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Optics Type: achromatic refractorLens diameter: 120mmFocal length: 1000mmEyepiece: 2xBarlow lens 20mmAluminium/steel/plastic tripod
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Refractor telescopes have along lifespan
Refractingtelescopesare very durable unlike reflectors and catadioptric systems, which annually lose 1 to 1.5%mirror surface reflectivity because of slow oxidation of the aluminiumcoatings. Light transmission of a low-maintenance refractor telescopedoes not decrease over time. Refracting telescopes that are hundreds of yearsold are still being used and commended by discerning amateurs. The biggestrefractor in the world -the gigantic 40 inch telescope of the YerkesObservatory- has been professionally used since 1897!
Coma (an optical defect which shows stars as a kind of Vshape on the edges of the field of view) does not occur in refractingtelescopes. Refractors are better able to show stars that are close to eachother (the so-called binary star observations). As the moon and the planets areall brightly lit by the sun, the light collection capacity is not as importantas magnifications in the solar system. This is why the relatively small openingof a refractor is often an advantage for these kinds of observations, just likethe big magnifying capacity caused by their relatively long focal length is anadvantage of reflectors.
What is the result of lower diffraction and more lighttransmission? Under favourable weather conditions a simple refractor can showyou more subtle lunar and planetary features with a wider contrast range andsharper detail than reflectors and catadioptric telescopes can do. This isespecially true for less than ideal viewing nights, when turbulence in ouratmosphere blurs the details discernible with big telescopes. A small refractor looks through asmaller area, and, as a result, the images are simply less affected by theaforementioned blurring. By way of example, a good 80mm refractor can show youmore lunar details than you will ever be able to see with the naked eye.
Refracting telescopes have better light transmissioncharacteristics. Contrary to reflectors and catadioptric systems(Schmidt-Cassegrain, Maksutov-Cassegrain, and so on and so forth) refractors donot have secondary mirror obstruction or multiple reflection to brighten thebackground of the sky, reduce contrast and smear images. Refractor telescopessimply have the most light transmission – the percentage of the collected lightgathered by the telescope that really reaches your eye. Refractors can transmitmore than 90% of the collected light. This considerably differs from the 77 to80% light transmission of reflectors and the 64 to 75% transmission ofCatadioptric systems. We should add that the percentages for reflectors andcatadioptric systems only involve mirror reflectivity. They do not cover thelight blocked by a reflecting telescope or the diagonal or secondary mirror ofa catadioptric telescope. This blockage accounts for a spectacular 15 to 20%extra light loss in some telescopes.
Category: Cameras and Photography:Telescopes and Binoculars:Telescopes
Location: Reading
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