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Reflecting telescope

Reflecting telescope

A reflecting telescope (also called a reflector) is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century 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. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes uses mirrors, the design is sometimes referred to as a "catoptric" telescope.

History

The Italian professor Niccolò Zucchi is credited with making the first reflector in 1616. With it, in 1630 he observed two bands on Jupiter, and in 1640 observed spots on Mars. But his inability to shape the concave mirror accurately, and the lack of means of viewing the image without blocking the mirror, meant others did not adopt Zucchi's design. In 1663 James Gregory published Optica Promota which described the first practical design of a reflector using two concave mirrors. A working example was not built until 10 years later by Robert Hooke. Sir Isaac Newton is credited with constructing the first "practical" reflecting telescope after his own design in 1668. Newton's added a smaller "diagonal" mirror near the primary mirror's focus to reflect the image at 90° angle. This allowed the user to view the image without obstructing the incoming light. Newton invented his reflector to solve the problem of chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses.

Technical considerations

A curved primary mirror is the reflector telescope's basic optical element and creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or an eyepiece for visual observation or a mirror that reflects the image to an eyepiece.

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 thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective front surface. Early reflecting telescopes used a metal objective called a speculum.

Mirrors eliminate the risk of chromatic aberration but may still produce other types of aberrations: In general, on axis they may produce spherical aberration, in which case the outer and inner zones of the telescope do not share a common focus. This was the construction flaw in the Hubble Space Telescope mirrors. Spherical aberration can be eliminated with aspheric (non-spherical) mirrors. Off axis, additional aberrations may become apparent:

  • Coma - a variation of telescope magnification with radial zone on the mirror typically appears as a radial smudging of the images which gets worse at the edges of the field. Spherical aberration and coma are eliminated in two mirror Ritchey Chretien designs.
  • The best image plane is in general curved, which may not correspond to the detector's shape and leads to a focus error across the field.
  • Astigmatism, an azimuthal variation of focus around the aperture. Near the center of the field astigmatism is not usually a problem, but it gets rapidly worse once it becomes apparent - it varies quadratically with field angle.
  • Distortion over the field of view. Distortion does not affect image quality (sharpness) but does affect object shapes. It can be corrected by image processing.

There are reflector designs and modifications such as catadioptrics that correct some of these aberrations.

Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this:

  • In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
  • Light of different wavelengths travels through a medium other than vacuum at different speeds. This causes chromatic aberration in uncorrected lenses and creating an aberration-free large lens is a costly process. A mirror can eliminate this problem entirely.
  • Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or catadioptric.
  • There are structural problems involved in manufacturing and manipulating large-aperture lenses. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter. In contrast, a mirror can be supported by the whole side opposite its reflecting face, allowing for reflecting telescope designs that can overcome gravitational sag. The largest reflector designs currently exceed 10 meters in diameter.

While the Newtonian focus design is still used in amateur astronomy, professionals now tend to use prime focus, Cassegrain focus, and coudé focus designs.

Reflecting telescope designs

Newtonian

The Newtonian usually has a paraboloid primary mirror but for small apertures, say 12 cm or less, if the focal ratio is f/8 or longer a spherical primary mirror is sufficient for high visual resolution. 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.

See also Schmidt-Newton telescope.

The Cassegrain design and its variations

The Cassegrain (sometimes called the "Classic Cassegrain") has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Folding the optics makes this a compact design. On smaller telescopes, and camera lenses, the secondary is often mounted on an optically flat, optically clear glass plate that closes the telescope tube. This support eliminates the "star-shaped" diffraction effects caused by a straight-vaned support spider. The closed tube stays clean, and the primary is protected, at the cost of some loss of light-gathering power.

Ritchey-Chrétien

The Ritchey-Chrétien 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 flat focal plane if the primary and secondary curvature are equal, making it well suited for wide field and photographic observations. Almost every professional reflector telescope in the world is of the Ritchey-Chrétien design. It was invented by George Willis Ritchey and Henri Chrétien in the early 1910s.

Dall-Kirkham

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. It uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to grind than a classic Cassegrain or Ritchey-Chretien system, it does not correct for off-axis coma and field curvature so the image degrades quickly off-axis. Because this is less noticeable at longer focal ratios, Dall-Kirkhams are seldom faster than f/15.

See also Schmidt-Cassegrain telescope, Maksutov telescope.

Gregorian

The Gregorian telescope, invented by James Gregory, employs a concave, not convex, secondary mirror and in this way achieves an upright image, useful for terrestrial observations. Some small spotting scopes are still built this way. The Steward Observatory Mirror Lab has been making mirrors for large Gregorian telescopes at least since 1985. These telescopes include the Vatican Advanced Technology Telescope, the Magellan telescopes, the Large Binocular Telescope, and the Giant Magellan Telescope.

Off-axis designs

There are several designs that try to avoid obstructing the incoming light by eliminating the secondary or moving any secondary element off the primary mirror's optical axis, commonly called off-axis optical systems.

Herschelian

The Herschelian reflector is named after William Herschel, who used this design to build very large telescopes including a 49½ inch (126 cm) diameter telescope in 1789. The design is similar to Niccolò Zucchi's early attempts to build a reflector where the observer stands in front of the objective to view the image. In the Herschelian reflector the primary mirror is tilted so the observers head does not block the incoming light. Although this introduces geometrical aberrations, Herschel employed this design to avoid the use of a newtonian secondary mirror since the fast tarnishing speculum metal mirrors of that time could only achieve 60% reflectivity.

Schiefspiegler

A variant of the Cassegrain, the Schiefspiegler telescope ("skewed" or "oblique reflector"), which uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary. However, while eliminating diffraction patterns this leads to an increase in coma and astigmatism. These defects become manageable at large focal ratios - most Schiefspieglers use f/15 or longer, which tends to restrict useful observation to the moon and planets.

A number of variations are common, with varying numbers of mirrors of different types. The Kutter style uses a single concave primary and a convex secondary. One variation of a multi-schiefspiegler uses a concave primary, convex secondary and a parabolic tertiary. One of the interesting aspects of some Schiefspieglers is that one of the mirrors can be involved in the light path twice - each light path reflects along a different meridional path.

Yolo

The Yolo was developed by Arthur S. Leonard in the mid 1960s Like the Schiefspiegler, it is an unobstructed, tilted reflector telescope. The Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the main axis. The Yolo design eliminates coma, but leaves significant astigmatism, which is reduced by deformation of the secondary mirror by some form of warping harness, or alternatively, polishing a toroidal figure into the secondary.

Focal planes

Prime focus

In a prime focus design in large observatory telescopes, the observer sits inside the telescope, at the focal point of the reflected light. In the past this would be the astronomer himself, but nowadays CCD cameras are used. The space available at prime focus is severely limited by the need to avoid obstructing the incoming light.

Radio telescopes often have a prime focus design. The mirror is replaced by a metal surface for reflecting radio waves, and the observer is an antenna.

See also:

Nasmyth and Coudé focus

Nasmyth

The Nasmyth design is similar to the Cassegrain except no hole is drilled in the primary mirror; instead, a third mirror reflects the light to the side.

Coudé

Adding further optics to a Nasmyth style telescope that deliver the light (usually through the declination axis) to a fixed focus point that does not move as the telescope is reoriented gives you a Coudé focus. This design is often used on large observatory telescopes, as it allows heavy observation equipment, such as spectrographs, to be more easily used.

References

See also

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