Home / Guides / Schmidt-Cassegrain telescopes: a long focal length in a short tube
Schmidt-Cassegrain telescopes: a long focal length in a short tube
A Schmidt-Cassegrain, or SCT, folds a long focal length into a short, portable tube using a thin corrector plate at the front and two mirrors inside. It is the most popular compact catadioptric design.
How an SCT folds the light path
Light passes through a thin aspheric corrector plate at the front, falls on a spherical primary mirror at the back, reflects forward to a convex secondary mirror mounted on the corrector, and is sent back through a hole in the primary to the focuser behind the tube. Folding the path this way packs a long focal length, typically around f/10, into a tube less than half its focal length long.
The result is a telescope with a lot of reach for planets and small deep-sky objects that is still compact enough to carry easily. An eight inch SCT is a common, capable, transportable instrument.
What an SCT suits
The long f/10 focal length gives high magnification for the Moon, planets and small objects like planetary nebulae and galaxies without needing extreme eyepieces. The closed tube keeps the optics clean and holds collimation well.
SCTs are also a popular imaging platform, especially with a focal reducer that shortens the effective focal length and widens the field. Their compactness makes them a favourite for observers who travel to darker skies.
Things to know
The closed tube takes time to cool to the night air, and a long focal length means a narrow field of view, so an SCT is less suited to sweeping large star fields than a short refractor or fast Newtonian. Focusing is usually by moving the primary mirror, which can cause a slight image shift near focus.
Most SCTs use a standard rear cell thread, often the 2 inch SCT thread, that accepts visual back, diagonal, reducer and camera adapters. Confirm the specific thread on your model before buying accessories.
Eyepieces and reach
Because f/10 is a long focal ratio, an SCT reaches high magnification with ordinary eyepiece focal lengths, and it is forgiving of budget eyepieces. Its long focal length also makes a low-power, wide-field view hard to achieve, so a 2 inch diagonal and a long focal-length eyepiece, or a reducer, are worth having for deep-sky sweeps.
Get the exact magnification for your scope
Open the Fit My Telescope toolMatch eyepieces to your SCT's focal lengthFind schmidt cassegrain telescope on eBayaffiliateRelated
Sources: Catadioptric system (Wikipedia, CC BY-SA) ยท Telescope Optics, Rutten and van Venrooij; Sky & Telescope optics primers