Ray-tracing for coordinate knowledge in the JWST Integrated Science Instrument Module

Abstract

Optical alignment and testing of the Integrated Science Instrument Module of the James Webb Space Telescope is underway. We describe the Optical Telescope Element Simulator used to feed the science instruments with point images of precisely known location and chief ray pointing, at appropriate wavelengths and flux levels, in vacuum and at operating temperature. The simulator's capabilities include a number of devices for in situ monitoring of source flux, wavefront error, pupil illumination, image position and chief ray angle. Taken together, these functions become a fascinating example of how the first order properties and constructs of an optical design (coordinate systems, image surface and pupil location) acquire measurable meaning in a real system. We illustrate these functions with experimental data, and describe the ray tracing system used to provide both pointing control during operation and analysis support subsequently. Prescription management takes the form of optimization and fitting. Our core tools employ a matrix/vector ray tracing model which proves broadly useful in optical engineering problems. We spell out its mathematical basis, and illustrate its use in ray tracing plane mirror systems relevant to optical metrology such as a pentaprism and corner cube.

Stray light modeling of the James Webb Space Telescope (JWST) Integrated Science Instrument Module (ISIM)

“…Light is emitted from point sources from a Source Plate Assembly (1) located in a recessed back of Fold Mirror 1 (FM1). Light comes through one of a variety of holes in FM1 to the large spherical Primary Mirror (2), reflects back towards FM1 (3), to Fold Mirror 2 (4), through a dual wheel Pupil Select Mechanism (5), and off of a steerable Fold Mirror 3 (6), toward the ISIM Science Instruments (7). The Source Plate Assembly can move along and normal to the beam path providing control over focus and short-range steering in field space.…”