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( Reflecting telescope)
A reflecting telescope (also called a reflector) is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The Reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical of aberrations, it is a design that allows for very large diameter objectives and is used almost exclusively in major research telescopes. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes uses mirrors, the design is sometimes referred to as a "catoptric" telescope. The Italian professor Niccolò Zucchi is credited with making the first reflector in 1616. With it, in 1630 he observed two bands on Jupiter, and in 1640 observed spots on Mars. But his inability to shape the concave mirror accurately, and the lack of means of viewing the image without blocking the mirror, meant others did not adopt Zucchi's design. In 1663 James Gregory published Optica Promota which described the first practical design of a reflector using two concave mirrors. A working example was not built until 10 years later by Robert Hooke. Sir Isaac Newton is credited with constructing the first "practical" reflecting telescope after his own design in 1668.[1] Newton's added a smaller "diagonal" mirror near the primary mirror's focus to reflect the image at 90° angle. This allowed the user to view the image without obstructing the incoming light. Newton invented his reflector to solve the problem of chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses. A curved primary mirror is the reflector telescope's basic optical element and creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or an eyepiece for visual observation or a mirror that reflects the image to an eyepiece. The primary mirror in most modern telescopes is composed of a solid glass cylinder whose front surface has been ground to a spherical or parabolic shape. A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective front surface. Early reflecting telescopes used a metal objective called a speculum.
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