The distinction between mechanically and optically compensated zoom systems has an interesting counterpart in certain catadioptric systems. The simplest such system is very simple indeed: a front refractive air-spaced nearly afocal doublet near the front focus of a spherical concave mirror. The front positive lens and the mirror have a fixed separation and the following negative lens moves back and forth in the space between, while also being traversed again by the focused light from the mirror. By having the negative lens used in double pass during its zooming motion a quartic focus curve can be obtained. The image falls near the center of the front positive lens, where a detector can be placed. A sample design with a 2.5$MUL zoom ratio is described. The two front lenses are germanium and it zooms from an f/2.0, 5.0 degree diameter field to an f/5.0, 2.0 degree diameter field with good image quality.
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