The Einstein Observatory and Exosat have demonstrated that stellar coronae are a common feature of cool stars. The comparative study of the solar corona, which provides an example of a stellar corona that can be studied on the spatial scales of the physical mechanisms underlying coronal phenomena, and of stellar coronae, which allow the scaling of coronal phenomena with basic stellar parameters such as surface temperature, surface gravity, and rotation rate to be examined, is a powerful method for the investigation of coronal phenomena. We describe a `small explorer mission,' the Solar/Stellar Coronal Explorer (SSCE) designed to carry out an investigation of solar and stellar coronal phenomena. The primary instrumentation of the SSCE consists of six identical Ritchey-Chretien Telescopes of 127 mm aperture each, which produce images that are recorded by multi-anode microchannel array (MAMA) detectors. The mirrors of five of the telescopes are coated with multilayer reflecting structures that select narrow XUV wavelength bands corresponding to strong emission lines (or line multiplets) emitted by the solar or by stellar coronal plasmas. The lines observed vary from lines such as upper chromospheric He II ($lambda 304 angstroms), which is emitted by material at T approximately 80,000 K, to Fe XXIV ($lambda 192 angstroms), which is emitted by the hot thermal plasma associated with flares (T approximately 20,000,000 K). The sixth telescope utilizes conventional optics to reflect ultraviolet or visible light; the bandpass of this telescope will be determined by filters (e.g., H Lyman-$alpha@, the C IV resonance doublet at 1548/1550 angstroms, the standard U, B, or V bands, and H $alpha@). Two objective crystal spectrometers of approximately 125 mm aperture which utilize Wolter I imaging optics permit narrow band observations in 4 bandpasses between 1.7 and 25 angstroms. We also describe a somewhat larger explorer mission concept, the Solar/Stellar Coronal Observatory (SSCO), which is compatible with the proposed new NASA `Medium Class Explorer' and will permit more extensive spectroscopic observations to be carried out. The SSCO instrument compliment includes seven telescopes of approximately 400 mm aperture, which cover VUV, FUV, EUV, XUV, and soft x-ray wavelengths.
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