The solar bolometric imager Original Research Article

The balloon-borne Solar Bolometric Imager (SBI) will provide the first bolometric (integrated light) maps of the solar photosphere. It will evaluate the photometric contribution of magnetic structures more accurately than has been possible with spectrally selective imaging over restricted wavebands. More accurate removal of the magnetic feature contribution will enable us to determine if solar irradiance variation mechanisms exist other than the effects of photospheric magnetism. The SBI detector is an array of 320 × 240 ferro-electric thermal IR elements whose spectral absorptance has been extended and flattened by a deposited layer of gold-black. The telescope itself is a 30-cm Dall–Kirkham design with uncoated primary and secondary pyrex mirrors. The combination of telescope and bolometric array provides an image of the Sun with a flat spectral response between 0.28 and 2.6 μm, over a field of view of 917 × 687 arcsec, and a pixel size of 2.8 arcsec. After a successful set of ground-based tests, the instrument is being readied for a one-day stratospheric balloon flight that will take place in September 2003. The observing platform will be the gondola previously used for the Flare Genesis Experiment (FGE), retrofitted to house and control the SBI telescope and detector. The balloon flight will enable SBI to image over essentially the full spectral range accepted by non-imaging space-borne radiometers such as ACRIM, making the data sets complementary. The SBI flight will also provide important engineering data to validate the space worthiness of the novel gold-blackened thermal array detectors, and verify the thermal performance of the SBIʹs uncoated optics in a vacuum environment.