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In 1999, a third servicing mission will be required to restore the HST orbit which decays due to atmospheric drag. This will be the next opportunity to install an instrument on HST. We have assessed the imaging science that astronomers have proposed for or desire from HST and believe that the most valuable 1999 instrument would be a camera with both near UV/optical and far UV sensitivity and with both wide field and high spatial resolution options.
We have proposed such a camera for joint development by NASA and ESA. The HST Advanced Camera will operate between 1150\AA\ and 10000\AA. It will use a mosaic of 4 thinned 4096x4096 arrays to provide critical sampling in the optical and near UV, but normally will operate in a 2x2 binned mode providing 16 million pixels over the field. It will also use a 2048x2048 solar blind detector (most likely a photon counting array) in the far UV between 1150\AA\ and 2000\AA. The individual cameras will be equipped with a wide variety of filters and may have a coronagraph for the suppression of the PSF wings well below levels acheivable from the ground. The AC will provide significant improvements over the existing cameras in field-of-view, throughput, detective quantum efficiency and spatial resolution. These improvements will provide at least an order of magnitude gain over the capabilities of the cameras on board at launch.
