The Wilkinson Microwave Anisotropy Probe (WMAP) measures anisotropy or temperature differences in the Cosmic Microwave Background (CMB) radiation with high angular resolution and sensitivity, yielding unprecedented accuracy. To achieve this measurement, WMAP’s back-to-back Gregorian telescopes focus microwave radiation into 20 feed horns connected to 10 differential microwave radiometers. Proper alignment of the telescope reflectors, feed horns, and radiometers at flight temperatures was essential to the mission success.
This paper will present the WMAP instrument metrology requirements and associated challenges, discuss the opto-mechanical tooling utilized to accomplish these objectives, and then give an overview of the metrology effort. The WMAP instrument integration effort included the following key metrology tasks: alignment and clocking of 20 microwave feed horns and mating microwave differencing assemblies within a focal plane assembly; alignment of a pair of primary and secondary reflectors composing back-to-back Gregorian telescopes; and the placement of the focal plane assembly and reflector system relative to each other, and as a unit on the spacecraft. WMAP environmental test metrology efforts included: reflector and truss thermal stability at 80 K; reflector and feed horn position verification at 90 K, and pre and post vibration and acoustic test reflector and feed horn position verification. The WMAP instrument integration and test objectives required the use of a photogrammetric camera, a laser tracker, a portable coordinate measuring machine (PCMM), and theodolites utilizing an electronic theodolite metrology system (ETMS) and autocollimation. The synergy of these metrology systems facilitated the successful characterization of the WMAP scientific instrument mechanical performance data at room temperature and flight temperatures, and correlation of the data to the analytical model. WMAP was launched on July 1, 2001, and flight data has confirmed the proper on-orbit instrument alignment was achieved.