The Newtonian telescope, also called the Newtonian reflector or just a Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror. Newton's first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope. [1] The Newtonian telescope's simple design has made it very popular with amateur telescope makers. [2]
A Newtonian telescope is composed of a primary mirror or objective, usually parabolic in shape, and a smaller flat secondary mirror. The primary mirror makes it possible to collect light from the pointed region of the sky, while the secondary mirror redirects the light out of the optical axis at a right angle so it can be viewed with an eyepiece.
There are several variations on the Newtonian design that add a lens to the system creating a catadioptric telescope. This is done to correct spherical aberration or reduce cost.
A Schmidt–Newtonian telescope combines the Newtonian optical design with a full-aperture Schmidt corrector plate in front of the primary mirror that not only corrects spherical aberration but can also support the secondary mirror. The resulting system has less coma and secondary mirror support induced diffraction effects. [7]
Similar to a Schmidt–Newtonian, a Maksutov telescope's meniscus shaped corrector can be added to the Newtonian configuration, which gives it minimal aberration over a wide field of view, with one-fourth the coma of a similar standard Newtonian and one-half the coma of a Schmidt-Newtonian. [8] Diffraction can also be minimized by using a high focal ratio with a proportionally small diagonal mirror mounted on the corrector. [9]
A Jones–Bird Newtonian (sometimes called a Bird–Jones) uses a spherical primary mirror in place of a parabolic one, with spherical aberrations corrected by sub-aperture corrector lens [10] usually mounted inside the focusser tube or in front of the secondary mirror. This design reduces the size and cost of the telescope with a shorter overall telescope tube length (with the corrector extending the focal length in a "telephoto" type layout) combined with a less costly spherical mirror. Commercially produced versions of this design have been noted to be optically compromised, due to the difficulty of producing a correctly shaped sub-aperture corrector, and are targeted at the inexpensive end of the telescope market. [11]
Newton's idea for a reflecting telescope was not new. Galileo Galilei and Giovanni Francesco Sagredo had discussed using a mirror as the image forming objective soon after the invention of the refracting telescope, [12] and others, such as Niccolò Zucchi, claimed to have experimented with the idea as far back as 1616. [13] Newton may even have read James Gregory's 1663 book Optica Promota which described reflecting telescope designs using parabolic mirrors [14] (a telescope Gregory had been trying unsuccessfully to build). [15]
Newton built his reflecting telescope because he suspected it could prove his theory that white light is composed of a spectrum of colours. [16] Colour distortion (chromatic aberration) was the primary fault of refracting telescopes of Newton's day, and there were many theories as to what caused it. During the mid-1660s with his work on the theory of colour, Newton concluded this defect was caused by the lens of the refracting telescope behaving the same as prisms he was experimenting with, breaking white light into a rainbow of colours around bright astronomical objects. [17] [18] If this were true, then chromatic aberration could be eliminated by building a telescope which did not use a lens – a reflecting telescope.
In late 1668 Isaac Newton built his first reflecting telescope. He chose an alloy (speculum metal) of tin and copper as the most suitable material for his objective mirror. He later devised means for shaping and grinding the mirror and may have been the first to use a pitch lap [20] to polish the optical surface. He chose a spherical shape for his mirror instead of a parabola to simplify construction; even though it would introduce spherical aberration, it would still correct chromatic aberration. He added to his reflector what is the hallmark of the design of a Newtonian telescope, a secondary diagonally mounted mirror near the primary mirror's focus to reflect the image at a 90° angle to an eyepiece mounted on the side of the telescope. This unique addition allowed the image to be viewed with minimal obstruction of the objective mirror. He also made the tube, mount, and fittings. Newton's first version had a primary mirror diameter of 1.3 inches (33 mm) and a focal ratio of f/5. [21] He found that the telescope worked without colour distortion and that he could see the four Galilean moons of Jupiter and the crescent phase of the planet Venus with it. Newton's friend Isaac Barrow showed a second telescope to a small group from the Royal Society of London at the end of 1671. They were so impressed with it that they demonstrated it to Charles II in January 1672. Newton was admitted as a fellow of the society in the same year.
Like Gregory before him, Newton found it hard to construct an effective reflector. It was difficult to grind the speculum metal to a regular curvature. The surface also tarnished rapidly; the consequent low reflectivity of the mirror and also its small size meant that the view through the telescope was very dim compared to contemporary refractors. Because of these difficulties in construction, the Newtonian reflecting telescope was initially not widely adopted. In 1721 John Hadley showed a much-improved model to the Royal Society. [22] Hadley had solved many of the problems of making a parabolic mirror. His Newtonian with a mirror diameter of 6 inches (150 mm) compared favourably with the large aerial refracting telescopes of the day. [23]
off-axis performance of the paraboloidal mirror drops so quickly with the increase in relative aperture beyond ~ƒ/6
A parabolicreflector is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis. The parabolic reflector transforms an incoming plane wave travelling along the axis into a spherical wave converging toward the focus. Conversely, a spherical wave generated by a point source placed in the focus is reflected into a plane wave propagating as a collimated beam along the axis.
