At a glance
This lesson explains the biggest optical telescope in India and its neighbour.
- The biggest optical telescope in the country sits high in the Himalayas.
- The Prime Ministers of India and Belgium activated it remotely in March 2016.
- The ILMT, next door, uses liquid mercury as its mirror.
Source: BINA, DOT at ARIES.
Source: BINA, ILMT at ARIES.
High in the Kumaun Himalayas, India runs its biggest optical telescope. Belgium helped pay for it and build it. This lesson explains what it is, how it was built, what Belgium gets and what else sits on the same hill.
Contents
What is the Devasthal Optical Telescope?
The Devasthal Optical Telescope (DOT) is India’s largest optical telescope. It has a 3.6 metre main mirror.
1. What is the DOT and where is it?
The 3.6 metre DOT is the largest optical telescope in India. It stands at Devasthal Observatory in Uttarakhand. The site is in Nainital district, in the Kumaun region of the Himalayas, at 2,450 metres.
ARIES runs the observatory. ARIES is the Aryabhatta Research Institute of Observational Sciences in Nainital. It is an autonomous institute under the Department of Science and Technology.
Source: BINA, DOT at ARIES, item 1
Source: Royal Observatory of Belgium, ILMT first light, item 1
Source: ARIES, ILMT first light, item 1
2. Who built it?
The DOT is an Indo-Belgian telescope. Its optics were built with the Belgian firm AMOS. AMOS stands for Advanced Mechanical and Optical Systems. The Liege Space Centre in Belgium also took part.
The DOT is a Ritchey-Chretien design. That is a common design for large research telescopes.
3. When and how was it switched on?
The Prime Ministers of India and Belgium activated the DOT remotely. Narendra Modi and Charles Michel did it from Brussels. Neither leader went to the hill.
The date is given two ways. India’s science portal says 30 March 2016. The BINA site says 31 March 2016. So remember March 2016 and the two leaders.
The telescope began scientific work in April 2017.
What does Belgium get from it?
Belgium paid part of the cost. In return it gets part of the observing time.
4. How much did Belgium pay, and what did it get?
The Belgian Science Policy Office (BELSPO) put in 2 million euros. In return, 7 per cent of the DOT’s observing time goes to projects led by Belgian astronomers.
ARIES says India holds the other 93 per cent. Of that, 33 per cent is guaranteed to ARIES.
5. What is BINA?
BINA is the Belgo-Indian Network for Astronomy and Astrophysics. It links Indian and Belgian astronomers. Its aim is to make the best use of the Indo-Belgian telescopes.
The BINA site says it was set up in 2014. A Belgian observatory page says it was initiated in 2016. The sources differ, so do not memorise one year.
6. What instruments does the DOT use?
The DOT has three Cassegrain ports. A fold mirror switches between them in under a minute. Two instruments are always mounted. TIRCAM2 takes infrared pictures. SPIM takes optical pictures.
ADFOSC and TANSPEC take turns on the main port. Both take images and spectra.
What else sits on the Devasthal hill?
Devasthal hosts three telescopes. Two of them are Indo-Belgian.
7. Which telescopes are at Devasthal?
Devasthal hosts a 1.3 metre telescope, the 3.6 metre DOT and the 4 metre ILMT. The DOT and the ILMT are the two Indo-Belgian telescopes.
The site has a median seeing of 1.1 arcseconds. It has about 175 photometric nights a year. Seeing is how steady the air is. Photometric nights are clear enough for exact brightness readings.
Source: Royal Observatory of Belgium, ILMT first light, item 7
Source: ARIES, ILMT first light, item 7
8. What is the ILMT?
ILMT stands for International Liquid Mirror Telescope. Its 4 metre mirror is a thin film of liquid mercury in a spinning dish. The spin makes the liquid curve into a parabola, the right shape to focus light.
A thin mylar film covers the mercury to protect it from wind. Liquid mirrors are much cheaper than glass mirrors of the same size.
Source: ARIES, ILMT first light, item 8
Source: Royal Observatory of Belgium, ILMT first light, item 8
9. How does the ILMT observe?
The ILMT cannot tilt. It always points at the zenith, the point straight overhead. As the Earth turns, a strip of sky drifts across it each night.
It is built to spot things that change, like supernovae and asteroids. It reached first light on 29 April 2022. It was formally inaugurated on 21 March 2023.
Source: BINA, ILMT at ARIES, item 9
Source: ARIES, ILMT page, item 9
10. Who built the ILMT?
The ILMT is an international project. Partners come from Belgium, Canada, India, Poland and Uzbekistan. The University of Liege in Belgium leads it.
BINA calls it the world’s largest liquid mirror telescope. It also calls it the first of its kind in India.
What do exams ask about Devasthal?
Exams mix up the DOT and the ILMT. Both are at the same place.
11. How do I tell the DOT from the ILMT?
The DOT is 3.6 metres wide. It is a conventional, steerable telescope. The ILMT is 4 metres wide and uses liquid mercury. It looks only straight up.
The Combined Defence Services (CDS) CDS II 2022 paper, Q57 asked where India commissioned its first liquid mirror telescope. The answer was Uttarakhand.
Source: ARIES, ILMT page, item 11
Source: BINA, ILMT at ARIES, item 11
12. Which "largest" claims are safe?
The DOT is the largest optical telescope in India. India’s science portal calls it the largest steerable imaging telescope in Asia. That wording is narrower than “largest reflecting telescope in Asia”.
Say “in India” for the safe claim. The ILMT is the one called the world’s largest liquid mirror telescope.
Four questions to think through
Why does Belgium get 7 per cent of the DOT's time?
Belgium paid 2 million euros toward the build. Its share of observing time is the return on that money. Belgian-led projects use that time. India keeps the other 93 per cent.
Why does a spinning liquid make a good mirror?
A spinning liquid curves into a parabola by itself. A parabola focuses light to one point. Liquid mirrors cost much less than glass ones. The catch is that they cannot tilt.
Why does the ILMT only look straight up?
Tilting a dish of liquid would spill it and spoil its shape. So the dish stays level. The Earth's spin moves new sky across the mirror every night. That gives a strip of sky to study.
What should I do when two sources give different dates?
First, find what both sources agree on. Here both say March 2016. Learn that, and note the two leaders. If a question gives only one of the dates, pick it carefully.
Check your understanding
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✅ Test Yourself
Try practice questions on the Devasthal Optical Telescope and the ILMT.
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