Another Cassegrain variety, the Maksutov Cassegrain telescope shares the advantage of being able to look into one end of the telescope to see out the other end of the telescope.  These use a thick spherical corrector lens within the primary mirror's radius of curvature. A small spot at the center of this lens, facing the primary mirror is silvered or aluminized to reflect the image back through the hole in the primary mirror.
This secondary mirror is significanly smaller than the convex mirror sported by the Schmidt Cassegrain telescope, making the Maksutov brighter than the Schmidt, and giving it better resolution as well.   These telescopes are heavier than the Schmidt Cassegrains are, because of the heavier corrector lens, but they have the shortest tube length of all telescope designs. Pictured in the background of this page, is a Maksutov Cassegrain type telescope.