Herschelian telescope

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A reflecting telescope in which the primary mirror is tilted so that it focuses light to the top edge of the tube. The eyepiece is located at the top edge, looking down at the mirror. This design avoids light loss caused by a second reflection, which was particularly important when mirrors were made of speculum metal. But it results in astigmatism unless the focal ratio is large. The design was invented by F. W. Herschel, who used it for his giant 48-inch (1.2-m), but it is now of historical interest only.

Subjects: Astronomy and Astrophysics.

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