CDS (II) 2022 General Knowledge · Question 57 of 120

Correct — B, Uttarakhand. The instrument is the 4-metre International Liquid Mirror Telescope, built on the Devasthal ridge in Nainital district of Uttarakhand at the observing campus of the Aryabhatta Research Institute of Observational Sciences. A liquid mirror telescope has no ground and polished glass: a shallow dish of liquid mercury is spun on a precise bearing, and the spinning surface settles into a paraboloid all by itself, giving a 4-metre f/2 primary at a fraction of the cost of a solid mirror. The price of that trick is that the telescope can only look straight up, so it surveys the same strip of sky night after night as the Earth turns — which is exactly what you want for spotting things that change, such as supernovae, variable stars and moving objects. It saw first light in the spring of 2022, a few months before this paper was written, and was formally inaugurated on 21 March 2023. It is the first liquid mirror telescope built for astronomy in Asia and the largest in the world.

• (a)Himachal Pradesh — Himachal Pradesh has no national optical observatory of this class. The confusion usually comes from lumping all Himalayan hill stations together; the observing sites that matter are Devasthal in Uttarakhand and Hanle in Ladakh.
• (c)Ladakh — Ladakh is a real observing site — the Indian Astronomical Observatory at Hanle, one of the highest in the world, runs the 2-metre Himalayan Chandra Telescope and gamma-ray and infrared instruments. But Hanle's telescopes use conventional solid mirrors, and the liquid mirror project is not there.
• (d)Arunachal Pradesh — Arunachal Pradesh hosts no major optical telescope. Its cloud cover and monsoon make it a poor optical site, which is why Indian observatories cluster in the drier western Himalaya and the peninsular hills.

A liquid mirror telescope exploits a fact from rotational mechanics: the free surface of a liquid spun steadily about a vertical axis takes the shape of a paraboloid, and a paraboloid is precisely the figure a telescope's primary mirror needs. Fill a dish with mercury, spin it slowly and evenly, and you have a large, cheap, optically excellent mirror — with the single restriction that it must stay horizontal, so the telescope stares permanently at the zenith and lets the sky drift past.

The item can be solved without recognising the technology, by knowing where India's optical observatories actually are. Devasthal in Uttarakhand hosts both the 3·6-metre Devasthal Optical Telescope and this 4-metre liquid mirror instrument; Hanle in Ladakh hosts the 2-metre Himalayan Chandra Telescope; Kodaikanal in Tamil Nadu and Girawali in Maharashtra are the other names worth carrying. Between the exam's date and now the position has moved on: the ILMT was still being commissioned in September 2022 and was formally inaugurated only in March 2023, so a candidate reading about it today will find inauguration dates that postdate this paper. The project is a three-country collaboration between the Indian institute, the University of Liège in Belgium and several Canadian universities.

• The International Liquid Mirror Telescope has a 4-metre f/2 primary formed by spinning liquid mercury.
• It stands at Devasthal, in Nainital district of Uttarakhand, on the campus of the Aryabhatta Research Institute of Observational Sciences.
• It observes only at the zenith, scanning about 40 square degrees of sky per night in survey mode.
• First light came in the spring of 2022 and the formal inauguration was on 21 March 2023.
• Partners are ARIES in India, the University of Liège in Belgium and Canadian universities including British Columbia, Laval, Montreal, Toronto, Victoria and York.
• Devasthal also hosts India's 3·6-metre Devasthal Optical Telescope, the largest conventional optical telescope in the country.

• Assuming any Himalayan telescope must be at Hanle in Ladakh; Hanle and Devasthal are different sites with different instruments.
• Reading 'liquid mirror' as a liquid lens; the mercury forms the reflecting surface, not a refracting one.
• Dating the telescope from its inauguration in March 2023 when answering a question set in 2022 — the commissioning came first.

As a where-is-it item on a newly commissioned facility, or as a statements item on what a liquid mirror telescope can and cannot do.

• practice — not a real PYQ

  A liquid mirror telescope can observe only a narrow strip of sky because

  • (a)mercury reflects only a narrow band of wavelengths
  • (b)the spinning liquid surface must remain horizontal, so the telescope points at the zenith
  • (c)the mirror must be cooled to cryogenic temperatures
  • (d)the Earth's rotation blurs images away from the horizon

  Answer(b) the spinning liquid surface must remain horizontal, so the telescope points at the zenith — tilting it would spill the paraboloid.

• practice — not a real PYQ

  The Indian Astronomical Observatory at Hanle, one of the highest observatories in the world, is located in

  • (a)Uttarakhand
  • (b)Himachal Pradesh
  • (c)Ladakh
  • (d)Sikkim

  Answer(c) Ladakh — Hanle hosts the 2-metre Himalayan Chandra Telescope, while Devasthal in Uttarakhand hosts the liquid mirror telescope.