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Re: Mirror Testing

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Posted by Tom on March 23, 1999 13:21:25 UTC

Scott:

How exactly, did you "star test" your scope? From your description, you sound like you are testing the image in focus. True star testing compares the out of focus "disk" of a star on both sides of focus at relativly high power. You must compare a equal distances on either side of focus - a good mirror showing identical images with no variations in the light intensity accross the disk. Errors on the mirror will show as bright rings in the "disk" on one side of focus and dark rings in the same place on the other side of focus, because light from the star comes to a focus at differnt places, focusing either too close or too distant from the main focal point. A "turned edge" if I recall right, will show as a "fuzzy edge" of the disk inside focus because the edge is throwing light out past the main focal point. Spherical abberation manifests as a brighter center inside of focus and a brighter edge outside of focus. (keep in mind the diagonal will block the middle of the mirror so by center I mean inner area) Check out a book - Star Testing Astronomical Telescopes by Suiter available from Willman-Bell Inc (they're on the web ar www.willbell.com). While a heady book, it does explain the startest in depth and has pictures of how the star should look with various abberations. Another thing to watch out for is be sure you are testing you mirror and not the local seeing. Zones on the mirror do not move! Lousy seeing will trash a star test since the image will boil way too much to see subtle variations in the disk. Be aware that the star test done right is brutally sensitive to certain abberations, ones that would never be noticed in normal observing so don't be quick to pronounce your mirror Language Removed, just cause the star test shows some problems.

All tests are going to be difficult and time consuming, requiring you to build/purchase test equipment. Just about any mirror making book will explain how to make and use a Focault tester. A REAL loose rule of thumb is objects will snap into focus in scope with good optics with a very definite spot of best focus. Lousy mirrors will "mush" through focus where its hard to tell just where best focus is. Just remember don't test your seeing! Pick a steady night and repeat it the test many times. Startest as many scopes as you can and look through a scope you know is good (don't take the owners word for it being good - I've looked through some crappy optics that their owners touted as being excellent [and kept my mouth shut]).

Good luck

- Tom

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