The 3.6m Devasthal Optical Telescope is a custom-built instrument of great complexity. This telescope has the distinction of being largest telescope in India for study of celestial objects at optical wavelegths. It is a national facility installed at Devasthal in the district of Nainital, India. It was commissioned in the year 2016 and is being maintained and operated by ARIES.

The 3.6m DOT facility consists of a modern 3.6 meter optical new technology telescope, a suite of instruments, an Aluminium coating plant, a control room and a data center. The telescope has a number of instruments providing  spectral and imaging capabilities at visible and near-infrared bands. In addition to optical studies of a wide variety of astronomical topics, it can be used for follow-up studies of sources identified at the radio region by Giant Meter Radio Telescope (GMRT) and at UV/X-ray by Indian Space Observatory (ASTROSAT).

The Devasthal, meaning abode of God, is located in the himalayan region.The characterisation of devasthal site was carried out on 80 nights during 1998-1999 with a Differential Image Motion Monitor (DIMM) using a 38-cm telescope with the mirror about 2 m above the ground, and it yielded a median seeing estimate of about 1.1 arcsec with the 10 percentile values lie below 0.7 arcsec. The microthermal measurements at Devasthal indicated that the slab of 13 to 18m above the ground can contribute seeing in the range 0.2 to 0.3 arcsec.The key parameters of Devasthal site is summarised in following Table.