Absolute Photometry: Past and Present

Abstract

I review efforts to make absolute measurements in astronomy over the wavelength interval from UV to mid-infrared (MIR). These have involved a variety of facilities: space-based, airborne, and ground-based, and a range of techniques. Of interest are the absolute uncertainties that have been attained by these techniques and the degree to which linkage between different spectral regimes has been achieved. From these efforts it has been possible to develop large networks of calibration stars that have supported MIR missions and observatories that require photometric standards and low-resolution spectroscopy. I discuss the assumptions inherent in these networks, external methods to test these products, and their absolute validation by the Midcourse Space Experiment (MSX). These networks provide a common absolute framework within which the calibration of multiple instruments on a given platform or sensors on different platforms can be bound together consistently. These stars offer an environment in which absolute multi-wavelength astronomical data can be intermingled without the need to apply photometric transformations or offsets between datasets from distinct facilities or wavelength regions. These stellar networks currently underpin NASA’s Spitzer Space Telescope, Japan’s AKARI, and are currently supporting the next NASA’s Wide-field Infrared Survey Explorer (WISE). Another approved development addresses the critical need to link optical and near-infrared calibrations for future dark energy missions.