A digital high-definition imaging system for spectral studies of extended planetary atmospheres. I. Initial results in white light showing features on the hemisphere of mercury unimaged by Mariner 10

We present an instrumentation plan for spectral imaging of Mercury's extend ed atmosphere. The approach depends upon simultaneous short-exposure images in white light and sodium, with the former used to select the frames for p ostintegration of the sodium images. The effects of atmospheric seeing are thus minimized by the combination of high-speed exposures and subsequent se lective integration. The instrumentation to be used is a long-slit imaging echelle spectrometer equipped with an image slicer and an imaging photon de tector. A test of the white-light component of the technique has yielded a best-to-date image of a portion of Mercury's surface not photographed durin g the Mariner 10 mission.