Catadioptric system
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors (catoptrics). Catadioptric combinations are used in focusing systems such as search lights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses. Other optical systems that use lenses and mirrors are also referred to as "catadioptric" such as surveillance catadioptric sensors.Catadioptric telescopes are optical telescopes that combine specifically shaped mirrors and lenses to form an image. This is usually done so that the telescope can have an overall greater degree of error correction than their all-lens or all-mirror counterparts, with a consequently wider aberration-free field of view. Their designs can have simple all-spherical surfaces and can take advantage of a folded optical path that reduces the mass of the telescope, making them easier to manufacture. Many types employ “correctors”, a lens or curved mirror in a combined image-forming optical system so that the reflective or refractive element can correct the aberrations produced by its counterpart.
Catadioptric lenses do, however, have several drawbacks. The fact that they have a central obstruction means they cannot use an adjustable diaphragm to control light transmission.[13] This means the lens's F-number value is fixed to the overall designed focal ratio of the optical system (the diameter of the primary mirror divided into the focal length). Exposure is usually adjusted by the placement of neutral density filters on the front or rear of the lens. Their modulation transfer function shows low contrast at low spatial frequencies. Finally, their most salient characteristic is the annular shape of defocused areas of the image, giving a doughnut-shaped 'iris blur' or bokeh, caused by the shape of the entrance pupil.
Several companies made catadioptric lenses throughout the later part of the 20th century. Nikon (under the Mirror-Nikkor and later Reflex-Nikkor names) and Canon both offered several designs, such as 500 mm 1:8 and 1000 mm 1:11. Smaller companies such as f.e. Tamron, Samyang, Vivitar, and Opteka also offered several versions, with the three latter of these brands still actively producing a number of catadioptric lenses for use in modern system cameras. Sony (formerly Minolta) offered a 500 mm catadioptric lens for their Alpha range of cameras. The Sony lens had the distinction of being the only reflex lens manufactured by a major brand to feature auto-focus (aside from the identical Minolta-manufactured lens that preceded Sony's production).
Refracting telescope
A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes the refracting telescope has been superseded by the reflecting telescope which allows larger apertures. A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece.
A Refracting Telescope
Refractors suffer from residual chromatic and spherical aberration. This affects shorter focal ratios more than longer ones. A 100 mm (4 in) f/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 100 mm (4 in) f/16 has little color fringing.
In very large apertures, there is also a problem of lens sagging, a result of gravity deforming glass. Since a lens can only be held in place by its edge, the center of a large lens sags due to gravity, distorting the images it produces. The largest practical lens size in a refracting telescope is around 1 meter (39 in).
There is a further problem of glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished in reflecting telescopes, which can be made in far larger apertures and which have all but replaced refractors for astronomical research.
Reflecting telescope
A reflecting telescope (also called a reflector) is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a "catoptric" telescope.
a reflector telescope
Drawbacks
Reflecting telescopes, just like any other optical system, do not produce "perfect" images. The need to image objects at distances up to infinity, view them at different wavelengths of light, along with the requirement to have some way to view the image the primary mirror produces, means there is always some compromise in a reflecting telescope's optical design.
Because the primary mirror focuses light to a common point in front of its own reflecting surface almost all reflecting telescope designs have a secondary mirror, film holder, or detector near that focal point partially obstructing the light from reaching the primary mirror. Not only does this cause some reduction in the amount of light the system collects, it also causes a loss in contrast in the image due to diffraction effects of the obstruction as well as diffraction spikes caused by most secondary support structures.
Which one you should consider before buying a Telescope ?
The catadioptric or compound telescope. These were invented in the 1930s out of a desire to marry the best characteristics of refractors and reflectors: they employ both lenses and mirrors to form an image. The greatest appeal of these instruments is that, in their commonly encountered forms (the Schmidt-Cassegrain and Maksutov-Cassegrain), they are very compact. Their tubes are just two to three times as long as wide, an arrangement allowed by "optical folding" of the light. The smaller tube can use a lighter and thus more manageable mounting. The upshot is that you can obtain a large-aperture, long-focus telescope that's very transportable. Its the combination of both reflector and refractor telescope.
The second type of telescope, the reflector, uses a mirror to gather and focus light. Its most common form is the Newtonian reflector (invented by Isaac Newton), with a specially curved concave (dish-shaped) primary mirror in the bottom end of the telescope. Near the top a small, diagonal secondary mirror directs the light from the primary to the side of the tube, where it's met by a conveniently placed eyepiece.
A refractor is the stereotype of how a telescope is supposed to look — a long, gleaming tube with a large lens in front and an eyepiece at the back. The front lens (the objective) focuses light to form an image in the back. The eyepiece is a little magnifying glass with which you look at the image.
High-quality refractors are often sought out by lunar and planetary observers who value their crisp, high-contrast images that can take high magnification. In fact, when well made a refractor can provide the finest images attainable with a given aperture.
Bottomline
In practice, many people seeking a highly versatile, very portable (for the aperture) scope that can be used for all sky subjects and for astrophotography will tend to opt for some form of compound instrument. Scopes of this type also tend to be the most highly "technologized," with many options such as computerized pointing and photographic adaptations. In short, they're excellent general-purpose scopes that can use a wide variety of accessories.
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