Maksutov-Cassegrain Telescopes

A Questar 3.5" Maksutov-Cassegrain telescope.

A Questar 3.5″ Maksutov-Cassegrain telescope.

Small Maksutov-Cassegrain telescopes are a godsend for urban observers who need a compact scope with good optics. And “Maks” are back in style, so there’s a good selection on the market. But Maks aren’t for everyone. Here’s how to tell if a Mak is right for you.

Maksutov-Cassegrains are another type of compound telescope, similar to Schmidt-Cassegrains. They have a spherical mirror to collect light and a curved lens up front to correct for aberrations. But the corrector lens on a Mak has a simple spherical curvature which is easy to manufacture. And the secondary mirror is simply a thin layer of aluminum deposited on the back of the lens. So unlike a Newtonian or SCT, a Mak requires no alignment.

The downside of the Mak’s optics? To keep aberrations small, Maks are made with a long focal ratio… typically f/12 to f/15. That means you get a higher magnification with a given eyepiece and a narrower field of view than with an f/10 Schmidt-Cass or f/6 or f/8 Newtonian. So Maks aren’t great if you want wide, sweeping views of the Milky Way.  They’re much better for objects that require high magnification such as planets, the Moon, double stars, globular clusters, and planetary nebulae.

A Skywatcher 150mm (6") Maksutov-Cassegrain telescope on an equatorial mount.

A Skywatcher 150mm (6″) Maksutov-Cassegrain telescope on an equatorial mount.

Maks are great for urban observers for two reasons. They are compact and easy to transport. And the higher magnification will darken the washed-out city sky and bring out more contrast in deep-sky objects.

They are rugged and robust, so Maksutov-Cassegrains are used in harsh environments in industrial and military applications. More than a few field photographs in National Geographic have been made with Maks. But because the corrector lens on a Mak is quite thick, these scopes get heavy at higher apertures. That’s why you won’t find commercially-made Maks with apertures larger than 7 inches (175 mm).

The most famous (and expensive) Maksutov telescope is the Questar. First made in 1954, a Questar is like a fine Swiss watch. These scopes have superb mechanics and razor-sharp optics almost without aberration. Questars are widely used for terrestrial observing and nature photography. And NASA used Questar telescopes on its early space missions. They are, however, outrageously expensive for their small apertures. A basic 3.5″ Questar costs $4,000 and up.

Less elaborate Mak-Cass scopes, such as the 6″ Skywatcher version shown above, go for just over $1,000 (without the mount). That’s still more expensive than a Schmidt-Cass of the same size.

Maksutov-Cassegrain Pros

  • Compact and versatile
  • Very little chromatic aberration
  • Large aperture
  • Alignment is rarely required

Maksutov-Cassegrain Cons

  • More expensive than Newtonians or Schmidt-Cassegrains for the same aperture
  • Narrow field of view in Maksutov design
  • Not available in large aperture

Maksutov-Cassegrain are best for

  • Observing of the Moon, planets, double stars, and narrow-field views of deep-sky objects
  • Observers with a larger budget who still want aperture but who favor portability
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