We present the design and performance of the TANSPEC, a medium-resolution 0.55–2.5 μm cryogenic spectrometer and imager, now in operation at the 3.6 m Devasthal Optical Telescope (DOT), Nainital, India. The TANSPEC provides three modes of operation, which include photometry with broad- and narrowband filters, spectroscopy with short slits of 20″ length and different widths (from 0.″5 to 4.″0) in cross-dispersed mode at a resolving power R of ∼2750, and spectroscopy with long slits of 60″ length and different widths (from 0.″5 to 4.″0) in prism mode at a resolving power R of ∼100–350. TANSPEC’s imager mode provides a field of view of 60″ × 60″ with a plate scale of 0.″245 pixel−1 on the 3.6 m DOT. The TANSPEC was successfully commissioned during 2019 April–May, and the subsequent characterization and astronomical observations are presented here. The TANSPEC was made available to the worldwide astronomical community for science observations from 2020 October.
