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Eyepieces And Focusers

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Posted by Steven A. Crosby on September 25, 2001 21:15:15 UTC

Thanks for the reply. I find that the more I learn, the more questions I have.... :-)

You say that not every ocular has a field stop... What happens if an ocular doesn't have a field stop? I know my TV Radian does, but I don't know if my Celestron Ultima does - it's not listed anywhere. If I knew what to look for inside the ocular, I could do some measurements of my own.

With that said, I suspect that the field stop of the longer focal length eyepieces sits somewhere below the top of the focuser tube. To get that type of ep to focus, you have to rack the focuser further out than a shorter focal length ep. I think what I have to do is to get the focal plane at a point somewhere between my shortest focal length ep with the focuser racked in and the longest focal length ep with the focuser racked out. But, to me, that would seem to be a point near or below the edge of the focuser when it's racked in. The Focuser Window view in NEWT shows the focal plane significantly above the top edge of the racked in focuser. If I set the "Spare Focuser In Travel" at 1/2", does that mean that the focal plane will be located at a point 1/2" above the top edge of the focuser when it's racked in?

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