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Paul, I was thinking the same thing. It's a bit tricky - it seems that it doesn't always happen even with reflecting scopes from the comments above.
Ambermile - I keep trying to convince myself that you're correct, and that it's the central obstruction. Unfortunately I don't know enough optics to really solve it for sure. There are a couple of things that I keep coming back to:
- earlier posts mention that this shadow moves around, and that you can position the camera so that it's "off the screen". I can't see how that is possible if it's really the central obstruction causing the shadow.
- When you are in focus there is no shadow in the middle of the image from the central obstruction. none. Not even a tiny little one. The light rays forming the image in the centre of the fov are using just as much of the mirror as light rays that form an image at the edge (assuming no vignetting).
Bird
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