The Schmidt-Cassegrain class of telescope (also called SCT)
was named for the German astronomer Bernhard Schmidt and French
sculptor Sieur Cassegrain. This hybrid telescope design has
outpaced both the reflectors and refractors in the last decsdes
to become the most popular telescope among amateur astronomers,
espcially among astrophotographers.
The Schmidt Cassegrain can be seen as a mixture of the Cassegrain telescope with a very short tube (since it uses a secondary mirror) and of a Schmidt design (due to the presence of the corrective plate).
But let's see hoe light travels into this dtelescope: light
first enters from the right, passing through a correcting plate
which is a thin, two sided correction lens that corrects spherical
aberration (like in the Schmidt
telescope design). The corrected light passes the entire
length of the optical tube to a spherical primary mirror where it
is reflected back up the tube to the convex secondary mirror. The
convex secondary mirror increases the effective focal length of
the primary mirror. The resulting light is returned back down the
scope into the primary baffle tube and focuser to the eyepiece
set at the focal point located on the end of the focuser . The
primary baffle tube passes through a hole in the primary mirror.
The main SCT advantage is its compact design. No other class of telescope fits as large of aperture and long focal length
into such a short tube assembly, making it easy to store and transport.
The optics tend to stay cleaner longer since they are enclosed within the telescope assembly.
However, SCTs produce fainter images with less contrast than other telescope designs with similar aperture sizes. This is due to the comparatively large secondary mirror required to reflect the light back the eyepiece. Whatsmore, SCTs are notorious for dewing up very quickly on the correcting plate.