The 3.6 Metre Devasthal Optical Telescope:From Inception to Realization

India’s largest 3.6 metre Devasthal Optical Telescope (DOT) was commissioned in 2016, though the idea of building it germinated way back in 1976. This article provides research accounts as well as glimpses of its nearly four decades of journey. After a decade of site surveys, Devasthal in the central Himalayan region of Kumaon, Uttarkhand was identified. Thereafter, a detailed site characterization was conducted and project approvals were obtained. The telescope is designed to be a technologically advanced optical astronomy instrument. It has been demonstrated to resolve a binary star having angular separation of 0.4 arc-sec. After technical activation of the telescope on 30 March 2016, it has been in regular use for testing various back-end instruments as well as for optical and near-infrared observations of celestial objects. Back-end instruments used for these observations are 4K × 4K CCD IMAGER, faint object imager-cum-spectrograph and TIFR nearinfrared camera-II. A few published science results based on the observations made with the telescope are also presented. Furthermore, routine observations show that for a good fraction of observing time the telescope provides sky images of sub-arc second resolution at optical and nearinfrared wavelengths. This indicates that the extreme care taken in the design and construction of the telescope dome building has been rewarding, since the as-built thermal mass contributes minimally so as not to degrade the natural atmospheric seeing measured at Devasthal about two decades ago during 1997–99 using differential image motion monitor. The overall on-site performance of the telescope is found to be excellent and at par with the performance of other similar telescopes located over the globe.