Although reflecting telescopes produce other types of optical aberrations, this is a design that can achieve large diameter targets. Almost all major telescopes used in astronomical research are such. Reflecting telescopes come in a variety of designs and may use additional optical elements to improve image quality or position the image in a mechanically advantageous position.
Characteristics of reflecting telescopes
The idea that curved mirrors behave like lenses goes back at least to Alphazen's 11th-century treatise on optics, a work that circulated widely in Latin translations in early modern Europe. Shortly after the invention of the refracting telescope by Galileo, Giovanni Francesco Sagredo and others, inspired by their knowledge of the principles of curved mirrors, discussed the idea of constructing a telescope using a mirror inas an imaging tool. Bolognese Cesare Caravaggi was reported to have built the first reflecting telescope around 1626. The Italian professor Niccolo Zucci, in a later work, wrote that he experimented with a concave bronze mirror in 1616, but said that it did not give a satisfactory image.
History of Creation
The potential benefits of using parabolic mirrors, primarily the reduction of spherical aberration without chromatic aberration, have led to many proposed designs for future telescopes. Most notable was James Gregory, who published an innovative design for a "reflecting" telescope in 1663. It took ten years (1673) before the experimental scientist Robert Hooke could build this type of telescope, which became known as the Gregorian telescope.
Isaac Newton was generally credited with building the first reflecting-refracting telescope in 1668. It used a spherical metal primary mirror and a small diagonal one in an optical configuration, called a Newtonian telescope.
Further development
Despite the theoretical advantages of reflector design, the design complexity and poor performance of the metal mirrors used at the time meant that it took over 100 years for them to become popular. Many of the advances in reflecting telescopes included improvements in the manufacture of parabolic mirrors in the 18th century.century, silver-coated glass mirrors in the 19th century, durable aluminum coatings in the 20th century, segmented mirrors to provide larger diameters, and active optics to compensate for gravitational deformation. A mid-20th century innovation was catadioptic telescopes such as the Schmidt camera, which use both a spherical mirror and a lens (called a corrector plate) as primary optical elements, mainly used for large-scale imaging without spherical aberration.
At the end of the 20th century, the development of adaptive optics and successful imaging to overcome the problems associated with observation and reflection of telescopes is ubiquitous on space telescopes and many types of spacecraft imaging tools.
The curvilinear primary mirror is the main optical element of the telescope, and it creates an image in the focal plane. The distance from the mirror to the focal plane is called the focal length. A digital sensor can be placed here to record an image, or an additional mirror can be added to change the optical characteristics and/or redirect light to the film, digital sensor, or eyepiece for visual observation.
Detailed description
The primary mirror in most modern telescopes consists of a solid glass cylinder whose front surface is ground to a spherical or parabolic shape. A thin layer of aluminum is evacuated onto the lens, formingreflective first surface mirror.
Some telescopes use primary mirrors that are made differently. The molten glass rotates to make its surface paraboloidal, it cools and solidifies. The resulting mirror shape approximates the desired paraboloid shape, which requires minimal grinding and polishing to achieve an accurate figure.
Image quality
Reflector telescopes, like any other optical system, do not create "ideal" images. The need to photograph objects at distances to infinity, to view them at different wavelengths of light, and to require some way of viewing the image that the primary mirror produces means that there is always some compromise in the optical design of a reflecting telescope.
Because the primary mirror focuses light to a common point in front of its own reflective surface, almost all reflective telescope designs have a secondary mirror, film holder, or detector near this focal point, partially preventing light from reaching the primary mirror. This not only results in some reduction in the amount of light the system collects, but also results in a loss of contrast in the image due to diffractive obstruction effects, as well as diffractive spikes caused by most secondary support structures.
The use of mirrors avoids chromatic aberration,but they create other types of aberrations. A simple spherical mirror cannot transmit light from a distant object to a common focus, because the reflection of light rays striking the mirror at its edge does not converge with those that reflect from the center of the mirror, a defect called spherical aberration. To avoid this problem, the most advanced reflecting telescope designs use parabolic mirrors that can bring all the light into a common focus.
Gregorian Telescope
The Gregorian telescope is described by the Scottish astronomer and mathematician James Gregory in his 1663 book Optica Promota as using a concave secondary mirror that reflects the image through a hole in the primary mirror. This creates a vertical image useful for terrestrial observations. There are several large modern telescopes that use the Gregorian configuration.
Newton's Reflector Telescope
Newton's apparatus was the first successful reflecting telescope, built by Isaac in 1668. It usually has a paraboloid primary, but at focal ratios of f/8 or more, a spherical primary, which may be sufficient for high visual resolution. A flat secondary reflects light at the focal plane on the side of the top of the telescope tube. This is one of the simplest and least expensive designs for a given raw material size, and is common among hobbyists. The ray path of reflecting telescopes was firstworked out precisely on the Newtonian sample.
Cassegrain Apparatus
The Cassegrain telescope (sometimes called the "classical Cassegrain") was first constructed in 1672, attributed to Laurent Cassegrain. It has a parabolic primary and a hyperbolic secondary that reflects light back and down through a hole in the primary.
The design of the Dall-Kirkham Cassegrain telescope was created by Horace Dall in 1928, and was named in an article published in Scientific American in 1930 after a discussion between amateur astronomer Allan Kirkham and Albert G. Ingalls, (the magazine's editor at the time time). It uses a concave elliptical primary and a convex secondary. While this system is easier to grind than the classic Cassegrain or Ritchey-Chrétien system, it is not suitable for off-axis coma. The curvature of the field is actually less than that of the classical Cassegrain. Today, this design is used in many applications of these wonderful devices. But it is being replaced by electronic counterparts. Nevertheless, it is this type of apparatus that is considered the largest reflective telescope.
