Episode 21: Amazing Reflectors
Dr. Arvind C. Ranade
Points to be covered:
- Limitations of refracting telescopes; spherical and chromatic aberrations
- Difficulties in making large achromatic lenses
- Advantages of reflecting telescopes; absence of spherical and chromatic aberrations,
ease of making and handling large mirrors
- Reflecting telescopes, Newtonian, and other types
- Multi-mirror telescopes
Emphasize and Comments:
- Principle of reflecting telescope
- Parabolic surface to eliminate spherical aberration
- World’s large telescopes; Hale, VLT, Keck, India’s Vainu-Bappu telescope at Kavalur
- Use of film, photomultipliers, and CCD to capture image
- Remote telescope
Outline of content:
From the beginning, astronomy has been an observational science. In comparison with
what was previously possible with the naked eye, Galileo’s invented telescope greatly
improved the ability to observe the universe. Today we continue to enhance our ability to
see the faint objects like nebulae, galaxy, star clusters, etc. Although observational
astronomy now covers the entire range of the electromagnetic spectrum, the most familiar
part of the field remains in the optical regime of the human eye. As a result, telescope and
detectors designed to investigate optical-wavelength radiation is at higher population. In
this episode we will see how reflectors are the best over refractor and will explore to the
different reflectors at national and international level. We will also talk about the present
day technology used to make the telescope with larger aperture size.
Telescope can largely be divided into two categories; refractor- use of lens only and the
reflector where lens and mirrors are used. Refracting telescopes suffers from a serious
optical distortion named aberration. When light is refracted through glass, shorter
wavelength bend more than longer wavelengths, and blue light comes to a focus closer to
the lens than does red light. If we focus the eyepiece on the blue image, the red light is
out of focus, producing a red blur around the image. If we focus on the red image, the
blue light blurs. This color separation is called chromatic aberration.
This kind of aberration can partially be corrected by replacing the single objective lens
with one made of two lenses, called achromatic lens, can be designed to bring any two
colors to the same focus. In reality not all colors can be brought to a single point but to
the maximum one can try to bring red and yellow to the maximum closer but blue and
violet would still be a challenge. This is because human eye are more sensitive to red and
yellow than the other colors.
Refracting telescopes were popular in nineteenth century, but they are no longer
economical for professional astronomy. A large achromatic lens is very expensive
because it contains four matched optical surfaces and must be made of high-quality glass.
Another problem with refractor is that one can not go for making the aperture size more
than 1 meter in diameter because such large lenses sag under their own weight. Also large
refractors have very long telescope tubes that require larger observatory domes.
Therefore, refractors are best used for amateur astronomy as layman tool to explore the
night sky by themselves. As seen in the last episode the present day available refractor is
of about 1.02 meter located at Yerke’s observatory at Wisconsin in United States, which
is world’s largest of today. Due to number of limitations, this kind of telescope is not
used for the real astronomical observations.
In reflecting telescope, the objective lens is replaced by a concave mirror which focuses
the starlight into an image that can be viewed with an eyepiece. Objective mirrors are
usually made of special kinds of glass or quartz covered with a thin layer of aluminum to
act as a reflecting surface. The objective mirror forms an image at the location called the
prime focus at the upper end of the telescope tube. In larger telescopes, astronomers can
ride inside a small compartment to photograph objects using prime focus, or they can
place cameras and instruments there which are controlled from a remote control room. To
use the telescope more conveniently, a small secondary mirror placed at 45 degree can be
used to reflect light out of telescope via a side, such arrangement is called Newtonian
telescope. The other alternative to get the light out of tube is through a hole in the
objective mirror; such arrangement is named as Cassegrain telescope. These kinds of
telescopes are used by the amateur as well as professional individuals; this is because
astrophotography and photometry of astronomical objects by a small amateur group or by
a single professional is possible by such telescope.
Nearly all recent built telescopes are reflectors. Because the light does not enter the glass,
there is no chromatic aberration, the glass need not be of perfect optical quality, and the
mirror can be supported over its back to reduce sagging. Another advantage is that the
reflectors tend to be shorter and thus require smaller mountings and smaller observatory
building. In present astronomy telescope mounting is more expensive than the telescope
itself. This is because mounting supports the optics, protect against vibration, move
accurately to designated objects and also compensate for the Earth’s rotation. Basically
there are two types of mounting used in the present day observatories, one alt-azimuth
and RA-DEC. In alt-azimuth system, the telescope moves perpendicular to horizon called
altitude and parallel to horizon called azimuth. Similarly, for fixed coordinate system of
stars that’s right accession parallel to azimuth and declination parallel to horizon
measured on the celestial equator is other type of coordinate system. Now-a-days, these
coordinates along with the different motion of telescopes are totally controlled by
sophisticated machines installed at the observatory which give the motions to the
precision of microns and track the object to the accuracy of arc seconds.
Astronomers have been developing new techniques to make large mirrors that weigh and
cost less. To give the example, the Steward Observatory Mirror Laboratory has built a
revolving oven at University of Arizona that can produce preshaped mirrors. The oven
turns like a merry-go-round, and the molten glass flows outward in the mold to form a
concave upper surface. The oven is currently able to cast mirrors as large as 8.4 meter in
diameter.
