Spectral Irradiance Calibration in the Infrared. XV. Absolute Calibration of Standard Stars by Experiments on theMidcourse Space Experiment

Abstract

Calibration experiments were conducted with the SPIRIT III infrared instrument on the Midcourse Space Experiment (MSX) against a number of infrared standard stars and five emissive reference spheres that were ejected at various times during the mission. The physical properties of the 2 cm diameter spheres, such as size and emissivity, were precisely measured in the laboratory. The energy balance equation between the total flux absorbed and that emitted by the sphere is solved to obtain the time-dependent temperature of the sphere under the assumption that the sphere radiates as a blackbody with the measured wavelength-dependent emissivity. The estimated uncertainties in the modeling of the sphere are about 1 K in the thermal component and 3% for the geometric contribution. MSX also measured over 150 mean fluxes for eight standard infrared calibration stars during the 10 month mission. The measurements were scaled to the absolute fluxes that Cohen et al. adopt for α CMa (Sirius). The measured spectral energy distributions of the calibration stars relative to Sirius are within the uncertainties that Cohen et al. assign to the absolute fluxes from these stars, with a few exceptions. However, the MSX measurement uncertainties are generally much smaller, and the mission-averaged fluxes reveal statistically significant deviations from the Cohen et al. values. Of the calibration stars, only β Peg was found to be variable. MSX also measured excess fluxes for α Lyr (Vega) in the 12.1, 14.7, and 21.3 μm spectral bands; the excesses in the latter two bands are consistent with the published thermal model for the dust ring around this star. The absolute calibration of the fluxes of the stellar standards based on the average of the measurements of the spheres over all MSX bands and the five experiments agrees with those predicted to within the 1.4% MSX measurement uncertainties. The zero-magnitude absolute fluxes proposed by Cohen et al. are validated if the flux from Sirius is increased by 1%.