SCALA upgrade: development of a light source for sub-percent calibration uncertainties
Résumé
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.1 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 (2–5) 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 ≤1 ̊A. 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