: 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