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 ( 2.5 \times 95 arcsec ) and wide ( 5 \times 25 arcsec ) options that are optimized for spectral resolution and flux calibration respectively .
On sky testing shows a wavelength range of 3200–6300 Å 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 < 2 Å 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 .