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At The Field Stop

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Posted by Walter Hamler on September 25, 2001 17:28:17 UTC

The light cone comes to a focus in the focuser at a point where the eyepiece can "look" at the image and magnify it for your observation. That point will be at the field stop of the eyepiece. Not every eyepiece has one, but in those that do it forms the sharply defined edge of the circular eyepiece field. You will note that with longer fl eyepieces the field stop is a greater distance from the eyelens, the shorter fl eyepieces it is much closer, maybe only a few mm's below. Since the shorter fl eyepieces require more "in" travel to obtain focus (place the field stop at the focus), as well as certain designs require more "in travel", it is a good idea to allow for variations, hense the additional 0.5" in travel in the formula. Hope this helps.

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