![]() SCTs, with their stable and sturdy tripods, are very useful for astrophotography. They are most popular in their 20cm (8") guise - I own a 20cm SCT myself. They have an ease of use not available with the other telescopes and there are plenty of readily available accessories, e.g. They are very portable as the tube does not have to be as long as that of a reflector - this is because of the light being 'folded over'. It has grown in popularity because it combines the best of a reflector and refractor. ![]() The third type that I mentioned was the Schmidt Cassegrain telescope. This can be very expensive, so very large apertures reflectors are more popular. The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to the optical systems primary mirror entrance aperture. It can be corrected using achromatic and apochromatic (modified) lenses. This, however, is only noticeable in refractors with an aperture (diameter of objective lens) of 10cm (4") or more. This is when the different wavelengths of light are brought to focus at different points due to the glass in the objective lens refracting (bending) the different wavelengths to different degrees. However, the refractor suffers from what is known as chromatic aberration. ![]() This type of telescope uses both lenses and mirrors in a compound system as can be seen in the third diagram.Įach telescope has its own advantage, for instance the refractor is better for observing the planets and the moon and the reflector for deep-sky objects (e.g. There are quite a few variations on this including the Georgian and Cassegrain reflectors.Ī third type of telescope, which has gained immense popularity over the last 30 years, is the Schmidt Cassegrain telescope (SCT). This exact system is known as a Newtonian Reflector. As the diagram illustrates, light travels down the tube where it is reflected (hence the name reflector) up to a secondary mirror near the top of the tube, which directs the light into the eyepiece. Rays of light travel through the objective (main) lens where they are focused at the focal length of the eyepiece.Ī reflector telescope, on the other hand, uses concave (bending inwards - like a cave) mirrors. If you are considering buying a telescope you would be well advised to read our Telescope Buyers' FAQ.Ī refractor telescope uses convex (bending outwards) lenses to collect, focus and magnify light. Also available are Schmidt-Cassegrain telescopes - a combination of reflector and refractor. Most telescopes fall into one of two main categories: refractor or reflector. Also, slight errors in grinding the mirrors can distort the image.A telescope is an optical instrument used to magnify and enhance the view of faraway objects, be they astronomical or terrestrial. One disadvantage of reflectors is that you occasionally have to clean and align the mirrors. Reflectors have large light gathering capacities, and can produce bright images of faint, deep-sky objects for visual observing as well as astrophotography. Large aperture primary mirrors (greater than 10 inches or 25 centimeters) can be made easily, which means that reflectors have a relatively low cost per unit of aperture. ![]() The reflector is simple and inexpensive to make. Because of their large apertures and low price, Dobsonians are well-suited to observing deep-sky objects. Dobsonians can have large apertures (6 to 17 inches, 15 to 43 centimeters). They are inexpensive to build or buy because they are made of plastic, fiberglass or plywood. They offer wider fields of view than longer focal ratio telescopes, and provide bright, panoramic views of comets and deep-sky objects like nebulae, galaxies and star clusters.Ī view inside the barrel - note the primary mirror, and the image of the secondary mirror reflected back onto the primary.ĭobsonian telescopes are a type of Newtonian reflector with a simple tube and alt-azimuth mounting (see "Telescope Mounts"). The focal ratio, or f/number, is the focal length divided by the aperture, and relates to the brightness of the image. Rich-field (or wide-field) reflectors are a type of Newtonian reflector with short focal ratios and low magnification. The Newtonian reflector was a highly successful design, and remains one of the most popular telescope designs in use today. In 1722, John Hadley developed a design that used parabolic mirrors, and there were various improvements in mirror-making. ![]()
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