MARVEL: optical tolerances for manufacturing, alignment, and operational stability of the spectrograph

MARVEL is a new facility at the Mercator Observatory which comprises an array of four 0.8 metre telescopes, each feeding via fibre link into a single high-resolution spectrograph. The facility will provide dedicated target vetting and follow-up capability to support large exoplanet surveys through radial velocity measurements with precision at the metre-per-second level. The MARVEL spectrograph records five R~90,000 spectra simultaneously in a single exposure across a wavelength range of 380-950 nm. The optical design makes use of simple spherical optics wherever possible, and specifies high-transmission lithography glasses. To confirm that the design performance is within reach, a five-stage optical tolerancing study was undertaken. The resulting tolerances not only directly reveal optical manufacturing requirements, but also inform the optomechanical mounting schemes that will be implemented. The required operational stability of the instrument was calculated in terms of maximum allowable displacement or tilt for each individual component or co-mounted group, translating into requirements on the environmental stability of the instrument, inside and outside the vacuum vessel.