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Posted by Robert May on May 4, 1998 13:22:00 UTC

Yes, it can. The best way is really to do what is called a double pass test where you start with the EP end of the scope and send the light out to a flat mirror (1/4wave or better) and look at the light when it comes back. It is a nice test if you have a large flat (should be at least 1/3 of the dia. of the scope(less accurate this way but you can see things)) around to do the test with. The double pass test will give double the error. If you want to actually do a star test there is a formula (try the ATM book set) which will tell you how far away to put the artificial star in order to mimic close to an ideal star. It ends up for any decent size scope that the distance is a large number (I think that an 200mm is at least 200-300 meters). That makes the test best done outdoors as there usually isn't any source of hallways that long. A third way is to use a scope of known accuracy and use that as the light source (which should be set up to make a parallel beam) and shine that into the scope under test. With the double pass and the reference scope, remember to rotate the mirror or scope in order to insure that the test equipment isn't producing the error. The flat has errors and if you rotate it and take out the errors that move, then you will know what the error of the scope under test is. Have Fun and Keep Looking Up. Bob May

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