TANSPEC: TIFR-ARIES Near-infrared Spectrometer

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.