Maksutov–Cassegrain Telescopes
The Maksutov–Cassegrain is a compound optical system with vertical curvature elements. As a result, adjusting to ambient temperatures takes much longer than a refractor. If taken from inside to outside in winter, a Maksutov–Cassegrain will require at least an hour to form acceptable images. An 80-mm refractor will need only 15 minutes.
The effect of poor viewing of star images is to broaden the Airy disk and increase the brightness of the secondary rings of the diffraction pattern. It is similar to the effect of the secondary mirror on the image. These two effects reinforce each other. This effect makes a Maksutov more sensitive to atmospheric conditions than a refractor.
Aperture-dependent performance of the 90-mm Maksutov is similar to that of an 80-mm refractor. If well made, the The Maksutov–Cassegrain is essentially free of image aberrations. Its most significant deficiency is a lack of versatility, due to a long focal-length instrument with a narrow field of view.
The Maksutov–Cassegrain excels for observations of the Moon, planets, and binary stars. But is more difficult to use for variable stars and other wide-field applications. With good seeing and adjustment to ambient temperature, the Maksutov is also a highly portable instrument. One must always, however, be aware of its limitations.
The Maksutov–Cassegrain is an excellent choice for users in moderately light-polluted areas interested in planetary, and deep space star observation.
Newtonian Reflectors
Newtonian reflectors are perfectly achromatic. With a parabolic primary mirror, they are free of spherical aberration. The cost per millimeter of the aperture is less than other optical systems. But they also have limitations that restrict their versatility as practical small telescopes.
Although a parabolic mirror is free of spherical aberration, another type of image error, coma, restricts the proper field of view. This effect, which consists of elongated star images away from the center of the field. It becomes more significant with decreasing focal length. Another problem is that to produce a fully illuminated field, the size of the secondary mirror has to increase as the f/ratio decreases. Therefore introducing diffraction effects detrimental to image contrast at high magnification.
This combined effect of coma and diffraction by the secondary restricts short-focus Newtonian reflectors to low-power observations. One can marginalize these detrimental effects if the instrument has a f/ratio of f/8 or higher. At such focal lengths, a Newtonian can perform as well as other optical systems discussed here. The field of view of the 114-mm f/8 reflector is approximately the same as that of an 80-mm f/11 refractor. While it has a fainter limiting magnitude.
Cooling-down problem
The Newtonian has the same cooling-down problem as Maksutov–Cassegrain telescopes. In addition, its open tube leaves the mirrors susceptible to environmental deterioration. Causing the internal tube currents to affect the image. The mirrors must be cleaned and realigned periodically. Consequently, Newtonian requires substantially more maintenance than other types of telescope.
A major problem with the 114-mm f/8 Newtonians is that they are made with spherical rather than parabolic mirrors to reduce manufacturing costs. Their spherical aberration is not noticeable at low magnification. But it severely affects contrast and image sharpness at high magnification. Since most distributors do not mention this, the only way you can find out is to call and ask.
Eyepieces
The eyepiece is as essential to a Maksutov–Cassegrain telescopes performance as the objective lens. Having an excellent objective paired with an eyepiece that needs to be more adequately corrected for image aberrations makes little sense. It also makes little sense to use an eyepiece that costs as much and weighs half as much as the entire tube assembly.
The apparent field of an eyepiece would be its field of view if it were being used as a magnifying glass. The telescope’s field of view is equal to the apparent field of the eyepiece. This is divided by the magnification that would be obtained with that eyepiece.
For example, a telescope with a focal length of 900 mm will have a magnification of 36× when used with an eyepiece with a focal length of 25 mm. If the eyepiece’s apparent field is 52, the telescope’s field of view will be equal to 52/36 or 14
The eyepiece choice for a particular observation depends on what is being observed. The telescope will take on a given night for the Moon, planets, and binary stars using the highest effective magnification. For star clusters and nebulae, a low to medium magnification is best. The Plossl design with a 52 apparent field is a good choice for general use. For more information its best to ask your distributors or retailer.
Focusing
The ability to precisely focus an image is another design factor that eludes Maksutov–Cassegrain Telescopes manufacturers. Good telescopes have a single, precise focal point where a sharp, high-contrast image is formed. Finding this point while observing can be frustrating, if not impossible if the focusing device is poorly made.
These difficulties often obfuscate otherwise superb optics. Buy something other than a telescope with a cheaply made plastic focusing device. If you encounter vibrations induced by unsteady hands or mount instabilities, you can eliminate them by adding a battery-powered focusing motor.
The Finder
The finders with most commercial Maksutov–Cassegrain telescopes range from barely adequate to useless. Fortunately, most telescopes have a dovetail finder mount that easily replaces a faulty finder. Any straight-through finder, regardless of quality, is difficult to use for objects near the zenith with any telescope.
It should be replaced with an excellent right-angle finder with a minimum aperture of 30 mm. An 80-mm f/5 refractor doesn’t need a finder; its 32-field with a 25-mm eyepiece is sufficient.
Recommended Accessories
Many of the following telescope accessories are helpful, but not all are essential for the observations described in the book. If you are an experienced observer wanting to get the most out of a small telescope, you probably have them already and are familiar with their use. If you are a beginner, you can add them as your particular interests deem necessary.
An additional set of color filters for planetary observations provides higher contrast in certain aspects of their images. A 12.5-mm eyepiece with an illuminated crossline reticle is helpful for some types of visual observations and for tracking long-exposure film photography.
A piggyback mounting bracket for a camera equipped with a telephoto lens is necessary for some digital and film photography. An electronic stopwatch is essential for lunar occultation measurements. Timing eclipses and transits of Jupiter’s moons, and timing the transit of Jupiter’s red spot. A red flashlight aids in reading camera settings and recording data.
