  We describe the design, construction and commissioning of LOTUS; a simple,
low-cost long-slit spectrograph for the Liverpool Telescope. The design is
optimized for near-UV and visible wavelengths and uses all transmitting optics.
It exploits the instrument focal plane field curvature to partially correct
axial chromatic aberration. A stepped slit provides narrow (2.5x95 arcsec) and
wide (5x25 arcsec) options that are optimized for spectral resolution and flux
calibration respectively. On sky testing shows a wavelength range of 3200-6300
Angstroms with a peak system throughput (including detector quantum efficiency)
of 15 per cent and wavelength dependant spectral resolution of R=225-430. By
repeated observations of the symbiotic emission line star AG Peg we demonstrate
the wavelength stability of the system is less than 2 Angstroms rms and is
limited by the positioning of the object in the slit. The spectrograph is now
in routine operation monitoring the activity of comet 67P/Churyumov-Gerasimenko
during its current post-perihelion apparition.
