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The finished and coated mirror of Devasthal Telescope

India’s Devasthal Telescope

In the Himalayan foothills near Naintal, India, stands the largest single-mirror optical telescope in Asia: India’s new Devasthal Telescope, designed and built under the joint leadership of the Aryabhatta Research Institute of Observational Sciences (ARIES), the Indian Institute for Astrophysics, and the Tata Institute for Fundamental Research. This week is the first anniversary of Devasthal going live, when Indian Prime Minister Narendra Modi and Belgian Prime Minister Charles Michel activated it remotely from Brussels. Additional contributions to the telescope come from Belgium’s industrial and academic sectors, and also from Russian industry. Here’s a photo of the telescope building and enclosure.

Devasthal’s main scientific objective is to monitor flux variability in astronomical sources such as transient events, and to carry out photometric and imaging surveys of extended astronomical sources, e.g. star clusters and galaxies. The telescope makes observations in the optical and near infrared wavelengths (0.35 up 2.50 micrometres).

Devasthal’s 3.6-metre primary mirror has a diameter twice or more that of India’s next-largest optical telescopes, the one- and two-metre class telescopes located in the Himalayas on the Ladakh Plateau. A larger primary mirror means astronomers can see and photograph fainter objects, and also make observations more quickly, because more light can be gathered in a given amount of time. By splitting up the light into its different wavelengths, we can also look for the signatures of different chemical species because each species has a unique fingerprint.

The largest class of optical observatories currently in existence have primary mirrors with a diameter on the order of 10 metres. At 30 metres, the diameter of TMT’s primary mirror will be three times larger than that, giving of course a collecting area almost an order of magnitude larger. In debating whether to contribute toward TMT, one of the questions for India’s government was whether instead to construct its own version of the existing largest class of telescope. But for all of TMT’s partner countries and institutes, working on the next generation of telescopes, such as TMT, is an investment in their next generation of astronomers and engineers. Fortunately for TMT, India has chosen to be part of building it—not only to contribute expertise, but also to learn from other partners the technologies necessary eventually to build India’s own 10-metre class observatory.

In Hindi, Devasthal means “abode of God.” I was lucky enough to visit the site number of times during the telescope’s installation and commissioning phases, whilst visiting ARIES in helping to co-ordinate their contributions to TMT. The closest main airport is in Delhi, and from there, the most comfortable way to reach Devasthal is by overnight train to Kathgodam, followed by a circuitous taxi drive into the hills to an elevation of 2,450 metres (over 8,000 feet). The hills are lush and fertile, and food is grown without the need for pesticides because of the richness of the soil and the abundant snow melt.

The nearest town is called Nainital, which is spread along the shore of a lake and serves as a base for exploring the surrounding Himalayan hills.

In Kathgodam I found, of all things, a butterfly museum. That experience might one day the subject of another entry in this blog. Other local wildlife includes the troops of baboons that surround the guest house at ARIES, and also the Big Cat (the Himalayan Tiger) which I have yet to see, although when once visiting the Royal Chitwan Park just across the border in Nepal, I had once seen tiger prints in the mud along the river bank. The Corbett National Park is close to ARIES, and I hope that I will have the opportunity to visit one day. Thinking about TMT’s search for signs of life on other planets always reminds me of the sanctity of life on our own home planet.

The physical effort required to create Devasthal’s primary mirror mirrors what will be needed on a larger scale for TMT. For example, the mirror glass will have to be thoroughly cleaned before it can be coated with the reflective material needed to make it useful for observing the heavens. Here’s a picture of Devasthal’s primary mirror glass being cleaned, before being coated with its reflective surface. The mirror is made from a type of glass commercially known as Zerodur, and has the important property that its size changes very little during changes in ambient temperature, so that its optical properties do not change when making long observations.

The coating chamber at Devasthal was built by an Indian company called Hind High Vac, and the chamber is co-located with the telescope to minimize risk of damage to the mirror during transport. This is especially important considering that recoating is necessary several times over the telescope’s lifetime, in order to maintain the mirror’s reflectivity. The mirror is moved to the coating chamber and back using a crane. Once coated with the reflective material, the finished mirror looks something like a giant CD.

The current plan is for us to deliver the TMT primary mirror segments to the site in the uncoated condition, for coating using a chamber which will also be provided by Indian industry. We’ll have to re-coat the mirror segments of TMT every few years, and will extract and replace the segments using a beautifully engineered Segment Handling System designed and built by our colleagues at Mitsubishi Electric.

Notwithstanding how perfect the surface looks, no mirror is wholly without flaws. To counteract the effects of mirror deformations, Devasthal is equipped with an active optics system, which actively shapes the mirror to null out those deformations. TMT will have even more advanced adaptive optics systems to compensate for atmospheric turbulence and optical aberrations. We’ll shoot lasers into the sky, watch how atmospheric turbulence makes those lasers shift position, then actively shape the mirror in the opposite direction to compensate for that turbulence. Our goal is to achieve image quality comparable to that achievable from space, where space telescopes observe from above the Earth’s atmospheric turbulence. You can access real-time observing weather conditions for Devasthal at , and I’ll be describing TMT’s adaptive optics system in more detail in a future blog entry.

Among the many benefits of a project like TMT is the synergy it creates. The things TMT’s contributors have learned from their own endeavors—projects like Devasthal—will help them produce the various components of TMT, and in turn, what they learn from helping build TMT will enable them to take their own scientific enterprises to a level not achievable without the cooperation and contribution of all.

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