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
