We present total reflectance measurements and Lambertian characterization of various materials that are commonly (and uncommonly) used as a screen for imaging system calibration (such as flat fielding). We measure the total reflectance of the samples over a broad wavelength range (250 nm < λ < 2500 nm) that is of interest to astronomical instruments in the ultraviolet, visible, and near-infrared regimes. A Helium-Neon laser was used to determine how closely the various materials' diffuse reflectance characteristics match that of a Lambertian surface.
We report on an expanded catalog of total and specular reflectance measurements of various common (and uncommon) materials used in the construction and/or baffling of optical systems. Total reflectance is measured over a broad wavelength range (250 nm < λ < 2500 nm) that is applicable to ultraviolet, visible, and near-infrared instrumentation. Characterization of each sample's specular reflection was measured using a helium-neon laser in two degree steps from near normal to grazing angles of incidence. The total and specular reflection measurements were then used to derive the specular fraction of each material.
We describe TCal, a mobile spectrophotometric calibration system that will be used to characterize the throughput as a function of wavelength of imaging systems at observatories around the world. TCal measurements will enhance the science return from follow-up observations of imaging surveys such as LSST (Large Synoptic Survey Telescope) and DES (Dark Energy Survey) by placing all tested imaging systems on a common photometric baseline. TCal uses a 1 nm bandpass tunable light source to measure the instrumental response function of imaging systems from 300 nm to 1100 nm, including the telescope, optics, filters, windows, and the detector. The system is comprised of a monochromator-based light source illuminating a dome flat field screen monitored by calibrated photodiodes, which allows determination of the telescope throughput as a function of wavelength. This calibration will be performed at 1-8m telescopes that expect to devote time towards survey follow-up. Performing the calibration on these telescopes will reduce systematic errors due to small differences in bandpass, making follow-up efforts more precise and accurate.