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

The 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. We thereby remove stellar atmospheric models from the calibration chain. Measurement results for supernova cosmology are directly improved, as systematic uncertainties in the flux calibration limit them. Using the existing SCALA setup we achieved a calibration that agrees with the CALSPEC and Hayes6 systems to within ~4 mmag / 1000 A over a wavelength range from 4500 A to 9000 A. We are now upgrading the SCALA system to reach measurement uncertainties below 0.5 %. To provide the flux references for the new system and to perform tests of the improved components, we have built a laboratory light source enabling measurements with sub-percent uncertainties. The light source provides monochromatic light (FWHM 1.8/3.6 nm) spanning UV to IR, with wavelength accuracy and reproducibility of ≤ 1A. Neutral density filters enable fluxes that induce photodiode currents between fA and µA. A subsystem allows linearity testing for detectors with their readout system. Using a gantry robot, we can measure our detectors’ spatial response and angular acceptance with active areas up to 0.5 m2.