To provide a lens system having a variation of high magnification ratio in a relay lens system, in an optical system having an objective lens system, an optical system for relaying and re-imaging an image of an objective lens system, and an eyepiece to observe the image, sequentially from the objective side in a ground telescope.

In the ground telescope, the relay optical system includes a condenser lens with a positive power, a second group with a positive power, a third group with a positive power, and a field lens with a negative power from a first focal surface image. The lens system of the ground telescope having the lens groups is characterized by relatively moving some lenses in an optical axis direction for variable magnification, while fixing the positions of the first and second focal surface images.

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