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The Space Telescope program is a major NASA undertaking, the purpose of which is to design, build and operate a National Observatory (known as the Hubble Space Telescope) in space. After feasibility and concept studies lasting well over a decade, the design was begun in earnest in the fall of 1977. It is by far the most significant commitment to the science of astronomy since the 5-meter Hale telescope at Mount Palomar. Space Telescope is a 2.4-meter aperture telescope designed to be 50 times as effective in observing faint objects as the largest ground-based telescopes. Its resolution will be at least 10 times as good. It is to be a long lived facility, capable of on-orbit main-tenance and update as well as ground refurbishment, so that it will be providing useful scientific data well into the twenty-first century. Above the distortion and absorption of the Earth's atmosphere, ST will provide, for the first time, high resolution broad spectral coverage of the entire sky. During the Hubble Space Telescope thermal vacuum test, the measured temperature of the primary mirror was lower than expected, and exhibited an axial gradient, which indicated that heat was being lost from the front surface of the mirror. This is most easily explained by an increase in the surface emissivity of the mirror coating. It was proposed that the small amount of particulate contamination known to be on the surface of the mirror might be the cause of an increase in effective emissivity. However, it could be argued that the conductive coupling between the dust particles and the mirror surface would be very small, making their net effect negligible, thermally. The problem, though conceptually simple to analyze, is not amenable to calculation be-cause we know little or nothing about the physical contact between a dust particle and the host surface. The best way to resolve the question is to measure the effective total hemi-spherical emissivity of a mirror surface with measured levels of particulate contamination. A thermal measurement is preferred, because typical optical measurements, which measure normal emissivity by subtracting measured reflectance from unity, will simply measure the effect of particulate obscuration.
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