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S. Libonate, P. Foukal (CRI, Inc.)
The Solar Bolometric Imager (SBI) is a novel solar imaging system optimized for studying mechanisms of total irradiance variation. Uncertain broad-band photometric contrasts of spots, and especially faculae and network, currently present the main obstacle to improved modeling of total irradiance fluctuations. After 20 years of effort, accurate contrasts remain elusive because the photometric response functions of conventional camera and telescope systems are highly wavelength dependent, and difficult to remove from measurements of structures having non-black-body radiance distributions. The SBI can provide the required data in a single image because it has the same spectrally “flat” (i.e. constant) photometric response as pyrheliometers such as ACRIM over the wavelength range between approximately 0.26 um and 2.6 um, containing over 96% of the total solar irradiance. The prototype SBI system at CRI utilizes a 50,000-element uncooled thermal imaging array whose spectral absorptance has been flattened by gold-blacking, without significantly degrading its modulation transfer. We use a 30 cm-aperture Dall-Kirkham design with uncoated (i.e. bare glass) primary and secondary mirrors to provide uniform spectral response, and to avoid solar heating and saturation of the imager. The image quality (~5” resolution over a 13 X 7 arc minute FOV) is very satisfactory for our purpose of accurately discriminating the total irradiance contributions of photospheric magnetic structures, such as spots, faculae and network from other possible solar heat flow inhomogeneities. We are currently redesigning the (commercial) camera electronics to reduce non-linearities and improve calibration accuracy in the telescope. We expect the improved accuracy provided by the SBI to significantly improve the constraints on possible slow changes in solar irradiance that may drive secular climate variations. Balloon flight of the SBI is necessary to avoid the most serious atmospheric transmission variations; useful measurements could be obtained from a short-duration flight, and the full potential of the SBI would be realized with a long-duration underflight of a spaceborne pyrheliometer. This work has been performed at Cambridge Research and Instrumentation (CRI), under NASA research grant NAG5-6979.