Cryogenic system for interferometric measurement of dimensional changes at 40 K: Design and performance
2007
This report describes the facility, experimental methods, characterizations, and uncertainty analysis of the Cryo-
Distortion Measurement Facility (CDMF) at the Goddard Space Flight Center (GSFC). This facility is designed to
measure thermal distortions of structural elements as the temperature is lowered from 320K to below 40 K over multiple
cycles, and is capable of unattended running and data logging. The first measurement is the change in length and any
bending of composite tubes with Invar end-fittings. The CDMF includes a chamber that is efficiently cooled with two
cryo-coolers (one single-stage and one two-stage) rather than with liquid cryogens. Five optical ports incorporate
sapphire radiation shields - transparent to the interferometer - on each of two shrouds and a fused silica vacuum-port
window. The change in length of composite tubes is monitored continuously with displacement-measuring
interferometers; and the rotations, bending, and twisting are measured intermittently with theodolites and a surface-figure
interferometer. Nickel-coated invar mirrors and attachment mechanisms were developed and qualified by test in
the CDMF. The uncertainty in measurement of length change of 0.4 m tubes is currently estimated at 0.9 micrometers.
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