Spectrophotometric calibration system for DECam

Abstract

We present a spectrophotometric calibration system that will be implemented as part of the DES DECam project at the Blanco 4 meter at CTIO. Our calibration system uses a 2nm wide tunable source to measure the instrumental response function of the telescope from 300nm up to 1100nm. The system consists of a monochromator based tunable light source that is projected uniformly on a Lambertian screen using a broadband "line to spot" fiber bundle and an engineered diffuser. Several calibrated photodiodes strategically positioned along the beam path will allow us to measure the throughput as a function of wavelength. Our system has an output power of 0.25 mW, equivalent to a flux of approximately 100 photons/s/pixel on DECam. We also present results from the deployment of a prototype of this system at the Swope 1m at Las Campanas Observatory for the calibration of the photometric equipment used in the Carnegie Supernova Project.