Tools of the Astronomer
Part 1 - Ground-based Optical Telescopes
Telescopes
are time machines
-
Carl Sagan
Window to the
World
Long
before the telescope was invented, it was well known that a piece of glass with
a convex surface (later called a lens) could be used as a magnifying device
through its ability to concentrate or disperse light. It was not until the beginning of the
seventeenth century that a pair of lenses of differing surface curvatures were
used to make a distant object appear closer and
larger. This marked the birth of the
optical telescope, whose invention is generally attributed to the Dutch lens
maker Hans Lipperhey in 1608. It would
have remained largely a curiosity were it not for the genius of the great
Galileo Galilei (1564 - 1642) who exploited its potential fully, both for
terrestrial and celestial observations, thus opening up a window to the outside
world. It was he who not only fabricated
the first astronomical telescope but also used it to make most of the path
breaking discoveries in modern astronomy.
They eventually led to the acceptance of the Copernican heliocentric
system of planetary motions, and ushered in a far reaching revolution in
scientific thought.
As
with anything new in the world of Science and Technology, Galileo’s invention
was received initially with a lot of indifference, scepticism, disbelief and
even scorn as it challenged long-held beliefs and prejudices. Today it is looked upon as one of the
greatest inventions of all time and Galileo himself as the father of modern
science.
The Galilean Telescope
Galileo’s early telescopes consisted of a biconvex
objective lens and a biconcave eye piece as in the diagram below and formed a non-inverted
image of the distant object. Though
compact, they were very crude by later standards, had a narrow field of view,
with limited magnifications, and the images were quite fuzzy because of the poor
quality of the lens surfaces.
Despite these serious limitations Galileo made some epoch
making discoveries that included lunar surface features, four satellites of
Jupiter and the phases of Venus. He
observed that the four tiny satellites of Jupiter orbited the planet much the
same way that the planets themselves went around the Sun. This discovery gave a knockout blow to the
prevailing geocentric world view which had placed the earth at the centre of
everything observable.
Shown
in the picture below are a painting of Galileo and two of his earliest telescopes, displayed in a Science
museum in Florence, Italy.
Refractor Telescopes
The
Galilean telescope is one of several types of what are called Refractor Telescopes since they use
lenses. Image formation is by refraction and convergence of light through the
objective lens of a large focal length.
Eyepieces of much shorter focal lengths and small apertures are used to
magnify the image formed by the objective for comfortable viewing or
photography.
In a
refractor telescope, light from a distant object is brought to focus through a
biconvex objective lens. The intermediate
image so formed is magnified when viewed through the eyepiece to give a much
larger (inverted) image as depicted in the diagram here.
In most refractor telescopes the eyepiece is placed in an adjustable housing at right angles to the viewing end to facilitate comfortable viewing. A reflecting mirror or prism is used to direct the image to the eyepiece. An adjustable laser pointer can be mounted near the eyepiece holder to locate the desired object precisely, especially in the night sky.
Images
formed by refractors suffer from a variety of defects (called aberrations) during the process of
refraction and dispersion through the glass material. A few of them are mentioned here.
A blurring
of the image due to refraction, often noticeable at the edges, is called spherical aberration. Light rays near the edges of the lens are
focussed slightly closer to the lens than the axial rays as shown in the
diagram. This causes the smeared out appearance of the image.
A lens
refracts different colours present in white light to different extents depending
on the wavelength of the component and this results in what is called chromatic aberration. The inability of the lens to bring all of the colours into a
common focal plane results in a slightly different image size and focal point
for each of the component wavelengths. The result is a coloured halo
surrounding the image, with the colour changing as the focal point of the
objective shifts.
If
light rays refracting through a lens in two perpendicular planes come to a
focus at different points, the resulting aberration is called astigmatism. This is generally due to
imperfections in the lens surfaces. The
human eye is particularly vulnerable to astigmatism, which can be corrected
through properly shaped spectacle lenses.
These
and other optical aberrations result in a significant degradation of the image
produced by lenses. It is possible to overcome them and achieve a high quality
image through the use of combination
of two or more lenses of differing shapes, curvatures and densities. Such complex imaging systems are invariably
employed in modern cameras and other optical instruments where image
fidelity is an important consideration.
Optical
aberrations in small refractor telescopes are minimised with combination lenses
only in the case of small telescopes.
For large telescopes this is not attempted at all since reflector telescopes offer a vastly
superior alternative to refractors as discussed later.
