![]() These words refer to the photographic exposure times required. f/6) are called fast, while larger focal ratios (e.g. Small refractors tend to be around f/6, while Schmidt-Cassegrain Telescopes tend to be f/10. ![]() It is written in the form “f/6” and typical focal ratios are f/5 to f/10. Focal Ratioįocal ratio is a calculated value: the ratio of focal length to aperture. Longer focal lengths are, generally, harder for beginners to use for astrophotography since the long focal length also magnifies the imperfections in your system, especially instability and tracking errors in your mount. Shorter focal lengths will allow you to image very large objects that may not fit in the field with a longer focal length. We’ll discuss the correct terminology later, but for now this will give you the idea.) Longer focal lengths will give you larger images of given objects, or enable you to image smaller objects. (Note: serious astrophotographers hate the word “magnification” and never use it. Refractors usually have focal lengths in the 400mm to 1000mm range, Newtonian reflectors in the 1000mm to 1500mm range, and Schmidt-Cassegrains in the 1200mm to 4000mm range.įocal length determines the magnification of your telescope-camera system. Schmidt-Cassegrains) the light travels back and forth inside the tube more than once, so the focal length will be considerably longer than the main tube. For refractors and reflectors, this is approximately the length of the main tube. Focal LengthĪ telescope’s focal length is the distance the incoming light travels from the first lens or mirror to the detector (your eye at an eyepiece, or the film or electronics in a camera). Remember that large aperture reduces the portability of your gear, and may also create capacity and balance problems for your mount that make it less stable. So you should select an aperture that meets your visual observation needs, and not worry too much about it for astrophotography purposes. For beginner-level astrophotography this is not a critical factor but, as you become more advanced, you may want to image very small objects, or very fine detail on large targets, where the greater resolving power of a larger aperture is important.Īs a beginner to astrophotography, you can get good results with any aperture from about 80 mm 3 inches and up. Resolution: The diameter of the main lens also determines the resolving power of a telescope – the smallest or closest objects that can be distinguished from one another.That is why you can take images with a small aperture telescope that show far more detail that you can observe visually with even a considerably larger scope. For astrophotography it is slightly less important, because you can use longer exposures to gather more light - something your eye cannot do. using your eye and an eyepiece) because it determines what you can see. ![]() For example, a 200 mm 8-inch telescope does not gather twice as much light as a 100 mm 4-inch telescope – it gathers about four times as much.Īperture is very important for visual observation (i.e. The light gathering power is proportional to the area of the lens or mirror, which depends on the square of the radius, so it increases rapidly with increased aperture.
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