  We present the design and performance of the TANSPEC, a medium-resolution
$0.55-2.5~\mu$m 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 narrow-band
filters, spectroscopy with short slits of 20$^{\prime \prime}$ length and
different widths (from 0.5$^{\prime \prime}$ to 4.0$^{\prime \prime}$) in
cross-dispersed mode at a resolving power R of $\sim$2750, and spectroscopy
with long slits of 60$^{\prime \prime}$ length and different widths (from
0.5$^{\prime \prime}$ to 4.0$^{\prime \prime}$) in prism mode at a resolving
power R of $\sim$100-350. TANSPEC's imager mode provides a field of view of
60$^{\prime \prime} \times 60^{\prime \prime}$ with a plate scale of
0.245$^{\prime \prime}$/pixel on the 3.6-m DOT. The TANSPEC was successfully
commissioned during April-May 2019 and the subsequent characterization and
astronomical observations are presented here. The TANSPEC has been made
available to the worldwide astronomical community for science observations from
October 2020.
