Although reflex telescopes produce other types of optical aberrations, it is a design that can achieve large diameter targets. Almost all the main telescopes used in astronomical research are such. Reflecting telescopes come in many design options and can use additional optical elements to improve image quality or to place the image in a mechanically advantageous position.
Characteristics of reflex telescopes
The idea that curved mirrors behave like lenses dates back, at least, to Alfazen's treatise of the 11th century on optics, a work that was widely spread in Latin translations in early modern Europe. Soon after the invention of the refractive telescope, 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 as an imaging tool. It was reported that Bolognese Cesare Caravaggi built the first reflex telescope around 1626. Italian professor Niccolo Zucci wrote in a later work that he experimented with a concave bronze mirror in 1616, but said that he did not give a satisfactory image.
History of creation
The potential advantages of using parabolic mirrors, primarily the reduction of spherical aberration without chromatic aberration, have led to many proposed projects for future telescopes. Most notable was James Gregory, who published an innovative design for the “reflective” telescope in 1663. It was ten years (1673) before experimental scientist Robert Hooke could build this type of telescope, which became known as the Gregorian telescope.
Isaac Newton, as a rule, was credited with the creation of the first reflex-refractor telescope in 1668. It used a primary mirror of spherical metal and a small diagonal in the optical configuration, called the Newtonian telescope.
Further development
Despite the theoretical advantages of the design of the reflector, the complexity of the design and the low performance of the metal mirrors used at the time meant that it took more than 100 years to become popular. Many of the advances in reflex telescopes included improvements to the manufacture of parabolic mirrors in the 18th century, silver-coated glass mirrors in the 19th century, durable aluminum coatings in the 20th century, segmented mirrors to provide a larger diameter, and active optics to compensate for gravitational deformation. Mid-20th century innovations were 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 visualization to overcome the problems associated with observation, and the reflection of telescopes, were ubiquitous on space telescopes and many types of visualization tools for spacecraft.
A curved primary mirror is the main optical element of the telescope; 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 located here for recording images, or an additional mirror can be added to change the optical characteristics and / or redirection of light to a film, digital sensor or eyepiece for visual observation.
Detailed description
The primary mirror in most modern telescopes consists of a solid glass cylinder, the front surface of which is ground to a spherical or parabolic shape. A thin layer of aluminum is evacuated onto the lens, forming a reflective first surface mirror.
Some telescopes use primary mirrors, which are made in different ways. The molten glass rotates to make its surface paraboloidal; it cools and hardens. The resulting shape of the mirror approximates the desired shape of the paraboloid, which requires minimal grinding and polishing in order to achieve an exact figure.
Image quality
Reflex telescopes, like any other optical system, do not create “perfect” images. The need to photograph objects at distances to infinity, to view them at different wavelengths of light, and also require to have some way of viewing the image that the primary mirror produces, means that there is always some kind of compromise in the optical design of a reflecting telescope.
Since the main mirror focuses the light at a common point in front of its own reflective surface, almost all telescope reflective structures have a secondary mirror, a film holder, or a detector near this focal point, partially preventing the light from reaching the main mirror. This not only leads to some reduction in the amount of light that the system collects, but also leads to a loss of contrast in the image due to diffraction effects of obstruction, as well as diffraction 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, since the reflection of light rays striking a 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 devices of reflex telescopes use parabolic mirrors, which can focus all the light on a common focus.
Gregorian telescope
The Gregorian telescope is described by Scottish astronomer and mathematician James Gregory in his 1663 book Optica Promota as using a concave secondary mirror that reflects an image through a hole in the primary mirror. This creates a vertical image useful for ground-based observations. There are several large modern telescopes that use the Gregorian configuration.
Newton Reflex Telescope
The Newtonian apparatus was the first successful reflective telescope created by Isaac in 1668. Usually it has a paraboloidal primary mirror, but with focal ratios f / 8 or more, it has a spherical primary mirror, which may be enough for high visual resolution. A flat secondary mirror reflects light in 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 size of primary material, and it is common among amateur handsets. The path of the rays of reflex telescopes was first worked out precisely on the Newtonian sample.
Cassegrain apparatus
The Cassegrain telescope (sometimes called the "classic Cassegrain") was first constructed in 1672, attributed to Laurent Cassegrain. It has a parabolic primary mirror and a hyperbolic secondary mirror, which reflects light back and down through an opening 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 of amateur astronomer Allan Kirkham and Albert G. Ingalls (editor of the magazine at the time). It uses a concave elliptical primary mirror and a convex secondary. Although 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 areas of application of these wonderful devices. But it is being replaced by electronic counterparts. Nevertheless, it is this type of apparatus that is considered the largest reflex telescope.