Cryogenic emittance measurement and its accuracy for the James Webb space telescope

Details Cryogenic emittance measurement and its accuracy for the James Webb space telescope Cryogenic emittance measurement and its accuracy for the James Webb space telescope

Aug 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005spie.5904..143o&link_type=abstract

Cryogenic Optical Systems and Instruments XI. Edited by Heaney, James B.; Burriesci, Lawrence G. Proceedings of the SPIE, Volume

Statistics

Methodology

Scientific paper

NASA's James Webb Space Telescope-Integrated Science Instrument Module (JWST-ISIM) radiators and structures operate in the 30 to 40 K range. There is limited emittance data for coatings of interest in this temperature range. Calorimetric emittance tests performed at Goddard Space Flight Center in the past have used a transient technique, which results in large uncertainties (typically > +/-30%) at the lowest temperatures. These large uncertainties would practically require use of overly conservative emissivities in radiator sizing, which would in turn pose unnecessary area and mass penalties. There is thus a strong incentive to make highly accurate emittance measurements. A special liquid helium cryogenic facility was fabricated for this purpose, and a series of thermal balance tests were subsequently performed at NASA/GSFC to measure the emittance of selected ISIM coatings accurately at temperatures down to 25K. This paper discusses the test methodology, and the analytical methods used to calculate the emittance and its accuracy from the measured data. Preliminary results show that for relatively high emittance coatings, typical measurement accuracies at 30 K approach +/- 5%.

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