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24-677: Observations at the TNG can be proposed through the Italian Time Allocation Committee (TAC) which assigns, based on the scientific merit of the proposals, 75% of the available time. The rest of the time is at disposal of the Spanish and international astronomical communities. The TNG is open to new proposals two times a year, typically in March–April and September–October. The TNG is an altazimuthal reflecting telescope with

48-552: A Ritchey-Chretien optical configuration and a flat tertiary mirror feeding two opposite Nasmyth foci. It has a design derived from the New Technology Telescope (NTT), an ESO 4-meters class telescope located in La Silla (Chile). Therefore, the optical quality of the telescope is ensured by an active optics system performing real-time corrections of the optical components and compensating, in particular, for

72-733: A 102 cm (40 in) instrument constructed by Ritchey for the United States Naval Observatory ; that telescope is still in operation at the Naval Observatory Flagstaff Station . As with the other Cassegrain-configuration reflectors, the Ritchey–Chrétien telescope (RCT) has a very short optical tube assembly and compact design for a given focal length . The RCT offers good off-axis optical performance, but its mirrors require sophisticated techniques to manufacture and test. Hence

96-634: A Ritchey–Chrétien system, the conic constants K 1 {\displaystyle K_{1}} and K 2 {\displaystyle K_{2}} of the two mirrors are chosen so as to eliminate third-order spherical aberration and coma; the solution is: and Note that K 1 {\displaystyle K_{1}} and K 2 {\displaystyle K_{2}} are less than − 1 {\displaystyle -1} (since M > 1 {\displaystyle M>1} ), so both mirrors are hyperbolic. (The primary mirror

120-427: A bundle of parallel rays parallel to the optical axis will be perfectly focused to a point (the mirror is free of spherical aberration ), no matter where they strike the mirror. However, this is only true if the rays are parallel to the axis of the parabola. When the incoming rays strike the mirror at an angle, individual rays are not reflected to the same point. When looking at a point that is not perfectly aligned with

144-462: A dual lens system of a plano-convex and a plano-concave lens fitted into an eyepiece adaptor which superficially resembles a Barlow lens . Coma of a single lens or a system of lenses can be minimized (and in some cases eliminated) by choosing the curvature of the lens surfaces to match the application. Lenses in which both spherical aberration and coma are minimized at a single wavelength are called bestform or aplanatic lenses . Vertical coma

168-411: A larger usable field of view compared to the parabolic designs actually used. However, Ritchey and Hale had a falling-out. With the 100-inch project already late and over budget, Hale refused to adopt the new design, with its hard-to-test curvatures, and Ritchey left the project. Both projects were then built with traditional optics. Since then, advances in optical measurement and fabrication have allowed

192-561: A small optical device called a null corrector that makes the hyperbolic primary look spherical for the interferometric test. On the Hubble Space Telescope , this device was built incorrectly (a reflection from an un-intended surface leading to an incorrect measurement of lens position) leading to the error in the Hubble primary mirror. Incorrect null correctors have led to other mirror fabrication errors as well, such as in

216-416: A tail ( coma ) like a comet . Specifically, coma is defined as a variation in magnification over the entrance pupil . In refractive or diffractive optical systems, especially those imaging a wide spectral range, coma can be a function of wavelength , in which case it is a form of chromatic aberration . Coma is an inherent property of telescopes using parabolic mirrors . Unlike a spherical mirror ,

240-524: Is a specialized variant of the Cassegrain telescope that has a hyperbolic primary mirror and a hyperbolic secondary mirror designed to eliminate off-axis optical errors ( coma ). The RCT has a wider field of view free of optical errors compared to a more traditional reflecting telescope configuration. Since the mid 20th century, a majority of large professional research telescopes have been Ritchey–Chrétien configurations; some well-known examples are

264-651: Is possible to change instrument during the night with a loss of time limited to a few minutes. The science based on observational data from the TNG is varied. Proposed observing programs go from the study of the planets and minor bodies of the Solar System up to researches of cosmological interest (e.g. large-scale structure of the Universe and systems of galaxies ). The TNG is equipped with five instruments: Decommissioned instruments: Ritchey-Chretien A Ritchey–Chrétien telescope ( RCT or simply RC )

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288-460: Is typically quite close to being parabolic, however.) The hyperbolic curvatures are difficult to test, especially with equipment typically available to amateur telescope makers or laboratory-scale fabricators; thus, older telescope layouts predominate in these applications. However, professional optics fabricators and large research groups test their mirrors with interferometers . A Ritchey–Chrétien then requires minimal additional equipment, typically

