<P> A variant of the Cassegrain, the Schiefspiegler telescope ("skewed" or "oblique reflector") uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary . However, while eliminating diffraction patterns this leads to an increase in coma and astigmatism . These defects become manageable at large focal ratios--most Schiefspieglers use f / 15 or longer, which tends to restrict useful observation to the moon and planets . A number of variations are common, with varying numbers of mirrors of different types . The Kutter (named after its inventor Anton Kutter) style uses a single concave primary, a convex secondary and a plano - convex lens between the secondary mirror and the focal plane, when needed (this is the case of the catadioptric Schiefspiegler). One variation of a multi-schiefspiegler uses a concave primary, convex secondary and a parabolic tertiary . One of the interesting aspects of some Schiefspieglers is that one of the mirrors can be involved in the light path twice--each light path reflects along a different meridional path . </P> <P> Stevick - Paul telescopes are off - axis versions of Paul 3 - mirror systems with an added flat diagonal mirror . A convex secondary mirror is placed just to the side of the light entering the telescope, and positioned afocally so as to send parallel light on to the tertiary . The concave tertiary mirror is positioned exactly twice as far to the side of the entering beam as was the convex secondary, and its own radius of curvature distant from the secondary . Because the tertiary mirror receives parallel light from the secondary, it forms an image at its focus . The focal plane lies within the system of mirrors, but is accessible to the eye with the inclusion of a flat diagonal . The Stevick - Paul configuration results in all optical aberrations totaling zero to the third - order, except for the Petzval surface which is gently curved . </P> <P> The Yolo was developed by Arthur S. Leonard in the mid-1960s . Like the Schiefspiegler, it is an unobstructed, tilted reflector telescope . The original Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the main axis . Most Yolos use toroidal reflectors . The Yolo design eliminates coma, but leaves significant astigmatism, which is reduced by deformation of the secondary mirror by some form of warping harness, or alternatively, polishing a toroidal figure into the secondary . Like Schiefspieglers, many Yolo variations have been pursued . The needed amount of toroidal shape can be transferred entirely or partially to the primary mirror . In large focal ratios optical assemblies, both primary and secondary mirror can be left spherical and a spectacle correcting lens is added between the secondary mirror and the focal plane (catadioptric Yolo). The addition of a convex, long focus tertiary mirror leads to Leonard's Solano configuration . The Solano telescope doesn't contain any toric surfaces . </P> <P> One design of telescope uses a rotating mirror consisting of a liquid metal in a tray which is spun at constant speed . As the tray spins the liquid forms a paraboloidal surface of essentially unlimited size . This allows for very big telescope mirrors (over 6 metres), but unfortunately they cannot be steered, as they always point vertically . </P>

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