Here we present the methodology and results of transferring UV–NIR flux calibration from NIST photodiodes to a set of 20 picoammeters. These are to be deployed as flux reference sensors on the SCALA calibration system at the University of Hawaii 2.2m telescope on Maunakea as part of a systematic upgrade aimed at improving the existing flux calibration for dark energy and exoplanet host star measurements beyond the ∼ 4 mmag / 100 nm we have already achieved at optical wavelengths with SCALA. Our robotic light source for performing the photodiode calibration transfer provides monochromatic light spanning 230 to 1200 nm with a dynamic range of 106 , while our new picoammeters have a noise floor of 10 fA in 4 s at 25 ◦C, with saturation around 400 pA. Our robotic gantry enabled the measurement of the spatial and angular response of our picoammeters. In preparation for the calibration transfer, a number of tests were performed to establish the measurement uncertainties, and these tests revealed subtle systematic effects that required correction. These includes polarization effects, leading to the redesign of part of the optics in the gantry head, implementation of a Holmium-Didymium filter as a precision wavelength transfer between arc and continuum light sources, and further suppression of stray light. We find that our calibration transfers are consistent with the NIST calibration to within ∼0.1%
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.
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