312-526: The Hubble Space Telescope , the Keck telescopes and the ESO Very Large Telescope . The Ritchey–Chrétien telescope was invented in the early 1910s by American astronomer George Willis Ritchey and French astronomer Henri Chrétien . Ritchey constructed the first successful RCT, which had an aperture diameter of 60 cm (24 in) in 1927 (Ritchey 24-inch reflector). The second RCT was

336-484: The New Technology Telescope . In practice, each of these designs may also include any number of flat fold mirrors , used to bend the optical path into more convenient configurations. This article only discusses the mirrors required for forming an image, not those for placing it in a convenient location. Ritchey intended the 100-inch Mount Wilson Hooker telescope (1917) and the 200-inch (5 m) Hale Telescope to be RCTs. His designs would have provided sharper images over

360-615: The RCT design to take over – the Hale telescope, dedicated in 1948, turned out to be the last world-leading telescope to have a parabolic primary mirror. Coma (optics) In optics (especially telescopes ), the coma ( / ˈ k oʊ m ə / ), or comatic aberration , in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted, appearing to have

384-533: The Ritchey–Chrétien configuration is most commonly found on high-performance professional telescopes. A telescope with only one curved mirror, such as a Newtonian telescope , will always have aberrations. If the mirror is spherical, it will suffer primarily from spherical aberration . If the mirror is made parabolic, to correct the spherical aberration, then it still suffers from coma and astigmatism , since there are no additional design parameters one can vary to eliminate them. With two non-spherical mirrors, such as

408-504: The Ritchey–Chrétien telescope, coma can be eliminated as well, by making the two mirrors' contribution to total coma cancel. This allows a larger useful field of view. However, such designs still suffer from astigmatism. The basic Ritchey–Chrétien two-surface design is free of third-order coma and spherical aberration . However, the two-surface design does suffer from fifth-order coma, severe large-angle astigmatism , and comparatively severe field curvature . When focused midway between

432-514: The Schmidt requires a full-aperture corrector plate, which restricts it to apertures below 1.2 meters, while a Ritchey–Chrétien can be much larger. Other telescope designs with front-correcting elements are not limited by the practical problems of making a multiply-curved Schmidt corrector plate, such as the Lurie–Houghton design . In a Ritchey–Chrétien design, as in most Cassegrain systems,

456-470: The deformations of the primary mirror, which is too thin to be completely rigid. The interface between the telescope fork and the instruments at both Nasmyth foci is provided by two rotator/adapters. Their main function is to compensate for the field rotation by a mechanical counter rotation. The best quality of the TNG is that all the available instruments are permanently mounted at the telescope. This guarantees flexibility during an observing session, since it

480-668: The optical axis, some of the incoming light from that point will strike the mirror at an angle. This causes an image that is not in the center of the field to appear as wedge-shaped. The further off-axis (or the greater the angle subtended by the point with the optical axis), the worse this effect is. This causes stars to appear to have a cometary coma , hence the name. Schemes to reduce coma without introducing spherical aberration include Schmidt , Maksutov , ACF and Ritchey–Chrétien optical systems. Correction lenses, " coma correctors " for Newtonian reflectors have been designed which reduce coma in newtonian telescopes. These work by means of

504-711: The primary and secondary mirrors, respectively, in a two-mirror Cassegrain configuration are: and where If, instead of B {\displaystyle B} and D {\displaystyle D} , the known quantities are the focal length of the primary mirror, f 1 {\displaystyle f_{1}} , and the distance to the focus behind the primary mirror, b {\displaystyle b} , then D = f 1 ( F − b ) / ( F + f 1 ) {\displaystyle D=f_{1}(F-b)/(F+f_{1})} and B = D + b {\displaystyle B=D+b} . For

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528-506: The result is a three-mirror anastigmat . Alternatively, a RCT may use one or several low-power lenses in front of the focal plane as a field-corrector to correct astigmatism and flatten the focal surface, as for example the SDSS telescope and the VISTA telescope ; this can allow a field-of-view up to around 3° diameter. The Schmidt camera can deliver even wider fields up to about 7°. However,

552-407: The sagittal and tangential focusing planes, stars appear as circles, making the Ritchey–Chrétien well suited for wide field and photographic observations. The remaining aberrations of the two-element basic design may be improved with the addition of smaller optical elements near the focal plane. Astigmatism can be cancelled by including a third curved optical element. When this element is a mirror,

576-487: The secondary mirror blocks a central portion of the aperture. This ring-shaped entrance aperture significantly reduces a portion of the modulation transfer function (MTF) over a range of low spatial frequencies, compared to a full-aperture design such as a refractor. This MTF notch has the effect of lowering image contrast when imaging broad features. In addition, the support for the secondary (the spider) may introduce diffraction spikes in images. The radii of curvature of

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