The SNIFS CALibration Apparatus (SCALA) provides a physical calibration for the SuperNova Integral Field Spectrograph (SNIFS) at the University Hawaii 2.2m telescope on Mauna Kea. Light projected by SCALA into the telescope and SNIFS is monitored by a NIST-traceable photodiode, calibrating SNIFS within an achromatic (geometric) factor. SNIFS then observes CALSPEC standard stars, thereby transferring the NIST calibration to these standard stars. This removes the dependence on stellar atmospheric models from the calibration chain, which otherwise is a poorly-constrained source of systematic uncertainty in the use of Type Ia supernovae to measure the dark energy equation of state.
Using the original SCALA we achieved a calibration that agrees with the CALSPEC and Hayes systems to within 4 mmag / 100 nm over a wavelength range from 450 to 900 nm. We are now in the process of upgrading SCALA in order to further suppress internal reflections (through improved baffling) and out-of-band emission (by upgrading to a double monochromator). We are also adding NIST-traceable photodiodes to all of SCALA’s 18 beams. We will describe laboratory and in-situ testing of these upgrade components. With these improvements we expect standard star flux calibration measurement uncertainties to be less than 0.5%.