How Is a Maksutov-Cassegrain Different from an SCT?
The main difference between Schmidt-Cassegrains and Maksutov-Cassegrains is the corrector lens at the front of the telescope. Both scopes use spherical mirrors which induce spherical aberration. The corrector lenses eliminate this aberration, allowing the scopes to be relatively inexpensive compared to other all-reflective Cassegrain designs which require expensive aspherical mirrors. SCTs use a flat Schmidt corrector which has a very slight aspheric curve. Mak-Casses use a meniscus corrector, a highly curved spherical lens, to achieve the same effect. The usual drawback to the Maksutov design is that the corrector is much thicker (usually around 10% of the aperture, compared to about 3% for a Schmidt corrector). This leads to a longer time required for a Maksutov to thermally equalize to outside temperature, especially for large sizes (greater than 6″). Also, Maksutov-Cassegrains usually have longer focal ratios than SCTs, making them less well-suited for deep-sky photography. Longer
