  We present a near-infrared cooled grating spectrometer that has been
developed at the Arcetri Astrophysical Observatory for the 1.5 m Infrared
Telescope at Gornergrat (TIRGO). The spectrometer is equipped with cooled
reflective optics and a grating in Littrow configuration. The detector is an
engineering grade Rockwell NICMOS3 array (256x256 pixels of 40 $\mu$m). The
scale on the focal plane is 1.73 arcsec/pixel and the field of view along the
slit is 70 arcsec. The accessible spectral range is 0.95-2.5 $\mu$m with a
dispersion, at first order, of about 11.5 angstroms/pixel. This paper presents
a complete description of the instrument, including its optics and
cryo-mechanical system, along with astronomical results from test observations,
started in 1994. Since January 1996, LonGSp is offered to TIRGO users and
employed in several Galactic and extragalactic programs.