Some Characteristics of Telescopes
Why are
big telescopes preferable to smaller ones given the same image quality? Here are some characteristics of optical
telescopes which show why:
Light Gathering Power
Obviously,
the larger the aperture (diameter) of the objective the more the light gathering power of the
telescope. As an example, if two similar open trays of different sizes are placed under steady rain, the tray with the larger
exposed area will collect proportionately more water in a given time than the
smaller tray. For analysing the light
gathered from any faint distant object this is a major consideration. Since the exposed area is proportional to the
aperture, a 20 cm aperture telescope is four times as good as a 10 cm one … and
a 2 m aperture one 400 times as good!
Resolving Power
Resolving power is another feature of a telescope,
often more important than its light gathering power. It is the ability
of the instrument to distinguish clearly between two points
whose angular separation is less than the smallest angle that the normal human
eye can resolve. It is directly
proportional to the aperture of the telescope and can be calculated from the
formula RP = (D/1.22 λ), where D is the aperture and λ the wavelength of
light involved (600 nm can be used as a representative value for white light).
The factor 1.22 arises from the application of what is known as Rayleigh’s
criterion for two closely spaced objects to be treated as resolved and takes
into account the diffraction of light by the objects.
Magnification
The magnification that a telescope can
provide is another key feature to consider. It is given by m = (fo/fe),
where fo is the focal length of the objective and fe the focal length of the eyepiece. To
obtain different magnifications with a given telescope (of fixed objective)
eyepieces of different focal lengths can be used. The magnification is also
equal to Do/De, where Do is the aperture
(diameter) of the objective and De is the aperture of the
eyepiece.
Beyond
a certain magnification the telescope may not be of much use since the field of
view and the image brightness both decrease in inverse proportion. On the other
hand, reducing the magnification increases the image brightness while
maintaining the same resolution, which is a major advantage especially for
photography and photometry.
Field of View (FOV)
This is
the open observable area visible through the eyepiece of the telescope and is
generally specified in angular measure.
For any given instrument the FOV depends on the focal length of the
eyepiece used, decreasing as this increases.
Obviously, higher the magnification lower the FOV. To observe the whole of the full Moon, the
FOV required is just over half a degree. Incidentally, binoculars are designed to provide
much larger FOVs than telescopes.
f-number
In line
with the common usage associated with camera lenses, the f-number of the
objective of a telescope is defined as fo/D. Thus, the
f-number of a telescope whose objective lens has an aperture of 15 cm and focal
length of 1.5m is denoted f/10.
World’s Largest Refractor
Because of numerous optical
aberrations which are very difficult to eliminate and huge fabrication
challenges and costs, large refractor telescopes are very rare and only of
historic interest today. The largest
such telescope ever built was the 40” (1.02 m) aperture instrument built at the
Yerkes Observatory in Wisconsin, USA.
Sadly, it is no longer in use. Here is a picture taken against the background of this telescope when
the celebrated physicist Albert Einstein visited the facility in 1921. The
large proportion of women in the picture is appropriately indicative of the
appeal Astronomy has always carried for women in the USA and elsewhere!
Another great refractor telescope of historic importance is also worth mentioning here. It is the 26-inch "Great Equatorial" located on the grounds of the US Naval Observatory at Washington, DC and still in limited use, with a rich history behind it. Completed in 1873, it was the largest refracting telescope in the world for a decade. It was from here, in August 1877, that astronomer Asaph Hall discovered the two tiny moons of Mars, Phobos and Deimos, drawing the attention of the world to the contribution of US Navy to astronomical research. This author cherishes a fond memory of his visit to this observatory in December 1966.
Binoculars
Though
the binoculars is a familiar and widely used piece of optical equipment,
somewhat like the camera, its use for viewing the night sky objects and star
fields is not widely known. Effectively,
it is two ‘folded’ telescopes placed side by side on a common framework with a
focussing thumbwheel for hand-held viewing.
The compact size and re-inverted image arising from the ‘folding’ of the
light path is achieved through the use of a pair of prisms as shown in the
cut-away diagram. Binoculars carry numbered designations like 8x40, 7x50,
12x80, etc. The first number is the
magnification and the second one the aperture.
For astronomical use, 7x50 binoculars is probably best suited because of
the right balance between the two parameters. A
seriously interested sky watcher will do well to begin with such an instrument
before going for any telescope.
