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