related to the square of the focal ratio. That is, an f/4 is probably 4 times harder to figure than an f/8, and Dobs tend to be short focal ratio scopes.
It will probably be cheaper to make a Dob. So why not spend that money on an eyepiece or two to make it all worthwhile? Good eyepieces make your scope seem twice as good. Really.

--Dan Johnson

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