off-axis guider

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A device for following a guide star which is just outside the normal field of view of a telescope. The telescope is positioned so that the guide star is over a small mirror or prism, in the light path close to the focal plane. The mirror or prism reflects the guide star's light to an eyepiece with an illuminated reticle. The advantages of an off-axis guider over a guide telescope are that it eliminates the risk of flexure between the guidescope and the main instrument, it is lighter, and fainter guide stars can be used because of the usually larger aperture of the main instrument. However, the object being used for guiding cannot be photographed at the same time.

Subjects: Astronomy and Astrophysics.

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