Above: Optical layout of a typical Schmidt-Newtonian
Schmidt-Newtonian telescopes are a modified version of a standard Newtonian. They incorporate a Schmidt corrector lens like that found on aSchmidt-Cassegrain telescope. This lens minimizes some of the aberrations associated with a standard Newtonian. Schmidt-Newtonians are usually well-suited to both visual observing and photography. The name of these scopes is usually shortened to Schmidt-Newt.
Above: Optical layout of a typical Maksutov-Newtonian
Maksutov-Newtonians are very much like Schmidt-Newtonians except that they use a meniscus corrector lens like that of a Maksutov-Cassegrainto minimize the aberrations found in standard Newtonians. They tend to have excellent optical performance and are good for photography as well. The name Maksutov-Newtonian is often shortened to Mak-Newt.
How Schmidt-Newtonians and Maksutov-Newtonians Work
Schmidt- and Maksutov-Newtonians are catadioptric telescopes, meaning they use both mirrors and lenses. Like regular Newtonian scopes, there is a primary mirror located at the back of the telescope, which reflects light up to a secondary mirror at the front of the scope. This secondary mirror is tilted at a 45° angle to reflect the light out the side of the tube to where an eyepiece would be located for viewing (or a camera for photography). However, Schmidt-Newts and Mak-Newts add an additional optical element, a corrector lens, at the front of the telescope. The lens is used to minimize the aberrations inherent in the Newtonian design. The corrector lens also allows the secondary mirror to be supported without the metal spider vanes needed in a standard Newtonian. This eliminates the diffraction spikes that can slightly degrade the image in a Newtonian.
Like standard Newtonians, the catadioptric versions produce an inverted image which makes them unsuited to terrestrial observing, but they are excellent for stargazing. Because of the lesser aberrations relative to a standard Newtonian, Schmidt-Newts and Mak-Newts are better suited to photography than a regular Newtonian.
Schmidt-Newtonian and Maksutov-Newtonian Optics
Newtonian telescopes use parabolic primary mirrors to eliminate spherical aberration. While spherical mirrors are easier to make, they do not produce sharp images. The diagram below demonstrates spherical aberration resulting from a spherical mirror. Note that the outer parts of the mirror focuses light to a point closer to the mirror than do the inner parts. A parabolic mirror would focus light to a single point.
Above: A spherical mirror focuses light rays from different off-axis distances to different points, causing spherical aberration
Catadioptric Newtonians such as Schmidt-Newts and Mak-Newts still use spherical primary mirrors, but they also add a corrector lens over the fullaperture at the front of the telescope to eliminate spherical aberration. This lens also has the effect of minimizing the coma inherent in Newtonian design. Coma is an off-axis aberration, meaning it only affects the outer parts of the image and not the center of the image. Coma results in the stars at the edge of the field having a comet-like shape with the narrow end pointed toward the center of the field.
Above: Shape of an off-axis star affected by coma. The center of the field is down in this diagram.
Coma prevents Newtonians from being ideal for wide-field photography because the stars are not sharp across the full field of view. Schmidt-Newts and Mak-Newts produce less coma than equivalent Newtonians, so they are better suited to deep-sky imaging. For example, an 8″ f/4 Schmidt-Newtonian has 250% less coma than an equivalent Newtonian and produces stars that are 1.6 times smaller at the edge of the field. An equivalent Mak-Newt has 350% less coma and stars 3.6 times smaller than a Newtonian.
Above: Spot diagrams showing the size and shape of stars at the edge of a 1° field in a standard Newtonian and each catadioptric Newtonian. Spots generated using ATMOS software by Massimo Riccardi.
The corrector lens in a Schmidt-Newtonian is relatively thin (about 3% of the aperture). This allows the telescope to equalize to ambient temperature faster than the thick (10% of the aperture) meniscus corrector in a Maksutov-Newtonian.
Schmidt-Newtonian and Maksutov-Newtonian Prices
The most common sizes for catadioptric Newtonians are between 5″ and 10″ in aperture. Some larger Maksutov-Newtonians are manufactured, but these are specialized, expensive instruments. Schmidt-Newtonians are generally inexpensive, compared to equivalent-aperture Schmidt-Cassegrains, although they are a bit more expensive than an equivalent Newtonian. They are well-suited to deep-sky viewing and imaging, if mounted on an adequate equatorial mount. Schmidt-Newtonians usually cost about $700 to $1300 for computerized goto models. Maksutov-Newtonians are usually more expensive for a given size, but are excellent quality telescopes, providing exquisite views, often comparable to a similar-sized apochromatic refractor. A good quality 5″ Mak-Newt starts around $900 for the optical tube only (no mount). 7″ Mak-Newts can run from $2500-5000, again for the optical tube only. Instruments larger than 8″ in aperture are usually custom made items and can cost over $10,000.
Is a Schmidt-Newtonian or Maksutov-Newtonian Best for Me?
The main advantage of a Schmidt-Newtonian over a standard Newtonian is that it is better for photography. Fast focal ratio Newtonians, which might otherwise be good for imaging, suffer from very large amounts of coma. An equivalent fast Schmidt-Newt, on the other hand, produces much sharper star images across a wide field of view. So, for a relatively inexpensive, large-aperture imaging scope, a Schmidt-Newtonian can be a good choice. Keep in mind that most current models of Schmidt-Newtonian, especially in the larger sizes, are not adequately mounted for long-exposure photography and must be used with a larger German equatorial mount.
Maksutov-Newtonians are more expensive scopes, but for an observer who wants the advantages of a high-quality refractor (wide field of view, photographic capabilities, and excellent planetary images) but don’t like the high price, Mak-Newts are a good alternative. They are still pricey, but nowhere near as expensive as the best apochromatic refractors. They are typically slower than Schmidt-Newtonians (usually f/5 or f/6 instead of f/4), but are still fairly fast and good for imaging.