Non-centering error may result in uneven field illumination or vignetting but will not otherwise introduce optical aberrations into the system provided that accurate angular alignment is maintained. This defect will be unnoticed if the secondary is slightly oversized. For example, failure to achieve perfect lateral alignment or perfect positioning along the optical axis of the primary will result in the secondary mirror not being placed precisely at the center of the primary optical axis. The first and second types of adjustment described above are of relatively minor importance when compared to the angular alignment adjustment. The secondary mirror or prism is positioned with respect to the primary mirror at a carefully selected distance so that the focal plane may be conveniently accessed with an ocular or camera associated with a focuser 18 on the outside of the telescope tube. The Newtonian secondary mirror 16 is also commonly known as a diagonal or diagonal mirror. Before the converging reflected light beam reaches a focal or image plane however, the beam is reflected at a substantially 90° angle toward and through the telescope tube by a secondary mirror 16 or less commonly by a prism. The parabolic surface of the primary mirror reflects collected light back up the tube along the optical axis of the primary toward a focal plane, the position of which is determined by the diameter and focal ratio of the primary mirror. In the conventional Newtonian design, light is collected by a parabolic primary mirror 10 which is positioned in a telescope tube 12 opposite an opening 14 which is pointed at the sky. A typical Newtonian telescope is schematically illustrated in FIG. The Newtonian telescope is named after its inventor, Sir Isaac Newton. G02B23/16- Housings Caps Mountings Supports, e.g.binoculars Periscopes Instruments for viewing the inside of hollow bodies Viewfinders Optical aiming or sighting devices G02B7/1825- Manual alignment made by screws, e.g.G02B7/1822- Mountings, adjusting means, or light-tight connections, for optical elements for prisms for mirrors for mirrors comprising means for aligning the optical axis.G02B7/182- Mountings, adjusting means, or light-tight connections, for optical elements for prisms for mirrors for mirrors.
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