10/5/2023 0 Comments Refracting telescope images![]() The larger the lens, the more light the telescope can gather. The light-gathering power of a telescope is directly proportional to the area of the objective lens. The brightness of an image from a telescope depends partly on how much light is collected by the telescope. The longer the focal length, the larger the image. The size of an image produced by a lens is proportional to the focal length of the lens. Image credit: Chabot Space and Science Center Magnification The largest lens is called the objective lens, and the smaller lens used for viewing is called the eyepiece lens.ĭiagram of what happens to light in a refracting telescope.Įight-Inch refracting telescope. Most refracting telescopes use two main lenses. This type of telescope is called a refracting telescope. They focus the light and make distant objects appear brighter, clearer and magnified. The earliest telescopes, as well as many amateur telescopes today, use lenses to gather more light than the human eye could collect on its own. This point is also called the focal point, and its distance is measured in negative units. The rays will then appear to originate from a point in front of the lens. If a lens is concave or diverging, it takes parallel rays and bends them so that they spread out. The distance from the lens to the focal point is called the focal length of the lens.ĭiagram of a convex lens, showing how the light is refracted and converges to a focal point. If a lens is convex or converging, it takes parallel light rays from a distant object and bends them so that they converge to a single point called the focal point. They are ground so that their surfaces are either segments of spheres or planes. Lenses form images by refraction and are typically made of either glass or plastic. In the diagram below, light is leaving air and entering glass, so it bends towards the normal on the way in, and away on the way out of the glass.ĭiagram showing how light changes direction as it moves into and out of a medium with a higher index of refraction. Light follows the same same principle and bends towards the normal when traveling into a medium with a higher index of refraction, and away from the normal when traveling into a medium where it can go faster. If the you approach the beach at an angle, one of the tires will be slowed down by the sand before the other is, and the car will turn in the direction of the tire that touched the sand first. If you approach the beach straight on, the car will slow down, but not change direction. Imagine driving a car from smooth pavement onto a sandy beach. ![]() ![]() The direction the light takes depends on whether it travels faster or slower in the new medium. If it enters at an angle, its speed and its direction will change. Equally important in a choice of telescope type is the focal length and the resulting field of view.If light enters the new medium at a right angle to the surface, it will change speed, but not direction. Refractors generally have the edge over reflectors for a quality image, both from the standpoint of lens/mirror quality and precision optical mountings, and lack of central obscurations. Refractors don't suffer from this problem. While not aesthetically objectionable, it is true that any obstructions can degrade the image a little. Reflecting telescopes, with their secondary mirror (generally with mirror supports), often produce "diffraction spikes" around star images.However, the brightness of a nebula is proportional to D2/FL2 since the image of the nebula is also spread out over x and y dimensions by an amount proportional to the focal length (FL). For photographs of nebulae and galaxies, the focal ratio of the telescope is very important! The brightness of stars (point sources) is proportional to D2 (the Diameter of the telescope, and D2 is proportional to the collecting area, pi*R2).Plate scale (in arcseconds/mm) and the amount of the sky shown on your image depends only on the focal length of the telescope. Are you interested in a moderately wide field, say enough to capture the Orion nebula? Then a short focal length telescope (and correspondingly smaller plate scale) will get more of the sky onto film of a given size.Are you interested in photographing a very faint object? Perhaps a faint Cepheid variable star? A large reflector will collect a lot more light, and the resulting small field of view is not a problem.Is a reflector better for astrophotography? Or a refractor? Well.
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