Reflecting telescope Totally Explained

A reflecting telescope (reflector) is an optical telescope which uses a combination of curved or plane (flat) mirrors to reflect light and form an image (catoptric), rather than lenses to refract or bend light to form an image (dioptric).

History

The Italian monk Niccolò Zucchi is credited with making the first reflector in 1616, but his inability to shape the concave mirror accurately and the lack of means of viewing the image without blocking the mirror, caused Zucchi to give up on the idea. In 1663 James Gregory published Optica Promota which described the first practical design of a reflector using two concave mirrors. A working example wasn't 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. He designed his reflector, which used a concave objective and a smaller "diagonal" mirror, in order to solve the problem of chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses.

Technical considerations

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.
Mirrors eliminate the risk of chromatic aberration but may still produce other types of aberrations: In general, on axis they may produce spherical aberration, in which case the outer and inner zones of the telescope don't share a common focus. This was the construction flaw in the Hubble Space Telescope mirrors. Spherical aberration can be eliminated with aspheric (non-spherical) mirrors. Off axis, additional aberrations may become apparent:

Coma - a variation of telescope magnification with radial zone on the mirror typically appears as a radial smudging of the images which gets worse at the edges of the field. Spherical aberration and coma are eliminated in two mirror Ritchey Chretien designs.
The best image plane is in general curved, which may not correspond to the detector's shape and leads to a focus error across the field.
Astigmatism, an azimuthal variation of focus around the aperture. Near the center of the field astigmatism isn't usually a problem, but it gets rapidly worse once it becomes apparent - it varies quadratically with field angle.
Distortion over the field of view. Distortion doesn't affect image quality (sharpness) but does affect object shapes. It can be corrected by image processing.

There are reflector designs and modifications such as catadioptrics that correct some of these aberrations.
Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this:

In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.

Light of different wavelengths travels through a medium other than vacuum at different speeds. This causes chromatic aberration in uncorrected lenses and creating an aberration-free large lens is a costly process. A mirror can eliminate this problem entirely.

Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or catadioptric.

There are structural problems involved in manufacturing and manipulating large-aperture lenses. A lens can only be held in place by its edge, which means that the sag due to gravity can be sufficient to distort the image. In contrast, a mirror can be supported by the whole side opposite its reflecting face. While the Newtonian focus design is still used in amateur astronomy, professionals now tend to use prime focus, Cassegrain focus, and coudé focus designs. By 2001, there were at least 49 reflectors with primary mirrors having diameters of 2 meters or more.

Reflecting telescope designs

Newtonian

The Newtonian usually has a paraboloid primary mirror but for small apertures, say 12 cm or less, if the focal ratio is f/8 or longer a spherical primary mirror is sufficient for high visual resolution. A flat secondary mirror reflects the light to a focal plane at the side of the top of the telescope tube. It is one of the simplest and least expensive designs for a given size of primary, and is popular with amateur telescope makers as a home-build project. ====

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