Ramsden disk

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The disk of light seen through the eyepiece of a telescope pointed at bright sky, as observed from some distance behind the eyepiece. It is actually the image of the objective, reduced in size. The magnification given by an eyepiece can be estimated by dividing the diameter of the objective lens by that of the Ramsden disk when the telescope is focused on infinity. It is named after the English instrument-maker Jesse Ramsden (1735–1800).

Subjects: Astronomy and Astrophysics.

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