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This paper describes the derivation of cleanliness requirements and the development of contamination control procedures for the Space IR Telescope Facility (SIRTF), the fourth of the great observatories. SIRTF, scheduled for launch in December 2001 and designed to explore the IR universe, will be placed into a heliocentric orbit and will perform background-limited imaging and spectroscopic measurements of celestial objects in the 3-180 micrometers spectral range. The main components of the facility are the three scientific instrument packages, a liquid helium dewar to maintain the instruments as 2 to 5 K, a telescope which is maintained at 5 K, and a spacecraft. The instrument/dewar assembly, known as the MIC, will be cooled before launch, while the telescope will be launched warm and will be cooled on orbit by a combination of radiation to space and helium boil off. SIRTF has several unique contamination control drivers and design challenges. The mission lifetime requirement of 2.5 years will be achieved by minimizing the thermal load on the telescope and helium dewar using very low emittance surfaces, most of which operate below the condensation temperature of water vapor, and are hence very vulnerable to emittance degradation due to contaminant deposition. Contaminant films and particles on optical surfaces will reduce optical throughput, increase off axis point source transmittance, and degrade the point spread function of the telescope and scientific instruments, and hence degrade the observatory performance. Quantification of these effects is hampered by the lack of contaminant optical effects data at long wavelengths and uncertainty about the structure of cryolayers at temperatures less than 20 K. The operational temperatures of the SIRTF telescope and the cryostat will be low enough to suppress equilibrium outgassing rates to negligible values, but outgassing of the telescope, baffle assembly, thermal insulation, etc. during initial cooldown is significant. Assessment of molecular contaminant transport during this phase requires modeling the effect of transient source and deposition surface temperatures, as well as the measurement of transient outgassing rates of materials at temperatures below ambient. The study completed so far facilitates subsequent contamination analysis and control at the optical component level.
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