The invention is directed to an infrared Kepler telescope which includes an objective defining an optical axis and including a positive front group and a negative rear group all arranged on the optical axis. The objective further includes an interchangeable optic interposed between the positive front group and the negative rear group with the interchangeable optic being configured to operate as a magnification changer. An ocular is mounted on the optical axis rearward of the negative rear group and the rear group and the ocular are fixedly pregiven. The positive front group is a first positive front group and is exchangeable with at least a second positive front group. The interchangeable optic is a first interchangeable optic and is exchangeable with at least a second interchangeable optic. The first and second positive front groups are optically so configured that each one of the positive front groups undercorrects spherical aberration and coma forward of the negative rear group to the same extent with or without the interchangeable optic. It is also provided that each combination has an overall diffraction-limited correction except for the distortion. The interchangeable optics (W11, W21) generate a second field of view. The telescope having interchangeable optics (W11, W21) has a negative distortion and has a positive distortion without this interchangeable optics.
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