A reflecting telescope 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 use mirrors, the design is sometimes referred to as a "catoptric"
telescope. History
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. Soon after the invention of the
refracting telescope, Galileo, Giovanni Francesco Sagredo, and others, spurred
on by their knowledge of the principles of curved mirrors, discussed the idea of
building a telescope using a mirror as the image forming objective. 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. The potential advantages of using parabolic
mirrors, primarily reduction of spherical aberration with no chromatic
aberration, led to many proposed designs for reflecting telescopes the most
notable being James Gregory’s 1663 published ideas for what came to be
called the Gregorian telescope, but no working models were built until 1673 by
Robert Hooke. Isaac Newton has been generally credited
with building the first reflecting telescope in 1668. It used a spherically
ground metal primary mirror and a small diagonal mirror in an optical
configuration that has come to be known as the Newtonian telescope.
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. Many of the
advances in reflecting telescopes included the perfection of parabolic
mirror fabrication in the 18th century, silver coated glass mirrors in the 19th
century, long-lasting aluminum coatings in the 20th century, segmented mirrors
to allow larger diameters, and active optics to compensate for gravitational
deformation. A mid-20th century innovation was catadioptric telescopes
such as the Schmidt camera, which use both a spherical mirror and a lens as
primary optical elements, mainly used for wide-field imaging without spherical
aberration. The late 20th century has seen the
development of adaptive optics and lucky imaging to overcome the problems of
seeing, and reflecting telescopes are ubiquitous on space telescopes and many
types of spacecraft imaging devices. Technical considerations
A curved primary mirror is the reflector telescope's basic optical element that
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 a secondary mirror may be added to modify the
optical characteristics and/or redirect the light to film, digital sensors, or
an eyepiece for visual observation. 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 first
surface mirror. Some telescopes use primary mirrors
which are made differently. Molten glass is rotated to make its surface
paraboloidal, and is kept rotating while it cools and solidifies. The resulting
mirror shape approximates a desired paraboloid shape that requires minimal
grinding and polishing to reach the exact figure needed.
= Optical errors= Reflecting telescopes, just like any
other optical system, do not produce "perfect" images. The need to image
objects at distances up to infinity, view them at different wavelengths of
light, along with the requirement to have some way to view the image the
primary mirror produces, means there is always some compromise in a reflecting
telescope's optical design. Because the primary mirror focuses light
to a common point in front of its own reflecting surface almost all reflecting
telescope designs have a secondary mirror, film holder, or detector near
that focal point partially obstructing the light from reaching the primary
mirror. Not only does this cause some reduction in the amount of light the
system collects, it also causes a loss in contrast in the image due to
diffraction effects of the obstruction as well as diffraction spikes caused by
most secondary support structures. The use of mirrors avoids chromatic
aberration but they produce other types of aberrations. A simple spherical
mirror cannot bring light from a distant object to a common focus since the
reflection of light rays striking the mirror near its edge do not converge
with those that reflect from nearer the center of the mirror, a defect called
spherical aberration. To avoid this problem most reflecting telescopes use
parabolic shaped mirrors, a shape that can focus all the light to a common
focus. Parabolic mirrors work well with objects near the center of the image
they produce,, but towards the edge of that same field of view they suffer from
off axis aberrations: Coma - an aberration where point sources
at the center of the image are focused to a point but typically appears as
"comet-like" radial smudges that get worse towards the edges of the image.
Field curvature - 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.
It is sometimes corrected by a field flattening lens.
Astigmatism - an azimuthal variation of focus around the aperture causing point
source images off-axis to appear elliptical. Astigmatism is not usually a
problem in a narrow field of view, but in a wide field image it gets rapidly
worse and varies quadratically with field angle.
Distortion - Distortion does not affect image quality but does affect object
shapes. It is sometimes corrected by image processing.
There are reflecting telescope designs that use modified mirror surfaces or
some form of correcting lens that correct some of these aberrations.
Use in astronomical research Nearly all large research-grade
astronomical telescopes are reflectors. There are several reasons for this:
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 in a catadioptric telescope. 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. Reducing this to acceptable
levels usually involves a combination of two or three aperture sized lenses. The
cost of such systems therefore scales significantly with aperture size. An
image obtained from a mirror does not suffer from chromatic aberration to
begin with, and the cost of the mirror scales much more modestly with its size.
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. Reflecting telescope designs
= Gregorian= The Gregorian telescope, described by
Scottish astronomer and mathematician James Gregory in his 1663 book Optica
Promota, employs a concave secondary mirror that reflects the image back
through a hole in the primary mirror. This produces an upright image, useful
for terrestrial observations. Some small spotting scopes are still built this
way. There are several large modern telescopes that use a Gregorian
configuration such as the Vatican Advanced Technology Telescope, the
Magellan telescopes, the Large Binocular Telescope, and the Giant Magellan
Telescope. = Newtonian=
The Newtonian telescope was the first successful reflecting telescope,
completed by Isaac Newton in 1668. It usually has a paraboloid primary mirror
but at focal ratios of f/8 or longer a spherical primary mirror can be
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.
