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Parallel rays coming into a parabolic mirror are focused at a point F. The vertex is V, and the axis of symmetry passes through V and F. For off-axis reflectors (with just the part of the paraboloid between the points P 1 and P 3), the receiver is still placed at the focus of the paraboloid, but it does not cast a shadow onto the reflector.
A Paul design has a parabolic primary with spherical secondary and tertiary mirrors; a Paul–Baker design modifies the secondary slightly to flatten the focal plane. A three-mirror anastigmat is an anastigmat telescope built with three curved mirrors, enabling it to minimize all three main optical aberrations – spherical aberration , coma ...
In the presence of astigmatism, an off-axis point on the object is not sharply imaged by the optical system. Instead, sharp lines are formed at the tangential and sagittal foci. The image at the tangent focus is a short line, oriented in the direction of the sagittal plane; images of circles centered on the optic axis, or small lines tangential ...
Several different off-axis configurations are used for radio antennas. [10] Another off-axis, unobstructed design and variant of the Cassegrain is the 'Yolo' reflector invented by Arthur Leonard. This design uses a spherical or parabolic primary and a mechanically warped spherical secondary to correct for off-axis induced astigmatism.
It is possible to calculate how closely the mirror surface resembles a perfect parabola by placing a Couder mask, [7] Everest pin stick (after A. W. Everest) [8] or other zone marker [9] over the mirror. A series of measurements with the tester, finding the radii of curvature of the zones along the optical axis of the mirror (Y-axis).
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.
Maksutov's 1944 design was the first-published meniscus telescope design, and was published in the widely-read Journal of the Optical Society of America. [11] [12] [7] This led to professional and amateur designers almost immediately experimenting with variations, including Newtonian, Cassegrain, and wide-field camera designs.
The Crossed Dragone Telescope is an off-axis telescope design consisting of a parabolic primary mirror and a large concave secondary mirror arranged so that the focal plane is at right angles to the incoming light. In this configuration the polarization of light is preserved through the optics.