Very popular Schmidt-Cassegrain telescopes are reflecting telescopes with a unique Schmidt corrector lens which eliminates aberrations.
The tube of this type of telescope isn't very long, but because of the unique folding of the light, the focal length can be pretty long.
Schmidt corrector is a thin, aspheric lens used to correct the light paths – all light is brought to the same focus. The aspheric corrector lens is located at the focus or near the focus of the primary mirror.
Design of Schmidt-Cassegrain telescope. (Available from:http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/telescopes/phy217_tel_catadioptric.html. [accessed 29 Oct, 2019] )
The light enters the tube through the corrector lens and reflects from the primary mirror, which is located at the other end of the telescope.
Once reflected, the light travels to the secondary mirror.
From there, it passes through a small hole in the primary mirror to the eyepiece.
Schmidt-Cassegrain telescopes are great for lunar and planetary viewing. They can also be used for “deep-sky” observing and astrophotography.
Compare to refractors, they feature large apertures at reasonable prices. They are easy to use, very compact, and portable.