Off-Axis Designs

You can get an unobstructed telescope simply by using an off-axis segment or section.  However they are not so easy to make and it usually means coring a segment from a larger mirror.; kind of a waste.  An off-axis Newtonian is one example:

6.5 in F10.4

6.5 inch Off-Axis Newt

But other designs can be off-axis such as an off axis Gregorian:

Off-Axis Gregorian