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what is meant exactly by 33mm imaging circle

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just wondered what the above meant exactly cheers

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It’s the amount of even light that gets through the glass on your optics before the edges start to darken which is called Vignetting...it so imagers know whay light will hit the camera sensor..

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A bit vague term, but could be the following:

- Fully illuminated circle: 100% illumination within that region - so no vignetting as LightBucket pointed out

- Fully corrected circle: again region that does not suffer from optical aberrations. This term is even looser. It should mean diffraction limited field, but often scopes don't have that large diffraction limited field. That would be much much stricter requirement. You can understand fully corrected circle to be one where optical aberrations are small enough to be masked by usual suspects - like seeing and tracking precision. Ultimately, one should have evenly round and tight stars in this region (if not diffraction limited).

Imaging circle should, at least I believe so, satisfy above two conditions - to be both fully illuminated and fully corrected.

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