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Eyepiece Blackout?


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Sorry for what might be a stupid question but here goes ....

Managed to try out my Revelation Superview 26mm 2" eyepiece last night. I was very impressed with what I saw, but .....

Every so often the entire view would just "black out" and I couldn't see anything. No amount of shifting my head around would bring the image back.

Removing my head and returning to the eyepiece would resolve the problem.

I'm guessing this is a "known issue" with observation, but I can't find the answer as to why it happens - your comments are appreciated ;)

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large exit pupil on a reflector tends to be prone to this sort of thing, you have to be pretty accurate with your eye position if you're not going to have issues with the secondary shadow. That's why some of us prefer refractors for low power views.

If your EP has a twist up eyecup, try to set it so that your eye is resting loosely on the eyecup when it's in exactly the right position, that should help avoid "blackouts". Otherwise use a higher power. 4 to 5 mm exit pupil is large enough with reflectors.

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Thank you for the info. If I understand the numbering correctly, exit pupil is calculated as eyepiece size / f number? So 26 / 5.91 = 4.3 (ish)

The eyepiece has a two stage fold down rubber eyecup, wearing glasses makes things problematic anyway but if it's clear tonight I will experiment with adjusting the cup.

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The exit pupil diameter is the entrance pupil diameter divided by the magnification. The magnification is the scope effective focal length divided by the eyepiece effective focal length. The entrance pupil diameter is usually (but not always) the diameter of the main mirror, so in your case with a 26mm EP, the mag would be 1200/26=46.15 and the exit pupil diameter would be 203/46.15=4.4mm. Also a problem can be the eye relief, ie the distance of the exit pupil from the last lens element. This can vary even with EPs of the same effective focal length. The combination of a small exit pupil and short eye relief can lead to the eye pupil 'missing' the EP exit pupil and darkness decends.

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