SCALA update: deci-percent laboratory spectro-radiometric NIST calibration transfer tonew flux reference sensors

摘要

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%