Reflector Telescopes
The highly
popular reflecting telescope was invented by the great Isaac Newton in
1668. The big breakthrough he made was
in replacing the objective lens of a refractor with a curved reflecting mirror.
Since the primary image is formed by reflection
from a polished or coated surface, most of the optical aberrations either do
not exist or can be more easily minimised.
A replica of Newton’s earliest reflecting telescope is shown above together with a modern version of a handy commertial portable reflector on an equatorial mount. In Newton's instrument, the mirror was made of a copper-tin alloy. Near the other end of this polished primary mirror Newton placed a small flat secondary mirror at a 45-degree angle, to reflect the light into an eyepiece mounted on the side of the telescope tube. The quality of the image was highly disappointing initially. It improved greatly when the primary mirror was made of glass, with its curved surface coated with a reflective layer. Later, vacuum coating of the surface with silver or aluminium deposition marked a great advancement in telescope making.
Since
image formation is by reflection and not by refraction, there was no question
of chromatic aberration. Spherical
aberration could also be avoided by replacing the spherical mirror surface with
a parabolic one, though this was not easy to achieve.
Imaging in Reflector Telescopes
Depending
on how and where the final image is formed, reflector telescopes are classified
as indicated in the following ray diagrams:
The
Cassegrainian telescopes offered some significant operational advantages over the other
types, especially in large instruments.
Schmidt Corrector Plate
The
difference in image formation between a spherical
reflecting surface and a paraboloidal
reflecting surface can be seen in the following diagram:
The
paraboloidal surface produces an image without any spherical aberration, but it
is a great deal more difficult to fabricate than a spherical surface with its
attendant aberration.
In
1931, Bernhard Schmidt came up with an ingenious work-around to get the best
out of a spherical mirror. He designed a thin highly transparent corrector plate placed in front of the
spherical surface with one of its surfaces shaped in such a manner as to
effectively cancel out the spherical aberration and form a sharply focused
image. The following diagram illustrates
how this is achieved:
Both the thickness and the curvature of the corrector plate are shown highly exaggerated here. In actual fact it is very thin, with the curvature hardly noticeable. In addition to its primary role, the corrector plate also served to shield the mirror from the atmosphere by acting as a protective barrier and allow only light to get through.
Catadioptric Telescopes
The
combination of a thin Schmidt type corrector lens and a reflecting spherical
mirror gives a class of efficient (and relatively inexpensive) telescopes
termed Catadioptric. A great
additional advantage of such systems is the substantial reduction in the length
and mass of the instrument. Use of a small secondary reflecting mirror at the
centre of the corrector plate will result in a folded (triple) path for the
light rays inside the telescope tube. A sophisticated portable computerised Schmidt-Cassegrainian
telescope of this type with folded optics (owned by this author) is shown below
with its corresponding ray diagram:
Advantages of Reflector Telescopes
Reflector
telescopes present numerous advantages compared to refractor types, especially
where large telescopes are concerned.
Here are some of the major ones:
- While optical aberrations degrading the image quality are a major issue with refractors, these are either non-existent or greatly reduced in reflectors. In particular, the virtual elimination of chromatic aberration is a huge advantage.
- Considering that the resolving power and light gathering power both increase in proportion to the aperture of the telescope, size for size it is easier and less expensive to build large and medium sized reflectors as compared to refractors. This is true of portable telescopes also.
- Large refractors would also require large objective lenses and, apart from the engineering and cost factors, the significant absorption of light through the thick lenses involved is another major deterrent.
- While quite a number of reflectors are now operational with apertures of the order of 3-5 metres or larger, refractors with apertures even of the order of 1-2 metres are now virtually defunct.
- Reflector telescopes can be used effectively not only in the visible region of the electromagnetic spectrum but also in the ultraviolet and near infrared regions. This greatly enhances the utilitarian aspect of large reflectors.
Some of the largest reflector telescopes in the world
Hundreds
of reflectors with apertures in the range 2 – 10 metres are in use for
astronomical and astrophysical research and studies all over the world. Just a few of the more important ones are
outlined below:
Perhaps, of the greatest importance of all is the 100”
(2.5 m) Hooker Telescope at the
Mount Wilson Observatory in California, USA. Completed in 1917, it was the
world’s largest until 1949 and ushered in some of the most path breaking
discoveries in Astronomy. Much of the
work leading to the epoch making and now well understood theory of the origin
and expansion of the universe was done here by the celebrated astronomer Edwin
Hubble. Earlier, the fact that the
Andromeda Nebula was indeed a full-fledged galaxy like our own Milky Way, and
external to it, had been established with its use. Later, the first indication of the existence
and nature of dark matter in the
universe came from here.