A Ritchey–Chrétien telescope 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 the Hubble Space Telescope, the Keck telescopes and the ESO Very Large Telescope.
The history of the telescope can be traced to before the invention of the earliest known telescope, which appeared in 1608 in the Netherlands, when a patent was submitted by Hans Lippershey, an eyeglass maker. Although Lippershey did not receive his patent, news of the invention soon spread across Europe. The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. Galileo improved on this design the following year and applied it to astronomy. In 1611, Johannes Kepler described how a far more useful telescope could be made with a convex objective lens and a convex eyepiece lens. By 1655, astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces.
In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strike a spherical surface off-centre are refracted or reflected more or less than those that strike close to the centre. This deviation reduces the quality of images produced by optical systems. The effect of spherical aberration was first identified by Ibn al-Haytham who discussed it in his work Kitāb al-Manāẓir.
A refracting telescope is a type of optical telescope that uses a lens as its objective to form an image. The refracting telescope design was originally used in spyglasses and astronomical telescopes but is also used for long-focus camera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the reflecting telescope, which allows larger apertures. A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece.
An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electronic image sensors.
A reflecting telescope is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Many variant forms are in use and some employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a catoptric telescope.
In optics, the coma, 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 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.
A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric astrophotographic telescope designed to provide wide fields of view with limited aberrations. The design was invented by Bernhard Schmidt in 1930.
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors (catoptrics). Catadioptric combinations are used in focusing systems such as searchlights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses. Other optical systems that use lenses and mirrors are also referred to as "catadioptric", such as surveillance catadioptric sensors.
The Maksutov is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus lens in a design that takes advantage of all the surfaces being nearly "spherically symmetrical". The negative lens is usually full diameter and placed at the entrance pupil of the telescope. The design corrects the problems of off-axis aberrations such as coma found in reflecting telescopes while also correcting chromatic aberration. It was patented in 1941 by Soviet optician Dmitri Dmitrievich Maksutov. Maksutov based his design on the idea behind the Schmidt camera of using the spherical errors of a negative lens to correct the opposite errors in a spherical primary mirror. The design is most commonly seen in a Cassegrain variation, with an integrated secondary, that can use all-spherical elements, thereby simplifying fabrication. Maksutov telescopes have been sold on the amateur market since the 1950s.
The Schmidt–Cassegrain is a catadioptric telescope that combines a Cassegrain reflector's optical path with a Schmidt corrector plate to make a compact astronomical instrument that uses simple spherical surfaces.
The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to the optical system's primary mirror entrance aperture. This design puts the focal point at a convenient location behind the primary mirror and the convex secondary adds a telephoto effect creating a much longer focal length in a mechanically short system.
A Schmidt–Newtonian telescope or Schmidt–Newton telescope is a catadioptric telescope that combines elements from both the Schmidt camera and the Newtonian telescope. In this telescope design, a spherical primary mirror is combined with a Schmidt corrector plate, which corrects the spherical aberration and holds the secondary mirror. The resulting system has less coma and diffraction effects than a Newtonian telescope with a parabolic mirror and a "spider" secondary mirror support. The design uses a 45° flat secondary mirror to view the image, as in a standard Newtonian telescope.
The Houghton telescope or Lurie–Houghton telescope is a catadioptric telescope. Houghton's original design uses a two-lens corrector at the front of the telescope and a spherical mirror at the back; it was patented in 1944. Instead of the hard to make intricately shaped compound curve Schmidt corrector plate, or the heavy Maksutov-type meniscus corrector lens, the Houghton double-lens corrector is relatively easy to make.
The following timeline lists the significant events in the invention and development of the telescope.
In optics, a Mangin mirror is a negative meniscus lens with the reflective surface on the rear side of the glass forming a curved mirror that reflects light without spherical aberration if certain conditions are met. This reflector was invented in 1874 by a French officer Alphonse Mangin as an improved catadioptric reflector for search lights and is also used in other optical devices.
The first reflecting telescope built by Sir Isaac Newton in 1668 is a landmark in the history of telescopes, being the first known successful reflecting telescope. It was the prototype for a design that later came to be called the Newtonian telescope. There were some early prototypes and also modern replicas of this design.
A three-mirror anastigmat is an anastigmat telescope built with three curved mirrors, enabling it to minimize all three main optical aberrations – spherical aberration, coma, and astigmatism. This is primarily used to enable wide fields of view, much larger than possible with telescopes with just one or two curved surfaces.
