SCALA: In-situ calibration for Integral Field Spectrographs

TL;DR

This paper introduces SCALA, a calibration system for the SNIFS instrument that achieves stable, high-precision flux calibration traceable to detector standards, improving the accuracy of astronomical measurements.

Contribution

We developed and deployed SCALA, a novel in-situ calibration system that ensures stable, traceable flux calibration for the SNIFS spectrograph, enhancing systematic control in optical surveys.

Findings

Long-term repeatability better than 1% in calibration light output.

Color calibration stability at the percent level over one year.

Calibration system successfully deployed and operational at UH 88 telescope.

Abstract

The scientific yield of current and future optical surveys is increasingly limited by systematic uncertainties in the flux calibration. This is the case for Type Ia supernova (SN Ia) cosmology programs, where an improved calibration directly translates into improved cosmological constraints. Current methodology rests on models of stars. Here we aim to obtain flux calibration that is traceable to state-of-the-art detector-based calibration. We present the SNIFS Calibration Apparatus (SCALA), a color (relative) flux calibration system developed for the SuperNova Integral Field Spectrograph (SNIFS), operating at the University of Hawaii 2.2 m (UH 88) telescope. By comparing the color trend of the illumination generated by SCALA during two commissioning runs, and to previous laboratory measurements, we show that we can determine the light emitted by SCALA with a long-term repeatability better than 1%. We describe the calibration procedure necessary to control for system aging. We present measurements of the SNIFS throughput as estimated by SCALA observations. The SCALA calibration unit is now fully deployed at the UH\,88 telescope, and with it color-calibration between 4000 {\AA} and 9000 {\AA} is stable at the percent level over a one-year baseline.