![]() Close-up lunar and planetary photography generally requires such a mount and motor drive. If a family shares the telescope, however, an equatorial mount with a motor drive will keep objects centered in the field of view so all can share the same view. The relatively small aperture of a refractor therefore often has an advantage over a larger reflector-type scope for this kind of observing, as there is less glare from a larger scope's brightly lit planetary surfaces to wash out faint detail.įor purely visual lunar, planetary, binary and star cluster observing, an altazimuth refractor with manual slow motion controls may be perfectly adequate. Since the Moon and planets are all brightly lit by the Sun, a large light-gathering capacity is not as important as high magnification within the solar system. With no diffraction spikes to hide faint binary star components or smear globular clusters, refractors can often resolve close-spaced stars more precisely than the typical reflector. A good 80mm refractor, for example, can reveal more lunar detail than you can sketch in a lifetime of observing.ĭiffraction spikes on a reflector's star images, caused by its diagonal mirror's spider vanes, are absent in an unobstructed refractor. A smaller refractor looks through less of our unstable atmosphere and its images are consequently less affected by this turbulence. This is especially true on nights of less-than-perfect seeing, when the details visible in a larger scope are often blurred by turbulence in our atmosphere. The result of a refractor's lower diffraction and higher light transmission? Given favorable seeing conditions, a modestly-sized refractor can often show you subtle lunar and planetary features with a wider and more easily observed contrast range, and with more sharply etched detail, than is possible with the light-scattering optics of larger reflectors and catadioptrics. Century-old refractors are still used, and highly prized, by discerning amateurs, and the world's largest refractor - the Yerkes Observatory's massive 40" - has been in professional use since 1897. Unlike reflectors and catadioptrics, which can lose 1% to 1.5% of their reflectivity per mirror surface per year as their aluminum coatings gradually oxidize, the light transmission of a low-maintenance refractor rarely deteriorates significantly with age. They do not take into account the light blocked by a reflector or catadioptric's diagonal or secondary mirror, which can reach a hefty 15% to 20% additional light loss in some scopes.) (The reflector and catadioptric percentages only concern the reflectivity of standard aluminum mirror coatings. Refractors typically transmit 90% or more of the light they collect, compared with the 77% to 80% transmission of reflectors and 64% to 75% of catadioptrics. Refractors also have the highest light transmission - the percentage of the light gathered by the scope that actually reaches your eye. ![]() Why? Unlike reflectors and catadioptrics (Schmidt-Cassegrains, Maksutov-Cassegrains, etc.), refractors do not have a secondary mirror obstruction or multiple-reflection optical path to introduce light-scattering diffraction and internal reflections that brighten the sky background, reduce contrast, and smear images. Under average seeing conditions, a useful rule of thumb in astronomy is that a good quality 3" to 4" refractor can often outperform an average quality 6" to 8" reflector or catadioptric telescope for seeing details on the Moon and planets, splitting binary stars, and resolving globular clusters.
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