LOTUS: A low cost, ultraviolet spectrograph

TL;DR

LOTUS is an affordable, UV-optimized spectrograph for the Liverpool Telescope, capable of capturing spectra from 3200-6300 Angstroms with stable wavelength calibration and suitable for monitoring celestial objects.

Contribution

This paper introduces LOTUS, a low-cost, UV-sensitive spectrograph with a novel optical design and stable wavelength calibration, optimized for routine astronomical observations.

Findings

Wavelength range of 3200-6300 Angstroms achieved

Peak throughput of 15% including detector efficiency

Wavelength stability within 2 Angstroms rms

Abstract

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.