a large lens can sag or flex due to its own weight, distorting the image that it forms, in telescopes…

a large lens can sag or flex due to its own weight, distorting the image that it forms, in telescopes telescopes do not have this problem.

a large lens can sag or flex due to its own weight, distorting the image that it forms, in telescopes telescopes do not have this problem.

Answer

Brief Explanations:

Lenses in refracting telescopes must be supported only at their edges to allow light to pass through. Large, heavy lenses can sag under their own weight, causing optical distortion. In contrast, reflecting telescopes use mirrors that can be supported across their entire back surface, preventing sagging and allowing for much larger apertures without structural distortion.

Answer:

refracting; reflecting