Celestron C90 Maksutov Spotting Scope

Maksutov-Cassegrain Optical Design

The MAK is a catadioptric spotting scope, which means the optical design includes both a lens and a mirror. The light coming through a MAK passes through the corrector lens to the primary mirror and back to a spherical secondary mirror before it hits the eyepiece. Images are bright, sharp, and virtually free of chromatic aberration. 

The Maksutov-Cassegrain’s optical design offers several unique benefits. Instead of a secondary mirror, the MAK features a “secondary spot” of reflective coating applied directly to the inside of the corrector plate. Because of this, the spotting scope does not require frequent collimation. Also, the secondary spot is significantly smaller, giving the MAK its signature high-contrast views.

One important thing to note about Maksutov-Cassegrain spotting scope is that the larger, heavier Maksutov corrector plate takes longer to cool down and adjust to ambient air temperature.