The CANDLE MISSION

Today’s existing networks of standard stars have heterogeneous development and are daisy-chained employing a variety of methods to decades-old measurements of Vega which have unknown systematic uncertainties. For example, the spectrophotometric CALSPEC system is based on model atmospheres of three white dwarf stars, normalized to Vega’s spectral irradiance at 555.6 nm, whereas the Landolt UBV photometry traces back to Johnson & Morgan (1953) and the RI photometry to Cousins (1976), the UKIRT IR standards are on the Vega-based CIT system (Frogel+ 1978, Elias+1982). and the latest faint white dwarf flux standards (WDFS) (Narayan+ 2021, Calamida+2019) are on the CALSPEC system. Even when these systems have high internal precision, the absolute measurements dominate the error budgets, resulting in a cascade of systematic uncertainty that spreads across the wavelength range between 100 nm to 2500 nm (and beyond).

The CANDLE Mission is a proposed space observatory that will provide thousands of spectrophotometric standard stars around the sky whose fluxes are directly traceable to the International System of Units meaning there is “...an unbroken chain of calibrations to specified reference standards... ” and “... that the uncertainty associated with a measured value or calibration be sufficiently small to satisfy a particular measurement need”. I will describe a concept for a space-based hybrid project consisting of twin satellites, each consisting of a telescope and a calibrated artificial star, that can be deployed to establish an all-sky network of high accuracy, absolute flux standard stars. With NIST calibrated instruments and artificial stars above the earth’s atmosphere, we aim to achieve total uncertainties of less than 0.5%, a factor of 5x better than the current 2.5%, over the VIS/NIR range for stars with a range of spectral types and brightness.