The mirrors of 8 meter in diameter would be too heavy to support in a telescope, so
astronomers have devised ways to make the mirrors thinner; the technique is to make the
mirror in segments. Small segments are less expensive and sag less under their own
weight. The large general purpose optical telescopes in the world are the twin 10 meter
Keck-I and Keck-II telescopes in Hawaii at Mauna Kea. Each of these giant telescopes
uses 36 hexagonal mirror segments held in alignment by computer-controlled thrusters to
from a single mirror. These telescopes are operational since 1993 (Keck-I) and 1996
(Keck-II) respectively. The special purpose Hobby-Eberly telescope in Texas uses 91
hexagonal segments to make up a mirror of 9.2 meter in diameter. This observatory
named Mc Donald at Texas is operational since 1999. Another alternative to reduce the
weight of telescope mirror is to make them thin. Thin mirrors sag so easily under their
own weight that they are called floppy mirrors, but a computer can control their shape in
through active optics- in which small actuators are placed at the base of mirror. These
actuators are in the typical network placed very near to each other can operated by the
continuous electronics applied to it. The new technology telescope at the European
Southern Observatory in Chile contains a 3.58 meter mirror that is only 24 cm in
thickness.
The Very Large Telescope (VLT) built by the European Southern Observatory is located
high in the remote Andes mountains in northern Chile. The VLT consists of four
telescopes with computer controlled main mirrors 8.2 meter in diameter and only 17.5 cm
thick. These four telescopes are able to work together as a single giant telescope or
separately as well. The VLT is fully operational since 2000. The large telescope named
Gemini North and Gemini South each of 8.1 meter in diameter is located in Chile and
Hawaii. Bye this telescope one can cover the entire sky available for the night sky.
Gemini North located at Hawaii is in the operation since 1999 while Gemini South has
started its operation since 2002.
Largest of all large telescopes available in the world is single telescope of 11 meter in
diameter made out of 91 hexagon mirrors of 1 meter in size. Together it makes 11 X 9.8
meter size mirror for the observation. It is known as South African Large Telescope
(SALT) located at Sutherland, South Africa. The construction of SALT is funded by a
consortium of international partners from South Africa, the United States, Germany,
Poland, the United Kingdom and New Zealand. This telescope can see billion times faint
to be seen with the unaided eye - as faint as a candle flame at the distance of the moon. It
is operational since 2005.
The second largest optical telescope is Gran Telescopio Canarias (GTC) located at La
Palma in Canary Islands. It has the size of 10.4 meter in diameter is operational since
2005.
As long as the types of reflector telescope within the India are concerned, the situation is
not that bad. India too has more than five observatories located in different parts of India.
To list them, we must talk about the Venu Bappu Telescope (VBT) located at Kavalur in
the state of Tamilnadu. It has the aperture of 2.34 meter and made of single mirror. This
is the Asia’s largest telescope at present. It is in operation since 1980s. The observatory is
about 175 km from Bangaluru. It is operated by the Indian Institute of Astrophysics,
Bangaluru and works in optical region of electromagnetic spectrum. It stands at the
higher ladder of providing the clear sky for the astronomical photometry and
spectroscopy work. It is named after the Indian astronomer Prof Venu Bappu, who
contributed to the astronomy not just in the observational part but also in the theoretical
astronomy.
As far as the history of Indian astronomical observatories is concern then we must also
talk about the Sampurnanad telescope located at Nainital. It has the aperture of 1.04
meter in diameter and is the oldest of its kind. It is operational since 1972. It is installed
at the Manora peak hills at 2000 meter above mean sea level near Nainital and it works
only in optical part of EMR. It is operated by the Arryabhatta Research Institute of
Observational Sciences.
Gurushikhar Telescope is another name in the list of available telescopes in India. It has
the mirror of 1.2 meter in diameter and located at the Mount Abu in Rajastan. It is
installed at the height of 1680 meter above mean sea level and operates in optical as well
as the near infrared part of EMR. It is operated by the Physical Research Laboratory
(PRL), Ahmedabad. It is operational since 1994.
There are two more telescopes have come up in the last one decade in India. One of them
is 2 meter Himalayan Chandra Telescope (HCT) installed at the height of 4500 meter
above mean sea level. This is the World’s highest altitude observatory installed on the
Earth for the astronomical observations. It is located at the Mount Saraswati in the Leh
region of Jammu and Kashmir. One of the important features of this telescope is that it is
remotely controlled from Bangaluru (CREST). It is operated by the Indian Institute of
Astrophysics (IIA), Bangaluru. It works in optical as well as near infrared part of the
EMR. It is in operation since 2002. One of the crucial things is that, most of the time this
telescope remains in the negative temperature. The astronomically important feature is
that the Indian astronomers are getting more and clearer nights for the observations.
In astronomy, it is strongly recommended that the observatory should be located at
greater height to minimize absorption of star light by atmospheric constituents like CO2
gas, dust, aerosols, etc., it is also important to select the location which should be quite
far away from the city lights.
List will not be complete unless we include the name of IUCAA telescope located at
Girawali hills near Pune in Maharashtra. It has the aperture of 2 meters installed at the
height of about 1000 meter above mean sea level and operates in optical as well as near
infrared part of EMR. It is operated by Inter-University Center for Astronomy and
Astrophysics, Pune.
Even if we could build greater and greater telescopes we must understand that the looking
through a telescope does not tell us much about the object under observation unless we
have great quality instruments to collect and analyze the incoming light.
In early time astronomers use to collect and analyze the light through photographic
plates. But photographic plates have been almost entirely replaced in astronomy by
electronic imaging systems. Most astronomers use Charge Coupled Device (CCD) to
record images. A CCD is a specialized computer chip containing roughly a million
microscopic light detectors arranged in an array about the size of a postage stamp. It has
dramatic advantage of detecting bright and faint objects in a single exposure.
Another type of instrument used in astronomy is a spectrograph. To analyze light in
detail, we need to spread the light out according to wavelength into a spectrum through a
prism in the spectrograph. Nearly all modern spectrographs use a grating in place of
prism. A grating is a piece of glass with thousands of microscopic parallel lines scribed
onto its surface. Different wavelengths of light reflect from the grating at slightly
different angles, so white light is spread into a spectrum and can be recorded, often by
CCD camera.