![]() ![]() This design reduces the size and cost of the telescope with a shorter overall telescope tube length (with the corrector extending the focal length in a " telephoto" type layout) combined with a less costly spherical mirror. Jones–Bird Ī Jones–Bird Newtonian (sometimes called a Bird–Jones) uses a spherical primary mirror in place of a parabolic one, with spherical aberrations corrected by sub-aperture corrector lens usually mounted inside the focusser tube or in front of the secondary mirror. Diffraction can also be minimized by using a high focal ratio with a proportionally small diagonal mirror mounted on the corrector. Similar to a Schmidt–Newtonian, a Maksutov telescope's meniscus shaped corrector can be added to the Newtonian configuration, which gives it minimal aberration over a wide field of view, with one-fourth the coma of a similar standard Newtonian and one-half the coma of a Schmidt-Newtonian. This is done to correct spherical aberration or reduce cost. There are several variations on the Newtonian design that add a lens to the system creating a catadioptric telescope. For research telescopes, counterbalancing very heavy instruments mounted at this focus has to be taken into consideration. Some designs provide mechanisms for rotating the eyepiece mount or the entire tube assembly to a better position. For visual observing, most notably on equatorial mounts, tube orientation can put the eyepiece in a very poor viewing position, and larger telescopes require ladders or support structures to access it.
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