Commissioned in 1949, the Hale telescope, named after astronomer George Ellery Hale, and
located on Mount Palomar in California, USA, was the world’s largest optical
telescope following the Hooker Telescope.
With an aperture of 200 inches (5.2 m) it was twice as large as its
predecessor, incorporated some path breaking technologies in its construction
and contributed equally to the advancement of observational astronomy,
including the direct imaging of a few exoplanets in 2010. The mirror itself took about eleven years to
fabricate starting from the molten glass that was poured into the mould. Overall, the project took two decades to
complete. Pictured here are both the
telescope and its housing dome.
Appropriately named after the
inventor of the reflecting telescope, the Isaac Newton Telescope or INT is
a 2.54 m (100 in) optical telescope managed by the Isaac Newton Group of Telescopes at Roque de los Muchachos
Observatory on La Palma in the Canary Islands since 1984. Originally the INT was situated
at Herstmonceux
Castle in Sussex, England, which was the site of the Royal Greenwich Observatory after it moved away from Greenwich due to light
pollution. In 1979, the INT was shipped to La Palma, where it has remained ever
since.
India also
figures among the giant optical telescopes of the world through its 3.6 m Devasthal Optical Telescope (DOT) built
by the Aryabhatta
Research Institute of Observational Sciences (ARIES) and located at the Devasthal Observatory site
near Nainital, Uttarakhand. The telescope was activated remotely on 31
March 2016 by Indian Prime Minister Narendra Modi and Belgian Prime
Minister Charles Michel from Brussels. The telescope optics has
been built in collaboration with the Belgian firm Advanced Mechanical and
Optical System (AMOS). The 3.6m DOT is currently the largest reflecting
telescope in Asia. The telescope is also the first of its kind in India
that features an active-optics
system
(being discussed later).
There is a technological limit to the size and surface
area of primary mirrors made of a single piece of glass substrate. They cannot
be fabricated larger than about eight meters in diameter. Using a
monolithic mirror much larger than about 5 meters becomes prohibitively
expensive. The best way to overcome this limitation is to use multiple segmented mirrors to effectively
function as a single device, an idea pioneered in the 1980s by Jerry Nelson at
the Lawrence Berkeley National Laboratory of the University of California. Many of the recently built giant sized
telescopes have adopted this idea. The
mirrors need to be controlled individually with exceptional precision so that
they collectively function perfectly as a single mirror of the combined
aperture.
Keck
Twin Telescopes
The strategy of building a telescope from segmented
mirrors was soon realised in the historic 10 m aperture Keck twin Telescopes,
each made of a honeycomb of 36 hexagonal segments (as can be seen clearly in
the picture), each of 1.8 m wide and only 7.5 cm thick. These telescope are
part of the W M Keck Observatory setup on the summit of the 4145 m high Mauna Kea Mountain in Hawaii. The key technological development that made
possible the construction of such huge telescopes was the use of advanced real time computer feedback and control systems, like active optics and adaptive optics, to operate
smaller mirror segments together as a single contiguous
mirror.
Active and
Adaptive Optics
Large telescope mirrors are very
heavy and suffer from structural and thermal instabilities which can distort
their surface. Even very small distortions can degrade the image significantly.
This places a practical limit on the size of any type of telescope. Another
complication is that the atmospheric temperature and density both fluctuate
continually, causing light waves to be distorted from their original form. Locating the telescopes at high altitudes is a
partial solution to this problem. Light pollution near urban areas is another
serious and continually worsening problem.
The techniques of active and adaptive optics now allow astronomers to
overcome these limitations far more effectively.
Active optics provides a way of deforming a mirror to
compensate for its lack of structural rigidity. In adaptive optics, the optical elements of the telescope are
instantaneously and continually adjusted to compensate for, and effectively
neutralize, the blurring effect of the Earth's atmosphere. Both active optics
and adaptive optics use actuators (tiny pistons) and computers in a constantly
monitored electronic feedback system to make minute adjustments to the shape of
the primary and secondary reflective surfaces.
