Calibration of the SHERLOC deep UV fluorescence/Raman spectrometer on the Perseverance Rover

We describe the wavelength calibration of the spectrometer for the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument onboard NASA’s Perseverance Rover. SHERLOC utilizes deep-UV Raman and fluorescence (DUV R/F) spectroscopy to enable analysis of samples on the martian surface. SHERLOC employs a 248.6 nm deep UV laser to generate Raman scattered photons and native fluorescence emission photons from near-surface material to detect and classify chemical and mineralogical compositions. The collected photons are focused on a charge-coupled device (CCD) and the data are returned to Earth for analysis. The compact DUV R/F spectrometer has a spectral range from 249.9 nm to 355.4 nm (~200 cm-1 to 12000 cm-1) (with a spectral resolution of 0.296 nm (∼40 cm−1)). The compact spectrometer uses an off-nominal design to project a high-resolution Raman spectrum and a low-resolution fluorescence spectrum on a single CCD without the need for mechanized components. The natural spectral separation enabled by deep UV excitation enables wavelength separation of the Raman/fluorescence spectra. The SHERLOC spectrometer design optimized the Raman spectral region for resolution, while the fluorescence region was optimized for wavelength range. The resulting illumination on the CCD is curved, requiring a non-traditional, segmented, non-linear wavelength calibration in order to understand the mineralogy and chemistry of martian materials.