Reflecting telescope
Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives.
This type included Newton's first designs and the largest telescope of the 19th century, the Leviathan of Parsonstown with a 6 feet (1.8 m) wide metal mirror.
The idea that curved mirrors behave like lenses dates back at least to Alhazen's 11th century treatise on optics, works that had been widely disseminated in Latin translations in early modern Europe.
[4] There were reports that the Bolognese Cesare Caravaggi had constructed one around 1626 and the Italian professor Niccolò Zucchi, in a later work, wrote that he had experimented with a concave bronze mirror in 1616, but said it did not produce a satisfactory image.
Despite the theoretical advantages of the reflector design, the difficulty of construction and the poor performance of the speculum metal mirrors being used at the time meant it took over 100 years for them to become popular.
A curved primary mirror is the reflector telescope's basic optical element that creates an image at the focal plane.
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 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.
The Ritchey–Chrétien telescope, invented by George Willis Ritchey and Henri Chrétien in the early 1910s, is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary).
It is free of coma and spherical aberration at a nearly flat focal plane if the primary and secondary curvature are properly figured, making it well suited for wide field and photographic observations.
The Dall–Kirkham Cassegrain telescope's design was created by Horace Dall in 1928 and took on the name in an article published in Scientific American in 1930 following discussion between amateur astronomer Allan Kirkham and Albert G. Ingalls, the magazine editor at the time.
In the Herschelian reflector the primary mirror is tilted so the observer's head does not block the incoming light.
These defects become manageable at large focal ratios — most Schiefspieglers use f/15 or longer, which tends to restrict useful observations to objects which fit in a moderate field of view.
Stevick-Paul telescopes[20] are off-axis versions of Paul 3-mirror systems[21] with an added flat diagonal mirror.
The Stevick-Paul configuration results in all optical aberrations totaling zero to the third-order, except for the Petzval surface which is gently curved.
The original Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the main axis.
The addition of a convex, long focus tertiary mirror leads to Leonard's Solano configuration.
One design of telescope uses a rotating mirror consisting of a liquid metal in a tray that is spun at constant speed.
In a prime focus design no secondary optics are used, the image is accessed at the focal point of the primary mirror.
The space available at prime focus is severely limited by the need to avoid obstructing the incoming light.
Such instruments could not withstand being moved, and adding mirrors to the light path to form a coudé train, diverting the light to a fixed position to such an instrument housed on or below the observing floor (and usually built as an unmoving integral part of the observatory building) was the only option.
The development of echelle spectrometers allowed high-resolution spectroscopy with a much more compact instrument, one which can sometimes be successfully mounted on the Cassegrain focus.
Whilst transmission of the full field of view would require a standard coudé focus, spectroscopy typically involves the measurement of only a few discrete objects, such as stars or galaxies.
