A
couple of months ago, I had reported about a giant new astronomical
telescope that is being set up at Mauna Kea, the highest point in the
U.S. state of Hawaii, through international cooperation. This new
telescope with a 30 meter diameter mirror is being called as Thirty
Meter Telescope (TMT) It is expected that it would enable the user
astronomers to gaze 13 billion light years into the universe.
Last
week, India officially signed as a partner for the TMT project along
with the U.S., Japan, Canada and China. India will contribute about
Rs 1.3 Billion over the construction period 2014-23. with 30 percent
of it in cash and balance in kind, to be spent on vital components of
the telescope to be built in India in partnership with various
industries.
A
telescope's light gathering power and ability to resolve small detail
is directly related to the diameter (or aperture) of its objective
(the primary lens or mirror that collects and focuses the light). The
larger the objective, the more light the telescope collects and the
finer detail it resolves. The the largest telescope in India has
only a 2-metre mirror ( on Mt. Saraswati, Digpa-ratsa Ri, Hanle in
south-eastern Ladakh in the eastern Jammu and Kashmir state of
India), though India is building one at Devasthal, Nainital,
Uttrakhand state, that will have 3.6 meter diameter. Readers can
imagine how much more powerful the TMT telescope is likely to be.
Genaral internal arrangement of the telescope
Telescopes
can use either lenses or mirrors as objectives. The telescopes using
lenses are known as refracting telescopes (dioptrics). The ones that
use mirrors are called as reflecting telescopes (catoptrics) and
those using a combination of lenses and mirrors are known as
Catadioptric telescopes. Any large sized telescope has to be either
of the latter two types because of practical difficulties in making
optical glass flats of such large size, that would give distortion
free images and grinding the same to required convex objective lens
shapes. Comparatively, curved mirrors are easier to make and can be
given even parabolic or hyperbolic shapes.
The mirror consists of 492 hexagonal segments
However,
when we think of making a 30 meter diameter mirror objective, it is
impossible to create a single, workable 30-meter diameter mirror. It
is bound to be too heavy and will eventually sag from gravity
distorting the curve profile. There are other problems in maintaining
such a large mirror. At Mauna Kea, there is another large telescope
built by Japan and known as Subaru telescope. A mishap happened at
this telescope when there was a coolant leak onto the primary mirror
in July, 2011. The mishap had a significant impact on functioning of
Subaru Telescope and recovery work to put back telescope back in
shape including meticulous inspection and cleaning of the affected
areas took almost two years and was completed only in the summer of
2013.
To
avoid such problems, TMT design has decided to substitute the single
objective mirror with 492 mirror segments of hexagonal shape of 1.44
meters size, that will be assembled like a jig-saw puzzle to make on
mirror. Position of each of these mirrors, will be precisely
controlled along three axises using at least 3 precision actuators to
sense and correct any segment displacement even to an extent of a
micron ( one hundredth part of a millimeter), so that the entire
assembly would behave like a single, gigantic 30 meter-wide surface.
A Hexagonal segment with controllers
India
has agreed to fabricate, test and supply these 492 mirrors along with
1500 actuators to control the hexagonal mirrors along with the
segment support assembly to keep all the mirrors in position so they
function as a single mirror of hyperboloid shape. Most of these parts
would be fabricated in Bengaluru city in Karnataka state. Bengaluru’s
expertise in astrophysics, electronics and software development will
be put to test in this project.
Eswar
Reddy, programme director of TMT-India and associate professor at the
Indian Institue of Astrophysics feels that the “trickiest part”
is to get hundreds of segments behave like a gigantic monolithic
mirror and this involves developing several new technologies in
mechanics, electronics, optics and control software.TMT-India has
been formed by coming together of number of research institutions
comprising of Indian Institue of Astrophysics, the IIA, Aryabhatta
Research Institute for Observational Sciences (Nainital) and the
Inter-University Centre for Astronomy and Astrophysics (Pune).
Prof.
Eswar Reddy says that The TMT, which is expected to be completed in
2023, will be 150 times more powerful than the Hubble Space
Telescope, allowing astronomers to look well beyond our solar system,
to the near-beginning of time and will be able to observe the
universe when it was very young, just half a million light years.
Henry
Yang, Chair of the TMT International Observatory Board and Chancellor
of the University of California Santa Barbara says; "The
official signing today with India's investment reassures the success
of TMT, India's contributions in the areas of software systems,
segment production and the production of the very high precision
sensors and actuators that make the primary mirror possible are key
to the project."
11th
December 2014
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