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.

The telescope is a two-mirror Ritchey-Chretien system with the effective f-ratio of 9. The primary mirror (M1) is a thin meniscus of optical diameter 3.6 m, f-ratio of 2, 165mm thickness, and cast in Zerodur. The secondary mirror (M2) has optical diameter of 952 mm,f-ratio of 4, 120 mm thickness cast in Astro-Sitall. The back focal distance is 2.5m. The telescope has three Cassegrain ports.
The main axial port has science field of view of 30 arcmin diameter whereas the side portsprovide 10 arcmin diameter. The as-specified optical image quality is E80 is less than 0.45arcsec for wavelengths less than 1500 nm. The active optics system (AOS) controls the alignment of M1 and M2 using pneumatic actuators and hexapod mechanism respectively. The corrections can also be applied in closed loop using data from the Shack-Hartmann wavefront sensing system.

The 3.6m Telescope Enclosure building is a custom-built technical building. The enclosure involves structural (concrete and steel), mechanical and electrical engineering. The telescope enclosure building has three parts viz. the rotating dome, the stationary dome-support structure and an auxiliary building. The entire building is made up of steel. The dome is a cylindrical insulated structure with pitched roof having diameter of 16.5m and height of 13m. The dome has a 4.2 m wide opening slits and a wind screen. The telescope floor is at 11m level and a total of 12 ventilation fans are installed on the dome support structure for thermalisation of the telescope building at the start of observations. The ventilation duct has also been provided to flush the hot air of technical room. The dome building is also equipped with a 200 kg lift which is used to transport instrument components from the ground to the telescope floor. The auxiliary building is equipped with coating plant unit.

A rigorous on-sky tests on telescope were performed during October 2015 to February 2016. In particular, a set of four system level tests viz. pointing accuracy, tracking accuracy, optical quality and AGU guider sensitivity. A set of four instruments were used for the purpose of tests, viz. Test-Camera and Test-Wave Front Sensor  (WFS) which were used at Cassegrain ports of the telescope; the AGU-Camera and the AGU-WFS which are part of telescope.
An air-cooled Microline ML 402ME CCD with 9 micron pixel and chip size of 768×512 pixels is used for both test-camera and the AGU-Camera whereas they have a plate scale of 0.05768 arcsec/pix and 0.166 arcsec/pix respectively. The Microline ML4710-1- MB with 1024×1024 size and 13 micron pixels was used with both test-WFS and AGU-WFS, though they used lenslet array of 33×33 and 11×11 respectively. A comprehensive set of data were collected, analysed and thetest procedures and results were put in form of a professional technical report. The performance results were evaluated by a high-level technical committee.

The image quality of the telescope was found to be consistent with the specifications. The as-built specifications of four system level parameters are listed in Table. A total of six measurements resulted in E50 of 0.15+/-0.03 arcsec, E80 of 0.26+/-0.04 and E90 of 0.37+/-0.07 arcsec and these were better than the specifications of 0.3, 0.45 and 0.6 arcsec respectively. The pointing accuracy was found to be well withing the specifications. Some tracking measurements are marginally above the specification for these are very sensitive to seeing conditions, particulaly at low elevations, and local weather disturbances (.e.g. strong winds,passing clouds) and these affect the centroiding of stars. The images of a few binary stars with known separation were observed on different nights during November-December 2015 using Test-Camera. Stars having seperation of sub-arcsec were observed to be resolved clearly. In one of the observations, a binary with known angular sepeartion of about 0.4 arcsec was resolved in the night of 30th November 2015, see Figure 9. The PSF-FWHM of a star has three main contributions viz. seeing, tracking and optical quality. It has been observed that the optical quality of telescope can be maintained at the level of 0.2 arcsec FWHM. As the primary mirror of the telescope is about 15m above the ground level and the seeing contribution at this level is reduced to about 0.2 arcsec. Hence, considering the tracking accuracy results, it is expected that the telescope can deliver FWHM images of celestial sources as good as about 0.4 arcsec.