SCALA upgrade: development of a light source for sub-percent calibration uncertainties

International audienceThe SCALA system provides a physical flux calibration for the SuperNova Integral Field Spectrograph (SNIFS)mounted to the University of Hawaii 2.2m telescope on Mauna Kea by transferring the flux scale from a NIST-traceable photodiode to SNIFS. This calibration is then applied to CALSPEC standard stars.1 We therebyremove stellar atmospheric models from the calibration chain. Measurement results for supernova cosmology aredirectly improved, as systematic uncertainties in the flux calibration limit them.Using the existing SCALA setup (2–5) we achieved a calibration that agrees with the CALSPEC and Hayes6systems to within ∼ 4 mmag / 1000 ̊A over a wavelength range from 4500 ̊A to 9000 ̊A. We are now upgradingthe SCALA system to reach measurement uncertainties below 0.5 %. To provide the flux references for thenew system and to perform tests of the improved components, we have built a laboratory light source enablingmeasurements with sub-percent uncertainties.The light source provides monochromatic light (FWHM 1.8/3.6 nm) spanning UV to IR, with wavelengthaccuracy and reproducibility of ≤1 ̊A. Neutral density filters enable fluxes that induce photodiode currentsbetween fA and μA. A subsystem allows linearity testing for detectors with their readout system. Using a gantryrobot, we can measure our detectors’ spatial response and angular acceptance with active areas up to 0.5 m