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I have a Celestron astromaster130 eq. While viewing through its eyepiece, or using a smartphone camera for photography purpose there is always a definitive no light region( it is circular in curvature). I make sure that I look directly at the eyepiece, but this tunneling makes it really difficult to view things at times. I have googled this, but am not sure of what this effect is called. Is it a general problem or is there a need to rectify some fault in the telescope itself. If not, what are the ways in which this can be effectively reduced? Thanks. Edits are most welcome.

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  • $\begingroup$ "Vignetting" is the term I would probably use. (Dimming of the image at the edges, and only a limited circular area in the middle where the image is visible.) It may vary depending on exactly where your eye is placed behind the eyepiece, and you may find it only happens for lower power eyepieces. I can't answer though as I'm not sure how to reduce this, it depends on the exact arrangement you have. $\endgroup$ – Andy Apr 21 '16 at 9:21
  • $\begingroup$ It's not dimming that is experienced, the image is completely blocked as if there was a circular aperature further up the eyepiece, misaligned with it. $\endgroup$ – Abhinav Apr 21 '16 at 10:39
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    $\begingroup$ In that case I think you might be seeing the secondary shadow - the "exit pupil" of the telescope (visible at the eyepiece) has a black circle in the middle which is an image of the secondary mirror. This is normal if using a low power eyepiece in the daytime, when your eye's entrance pupil is fairly narrow and obstructed by that secondary shadow. If so, you will notice this problem less when using a higher power eyepiece, or when observing at night. (Not sure how to deal with the smartphone camera problem though, you may need to try different eyepieces.) $\endgroup$ – Andy Apr 21 '16 at 10:49
  • $\begingroup$ Please make a drawing and add it to your question. $\endgroup$ – user1569 Apr 21 '16 at 13:40
  • $\begingroup$ Sorry I deleted those images,but this question probably asks the same content, please check this out $\endgroup$ – Abhinav Apr 21 '16 at 16:08
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This is completely normal behaviour. What you are seeing is the edge of the projection angle for the lense. This is probably sort of what you're seeing? :

enter image description here

Typically this is more noticable with cheaper eyepieces as the individual elements are smaller, and have a smaller field of view.

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In a telescope (actually, in any optical system), the last optical element creates a circular area called the "exit pupil", this area must line up with the pupil in your eye (the "entrance pupil" for the optic system called your eye). If the two pupils don't line up, you won't see any of the light the telescope gathers.

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