Laser Radar Metrology Measurements Through a Chamber Window of James Webb Space Telescope Alignment Fixtures during Cryogenic Testing

Abstract

Eichhorn, J. Eric Mentzell, and Raymond G. Ohl IV, NASA Goddard Space Flight Center; Anthony R. Slotwinski, Nikon Metrology Inc.; Bente H. Eegholm and Joseph E. Hayden, Sigma Space Corp.; Gregory A. Scharfstein, Flexure Engineering LLC; and Randal C. Telfer, Orbital Sciences Corp. ASA’s James Webb Space Telescope (JWST), the scientific successor to the Hubble Space Telescope, will be the premier space science program for astrophysics in the coming decade. Testing of subsystems is currently underway. Metrology measurements of critical subsystems under vacuum and cryogenic conditions are needed to ensure proper operation. However, current noncontact, laser-based measurement systems cannot operate under these conditions. This article presents the results of measuring through a vacuum chamber window using a Nikon Metrology (formerly Metris) Laser Radar (LR). The accuracy of laser metrology measurements depends upon knowing the parameters of the media through which the measurement beam travels. Under normal conditions, this means knowledge of the temperature, pressure, and humidity of the air in the measurement volume. Typically, thermal gradients provide the most challenging aspect of achieving accurate measurements, but this can be overcome with temperature sensors along the measurement path to provide an average effect. Often, however, there is a need to measure an object in an environment unsuitable for the measurement instrument, such as in an extreme temperature environment.