November 11, 2008

In Today's One Minute Astronomer...

Maksutov-Cassegrain Telescopes

Small Maksutov-Cassegrain telescopes are a godsend for urban observers who need a compact scope with a go-to mount and 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.

The Basics

• Modern Maks are much like 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 curve 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 Schmidt, 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 particular eyepiece and a narrower field of view than with an f/10 Schmidt or f/6 Newtonian. (See Easy Math for a review of how this works).

• 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 like planets, the Moon, double stars, globular clusters, and planetary nebulae.

A Deeper Look

• 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 a deep-sky object.

• The corrector lens on a Mak is quite thick, which means Maks get heavy at higher apertures because the lenses become massive. That's why you won't find commercially-made Maks with apertures larger than 7 inches.

• The most famous (and expensive) Makustov 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.

Good To Know

Because they are rugged and robust, Maksutov-Cassegrains are used in harsh environments in industrial and military applications. And more than a few field photographs in National Geographic have been made with Maks.

Personal View

Maks have come back into style largely because of the success of Meade's ETX-90 and ETX-125 telescopes. These scopes are fantastic for astronomical and terrestrial viewing, with fine optics that rival far more expensive refractors. And they're quite easy to move around and set up. You can get a 5-inch ETX-125PE with tripod and Go-To computer for less than $1000. Not too shabby.

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