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  1. #1
    DennisS42's Avatar
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    Default Is this what they call field stop?



    I have a B&L ASP 30MM EP. I've been messing with oculars in stellarium and.....if I set AFOV at 27 and Focal Length at 30 and Field Stop 0 when I look at M8 that's almost exactly what I see if I look though my scope at M8. Stellarium say's my FOV = 0.675*.....
    I was looking at the EP and realized the hole in the bottom of the barrel is small compared to the bottom lense of the actual EP. Is that what creates the Field Stop? Any idea what would happen if I opened that up 1/8" or 3/16's". I hope this makes sense. Any idea's or thought's?????

    Thanks,

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    Yes, that is the field stop. If you enlarged it, you would probably increase your AFOV and FOV. However, instead of having a nice crisp edge to the field, it would probably be a fuzzy edge, and you might get weird optical effects - distortions or reflections - near the edge. They put the field stop there because that's as wide as they can reliably get good performance out of the lens design.

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    pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome;
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  4. #3
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    Default

    oh well, guess I'll live with it until I can get better EP's. Just confuses me why they would make a 30MM ep with only 27* afov. Maybe there just to old.

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    Quote Originally Posted by DennisS42 View Post
    why they would make a 30MM ep with only 27* afov.
    The AFOV is determined by the optical design of the lens. That is probably a Kellner. The Kellner design just has a narrow AFOV. The field stop isn't arbitrary. It's there because that's the edge of the decent performance.

    There's nothing you can do about it except get a different design that has a different AFOV. The reason they make EPs like that is because they're cheap. The bigger the AFOV, the more expensive the eyepiece.

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