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Maksutov-Cassegrain telescopes: a sharp, compact planet scope

A Maksutov-Cassegrain, often shortened to Mak, is a compact catadioptric telescope that uses a thick curved meniscus lens at the front. It is prized for sharp, high-contrast views of the Moon and planets.

How a Maksutov works

A Maksutov places a thick, strongly curved meniscus corrector lens at the front of the tube. In many designs the secondary mirror is simply an aluminised spot on the back of that corrector, which makes the optics rugged and stable. Light passes through the meniscus, reflects off the primary, returns to the secondary spot, and exits through a hole in the primary to the focuser.

The design gives a long focal ratio, commonly f/12 to f/15, in a very compact tube, and holds its alignment well because so little can move.

Why it excels on the Moon and planets

A Maksutov's long focal ratio and small secondary obstruction produce sharp, high-contrast images, which is exactly what lunar and planetary detail rewards. A 90 mm to 127 mm Maksutov is a compact, almost maintenance-free instrument that punches above its size on the Moon, planets and double stars.

The sealed tube keeps dust out and collimation stable, so a Maksutov is close to a grab-and-go planet scope.

The trade-offs

The thick meniscus corrector stores heat and takes longer than most designs to cool to the night air, and images are soft until it does. The long focal length gives high magnification but a narrow field, so a Maksutov is not the tool for wide star fields or large nebulae.

As with an SCT, focusing usually moves the primary mirror, which can cause a small image shift near focus.

Eyepieces for a Maksutov

The long focal ratio means ordinary eyepiece focal lengths already give high magnification, and the design is forgiving of inexpensive eyepieces. A low-power eyepiece gives the widest field the design allows, which is still narrow compared with a short refractor, so a Maksutov is best thought of as a Moon and planet specialist.

Get the exact magnification for your scope

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Sources: Catadioptric system (Wikipedia, CC BY-SA) · Telescope Optics, Rutten and van Venrooij; Sky & Telescope optics primers