= The Cassegrain design and its variations=
The Cassegrain telescope was first published in an 1672 design attributed
to Laurent Cassegrain. It has a parabolic primary mirror, and a
hyperbolic secondary mirror that reflects the light back down through a
hole in the primary. Folding and diverging effect of the secondary
creates a telescope with a long focal length while having a short tube length
Ritchey–Chrétien 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. 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. Almost every professional reflector
telescope in the world is of the Ritchey–Chrétien design.
Three-mirror anastigmat Including a third curved mirror allows
correction of the remaining distortion, astigmatism, from the Ritchey–Chrétien
design. This allows much larger fields of view.
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–Chrétien system, it does not correct for off-axis coma. Field
curvature is actually less than a classical Cassegrain. Because this is
less noticeable at longer focal ratios, Dall–Kirkhams are seldom faster than
f/15. Takahashi Mewlon telescopes are Dall-Kirkham instruments with f/12 and
are highly regarded. They require a corrector for wide field applications.
= 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 49.5 inch diameter telescope in 1789.
In the Herschelian reflector the primary mirror is tilted so the observer's 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 speculum metal mirrors of that time tarnished
quickly and could only achieve 60% reflectivity.
Schiefspiegler A variant of the Cassegrain, the
Schiefspiegler telescope 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, a convex secondary and
a plano-convex lens between the secondary mirror and the focal plane,
when needed. 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. Stevick-Paul
Stevick-Paul telescopes are off-axis versions of Paul 3-mirror systems with
an added flat diagonal mirror. A convex secondary mirror is placed just to the
side of the light entering the telescope, and positioned afocally so as
to send parallel light on to the tertiary. The concave tertiary mirror is
positioned exactly twice as far to the side of the entering beam as was the
convex secondary, and its own radius of curvature distant from the secondary.
Because the tertiary mirror receives parallel light from the secondary, it
forms an image at its focus. The focal plane lies within the system of mirrors,
but is accessible to the eye with the inclusion of a flat diagonal. The
Stevick-Paul configuration results in all optical aberrations totaling zero to
the third-order, except for the Petzval surface which is gently curved.
Yolo The Yolo was developed by Arthur S.
Leonard in the mid-1960s. Like the Schiefspiegler, it is an unobstructed,
tilted reflector telescope. The original Yolo consists of a primary and secondary
concave mirror, with the same curvature, and the same tilt to the main axis. Most
Yolos use toroidal reflectors. 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. Like Schiefspieglers, plenty of Yolo's
variations have been pursued. The needed amount of toroidal shape can be
transferred entirely or partially to the primary mirror. In large focal ratios
optical assemblies, both primary and secondary mirror can be left spherical
and a spectacle correcting lens is added between the secondary mirror and the
focal plane. The addition of a convex, long focus tertiary mirror leads to
Leonard's Solano configuration. The Solano telescope doesn't contain any
toric surface. = Liquid mirror telescopes=
One design of telescope uses a rotating mirror consisting of a liquid metal in a
tray which is spun at constant speed. As the tray spins the liquid forms a
paraboloidal surface of essentially unlimited size. This allows for very big
telescope mirrors, but unfortunately they cannot be steered, as they always
point vertically. Focal planes
= Prime focus= In a prime focus design no secondary
optics are used, the image is accessed at the focal point of the primary
mirror. At the focal point is some type of structure for holding a film plate or
electronic detector. In the past, in very large telescopes, an observer would
sit inside the telescope in an "observing cage" to directly view the
image or operate a camera. Nowadays CCD cameras allow for remote operation of
the telescope from almost anywhere in the world. 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.
= Nasmyth and coudé focus= Nasmyth
The Nasmyth design is similar to the Cassegrain except the light is not
directed through a hole in the primary mirror; instead, a third mirror reflects
the light to the side of the telescope to allow for the mounting of heavy
instruments. This is a very common design in large research telescopes.
Coudé Adding further optics to a Nasmyth-style
telescope to deliver the light to a fixed focus point that does not move as
the telescope is reoriented gives a coudé focus. The coudé focus gives a
narrower field of view than a Nasmyth focus and is used with very heavy
instruments that do not need a wide field of view. One such application is
high-resolution spectrographs that have large collimating mirrors and very long
focal lengths. Such instruments could not withstand being moved, and adding
mirrors to the light path to divert the light to a fixed position to such an
instrument housed on or below the observing floor was the only option. The
60-inch Hale telescope, Hooker Telescope, 200-inch Hale Telescope,
Shane Telescope, and Harlan J. Smith Telescope all were built with coudé foci
instrumentation. 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. However, since
inexpensive and adequately stable computer-controlled alt-az telescope
mounts were developed in the 1980s, the Nasmyth design has supplanted the coudé
focus for large telescopes. See also
Catadioptric telescopes Honeycomb mirror
List of largest optical reflecting telescopes
List of largest optical telescopes historically
List of telescope types Mirror support cell
PLate OPtimizer Refracting telescope
References External links