Here's my trusty Schmidt Cassegrain (with the Megrez 90 FD mounted on top). It has a 2030mm focal length and an aperture of 203mm to give an f10 focal ratio. This type of compound scope has become incredibly popular with amateur astronomer and hobbyists over the last 30-40 years due to their compact size and large aperture.

It uses two mirrors and a special corrector lens mounted at the front.

Advantages of the SCT

• Relatively portable, much more so than a Newt or Refractor of the same aperture.

• Relatively inexpensive – more expensive than Newts but far cheaper than a similar aperture refractor.

• Relatively light & short, so it is possible to match it to cheaper mounts than a similar refractor say.

Disadvantages

• The massive focal length of this design gives a very small field of view. Great for small objects, the moon and planets but a pain for deep sky viewing (by the way, look at the length of the two scopes, the SCT is the same length but has effectively more than 3 times the focal length of the refractor).

• Uses a larger central obstruction than a similar size newtonian, usually between 30% and 35%. This drastically limits the contrast of the image. Mind you, it would cost 10 times the price of an 8” SCT to get a decent 8” refractor!

• Collimation – as with all reflector type scopes SCTs must be regularly collimated. Not too much of a problem if you have an artificial star or live in an area with decent seeing.
• Slower optics - it takes WAAAAYYY longer to get a deep sky image off one of these as opposed to a fast refractor unless you have focal reducers or something like a Fastar or Hyperstar set installed.
• Due to the long focal length, if doing astrophotography you will have very little margin for error in your polar alignment. Autoguiding is definitely required.