英和和英

出典:『Wikipedia』 (2011/02/28 20:13 UTC 版)

英語による解説

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A star diagonal is an angled mirror or prism used in telescopes that allows viewing from a direction that is perpendicular to the usual eyepiece axis. It allows more convenient and comfortable viewing when the telescope is pointed at, or near the zenith (i.e. directly overhead). Also, the resulting image is right side up, but is reversed from left to right. Star diagonals are available in 1.25" and 2" diameters. The 2" diagonals allow larger longer focal length low power 2 inch barrel eyepieces for a greater field of view. Star diagonals come in all price ranges, from as low as thirty dollars up to hundreds of dollars. However , even an expensive star diagonal will deliver poor performance if it is not in alignment with the optical axis of the telescope. A telescope in perfect collimation will be thrown out of collimation by a misaligned star diagonal and often this misalignment will determine the image quality of the telescope to a larger extent than the surface accuracy of the prism or mirror. Since the mirror or prism of the star diagonal is located nearly at the focal plane of the instrument, surface accuracy of greater that 1/4 wave is more in the line of advertising than any increase in optical performance. A 1/10 wave mirror or prism star diagonal that throws off the collimation of the telescope will perform worse than a 1/2 wave star diagonal that is in proper alignment.