The eccentric pupil ("off axis") paraboloid is the most pop, ar optical system for the infrared because there are no obstructions in the light path. However, the field of view is limited owing to coma. Better overall image quality can sometimes be obtained by using a spherical mirror such as found in the quasi-Schmidt systems. Image quality not withstanding, we often find that we need a long focal length in a short space, and so we look toward more elaborate optics than a single mirror. Two-mirror reflectors, such as the Cassegrain, can be made with an eccentric pupil so that there is no obstruction, or with a Gregorian (concave secondary, intermediate image) reflector, which is commonly used in infrared telephoto systems.