The main
difference between the two techniques is that any changes made with active
optics are relatively slow and intended to correct for the sagging or wind
induced vibrations of the telescope as it tracks an object across the sky. In
contrast, adaptive optics is intended to remove the effect of turbulence in the
Earth's atmosphere and therefore makes much faster, continual, real time
adjustments.
Some giant Optical Telescopes of today
Top of the list
in terms of equivalent aperture is the Large
Binocular Telescope (LBT) located on top of Mount Graham in Arizona, USA. With an effective aperture of 11.8 m, each of
these two 8.4 m aperture mirrors is the single largest non-segmented mirror in
the world. Functional since 2005, it is
a joint project of a number of universities and incorporates both active and
adaptive optics. The primary use of this facility is in imaging through specialized techniques of aperture synthesis and long baseline interferometry.
One of the
largest multi segmented mirror telescopes in the world, the Hobby-Eberly Telescope (HET) located at
the McDonald Observatory in Texas,
USA, combines a number of features that
differentiate it from most telescope designs, resulting in significantly
reduced construction costs. The primary mirror is a honeycomb of 91
hexagonal segments adding up to an equivalent aperture of just over 10 m. It is fixed at a 55° angle and can only rotate around its base. A
target is tracked by moving the instruments at the focus of the telescope. This
provides access to about 70–80% of the sky at its location and allows a single
target to be tracked for up to two hours. The telescope is used primarily for
spectroscopic studies and does not need very sophisticated and expensive
control systems like other telescopes of the same class. It is actively engaged
in studies related to the properties and distribution of dark matter in the universe.
The author of this article fondly
recalls a whole day spent in visiting the numerous facilities at the sprawling
McDonald Observatory and interacting with some of its staff on a rainy and
gloomy day in August 2017 after viewing a total solar eclipse near faraway Nashville,
Tennessee a week earlier. He also had a long interaction with a
resident astronomer at the campus trying to understand the operational details
of the telescope and its efficacy in the ongoing cosmological studies. The composite picture below commemorates his visit to the Hobby-Eberly telescope.
The Gemini Observatories consist of two
identical 8.1 m aperture single mirror telescopes, one in the northern
hemisphere and the other in the southern hemisphere, at two of the best
locations for observational astronomy on the planet. Gemini North is part of the Mauna
Kea Observatories in Hawaii and Gemini
South is at Cerro Pachon in
Chile, both belonging to a global consortium of seven countries. Gemini South is close to the extensive Inter-American
Observatory at Cerro Tololo. Together they can cover all parts of the night
sky visible from the Earth.
A number of
giant telescopes are either already under various stages of construction or in
advanced stages of planning. They
include the following:
The Vera
C Rubin Observatory, named after a pioneering woman astronomer and formerly
known as the Large Synoptic Survey Telescope (LSST), is under construction at Cerro Pachon in Chile, close to the
existing Gemini South telescope. It has
a wide field 8.4 m primary mirror designed to photograph the entire available
night sky once every few days for building up maps in unprecedented detail. The telescope uses a novel three-mirror design, which allows
a compact telescope to deliver sharp images over a very wide 3.5-degree
diameter field of view. Images are recorded by a 3.2-gigapixel CCD imaging camera, the largest digital camera produced till recently. The
facility is being operated under the management of the Association of Universities for Research
in Astronomy (AURA). The picture below shows the top-end assembly being lowered
by a 500-ton crane. Work is expected to
be completed in another two years.
The Thirty Meter Telescope (TMT) is one of Astronomy's next generation telescopes, coming up on Mount Mauna Kea in Hawai, home to possibly the world's largest collection of giant telescopes. Being built by an international consortium that includes India, Japan and China, Its 30m diameter prime-mirror will enable observations from ultraviolet to mid-infrared wavelengths, with state-of-the-art adaptive optics systems that will compensate for the effects of Earth's atmosphere, and produce images rivaling those of the Hubble Space Teelscope and the James Webb Space Telescope as well.
Ground-based optical telescopes continue
to provide a huge impetus to the advancement of observational astronomy and
compliment the enormous contributions of space based instruments such as the
great Hubble Space Telescope and its successor, the James Web Space
Telescope. The following graphic presents a comparative picture of some of the
world’s best known optical telescopes that are in operation or under various
stages of completion. In concert with
telescopes operating in other regions of the electromagnetic spectrum, optical
telescopes are expanding the frontiers of astronomy, astrophysics and cosmology
at a breath taking pace and contributing to our continuing quest for
understanding our universe.
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