Simultaneous phase shifted digital speckle pattern interferometry

Abstract

The James Webb Space Telescope (JWST) is an anastigmatic, segmented cryogenic telescope. The stability of the primary mirror back-plane is a major concern in co-phasing the telescope and in the longer term stability of its image quality. The back-plane is a 6.6 meter structure constructed of carbon fiber, and is therefore a "diffuse" object, an object with surface roughness larger than the wavelength of the light. For this reason classical interferometeric measurements will not work. The objective is to design and characterize a Simultaneous phase shifted Digital Speckle Pattern Interferometer (SDSPI) that has the most potential to measure the JWST back plane structure within the required accuracy (15 nm RMS out-of-plane motion). SDSPI interferometry is a method of measuring the back-plane deformations that has the potential of reaching the required accuracy in the presence of vibrations within cryogenic chambers. 4D Technology Corporation, in Tucson, Arizona has an instantaneous phase shifted interferometer (PhaseCam) that is modified to a SDSPI interferometer. Repeatability, dynamic range, and accuracy of the SDSPI is characterized by measuring a 5cm x 5cm back-plane structure made of carbon fiber.