| REFRACTOR: A telescope design that uses lenses to refract(or bend)light into a converging cone. The cone comes to a point near the eyepiece at the near end of the tube. Refractors are generally slim and long for their aperture.The most common variations of the design are achromatic, apochromatic, and Petzval. The achromat generally uses 2 lenses of simple glass types. Light is actually a comination of all the colors of the spectrum. Achromats are unable to bring all the colors to the same focus point which results in chromatic aberration(false color). This will be seen as a purple haze across and around bright objects. Apochromatic refractors use exotic glass types, and often 3 lenses, to bring all colors to the same focus point. The result is sharp images without false color. Petzval refractors use a group of 2 lenses in the front and 2 lenses near the rear. Quality Petzvals also use exotic glass types to control false color, but the main goal of the design is to provide a wide field by providing a slow focal ratio without having to bend the light cone too tightly, which also contributes to false color. REFLECTOR: A telescope design that uses curved mirrors to bring light to a converging cone. Since the light is being reflected as opposed to refracted, no false color is created. The primary mirror reflects the light cone to a secondary mirror which in turn reflects it the eyepiece. The secondary mirror is usually placed in the middle of the tube which results in some light being blocked. This blockage is called the central obstruction(or CO). The larger the CO, the more image contrast is affected. The most common types are Cassegrain and Newtonian reflectors. Cassegrains use a secondary mirror that reflects light back towards the rear of the tube though a baffle tube in the middle of the primary mirror, and to the eyepiece. Newtonians use a secondary mirror on a 45* angle to reflect light out the side of the tube near the front. CATADIOPTRIC: A telescope design that uses both a lens and mirrors. Reflected light will exhibit blurred images(or coma)as the curve of the mirror gets deeper. Cats use deep curved mirrors to narrow the light cone in a relatively short distance to remain compact. The level of coma produced by these mirrors is compensated for by the front lens(or meniscus). The most common types are Schmidt Cassegrains, Maksutov Cassegrains, and Maksutov Newtonians. A Schmidt lens requires complex varying curves to be effective, which is extremely difficult to produce properly. A Maksutov lens is a simple spherical shape which is much easier to produce to exacting standards. Maksutov telescopes also use spherical surfaces on their mirrors. Spherical surfaces also less prone to micro scratching when they are made since no differentiating angles need to be blended. The result is less light scatter and higher contrast. APERTURE: This term basically means size. It refers to the diameter of the lens in a refractor or catadioptric, or the primary mirror size in a reflector. The bigger the aperture, the more light is gathered. The result is brighter images and more resolution for small or dim objects or features. FOCAL RATIO: This term refers to the distance a light cone takes to come to a point as compared to it's aperture. If a scope has an aperture of 100mm, and it's focal ratio is f/6, the light cone tapers down to a point in a distance of 600mm. The result is a scope's focal length. The shorter a scope's focal length, the more capable it will be to provide wide fields of view. The longer a scope's focal length, the narrower the field of view, but the easier it is for eyepieces to provide higher magnification views since they too have focal lengths of their own. The shorter an eyepiece's focal length, the higher the magnification. While the actual magnification provided by a given eyepiece is dependent upon it's field stop(a term used for it's aperture), it's safe to use this simple formula:scope focal length/eyepiece focal length=magnification power. |
