Above: Optical layout of a typical Maksutov-Cassegrain
Maksutov-Cassegrain telescopes are an alternative design to the similar Schmidt-Cassegrain telescope (SCT). While similar in most respects, there are a couple differences that have prevented Maksutov-Cassegrains from becoming quite as popular as SCTs. Probably not the least of which is the fact that SCTs have a nice abbreviation but no one uses the obvious MCT for Maksutov-Cassegrains. They are often just called Maks, but this can be confusing because there are also Maksutov-Newtonian telescopes. Here we will shorten the name to the popular “Mak-Cass”. Like SCTs, Mak-Casses are well-suited to a number of applications including photography and terrestrial observing as well as stargazing.
How Maksutov-Cassegrains Work
Maksutov-Cassegrain telescopes are a catadioptric design, meaning they use both lenses and mirrors. Mak-Casses are primarily reflectingtelescopes, but they use a corrector lens to eliminate aberrations that would result from the mirror design alone. In a Mak-Cass the incoming light passes through the Maksutov corrector lens (sometimes called a meniscus corrector, because of its shape) at the front of telescope. It is reflected from a concave primary mirror at the back of the scope which focuses the light to the front of the telescope where it is reflected again by a smaller, convex secondary mirror. Finally, the light travels back through a hole in the primary mirror to the rear of the scope where an eyepiece is located for visual observing (or a camera for photographing). By folding the light in this manner, a Mak-Cass can be made much smaller than an equivalent Newtonian or refractor.
At first it seems that an Maksutov-Cassegrain should have a focal length twice as long as the length of the tube, since the light travels twice the length of the tube. However, Mak-Casses actually have focal lengths about 5-7 times longer than the tube length. This is due to the convex curvature of the secondary mirror. This effectively magnifies the focal length, making the scope act much longer than it is. The diagram below shows how a Cassegrain type telescope (such as a Mak-Cass or similar design) works to increase the effective focal length. The primary mirror of a Maksutov-Cassegrain usually has a focal length only about three times its diameter. For example, a 7″ telescope would have a primary mirror focal length of about 21″. This focuses the light to a point 21″ in front of the mirror. By placing a convex secondary mirror between the primary mirror and this focal point, the light is reflected back toward the primary mirror but at a less steep angle. If you trace this light cone back to where it reaches the full width of the telescope’s aperture you can see how long the effective focal length is. Most Mak-Casses have secondary mirrors that provide a magnification factor of around 5x. In this example that would make the effective focal length 105″, or 15 times the diameter of the primary mirror, in a telescope only about 20″ long.
Above: How a Cassegrain telescope creates a long effective focal length in a short tube. The actual length of the telescope is barely longer than the separation between the two mirrors.
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 focal ratios require longer exposure times, which is always a disadvantage when it comes to imaging faint objects. While some Mak-Casses have focal ratios around f/10 (equal to the usual SCT design), most are slower, in the f/12 to f/15 range.
One other drawback is that many commercial Mak-Casses create the secondary mirror simply by aluminizing a small spot on the inside of the Maksutov corrector lens. While this is very convenient and less expensive, it is not as ideal as a separate secondary mirror with a slightly different (but more optimized) curvature. For small sizes this is not much of a problem, and small Mak-Casses tend to perform very well. Some high-end Mak-Casses use optimized, aspherical secondary mirrors instead of aluminized spots, and these instruments are excellent performers. They are especially well suited to planetary observing and imaging, but they are often quite expensive.
Maksutov-Cassegrains cover a fairly large price range. Small models (around 3-4″ in aperture) start around $300 on a small equatorial mount for automatic tracking. Small goto computerized models start around $600-700. Medium-sized scopes will run about $2000 for a computerized model. High-quality Mak-Casses can be quite pricey. An excellent 3.5″ scope can cost more than $4000–easily as much as, or more than, anapochromatic refractor–and a 10″ or 12″ scope can run upwards of $10,000-15,000. Such telescopes are amazing optical performers, though.
Is a Maksutov-Cassegrain Best for Me?
Although reasonably popular in small sizes, large Maksutov-Cassegrains are fairly uncommon. A well-built Mak-Cass can be an excellent telescope, but they are primarily intended more for visual use or planetary imaging and not so much for deep-sky photography. SCTs tend to be better suited to deep-sky imaging, making them a more popular choice. While a good-quality Mak-Cass will outperform an equivalent-size SCT, it will also be considerably more expensive. Within the same price range, performance is comparable between SCTs and Mak-Casses and SCTs tend to be more popular because of their versatility. High-end Mak-Casses are an excellent choice for someone who wants the visual performance of a large-aperture refractor in a much more compact package. The price of an excellent-quality, large-aperture Mak-Cass will not be much less than that of an equivalent refractor, but it will be considerably more portable and will require a smaller, less expensive mount.
For beginners, small Mak-Casses are popular choices for compact, portable telescopes and for people who want a telescope that is suited to both astronomical and terrestrial observing. These are very much like the popular SCT scopes, but Mak-Casses are available in smaller sizes so they start at lower prices.