Applying the tool: stray light cross-checks of the James Webb Space Telescope

System modeling of space observatories too large for end-to-end ground testing includes assessing levels of unwanted radiant energy on focal plane arrays, commonly called "stray light." The need for stray light analyses parallels the need for large telescope collecting apertures; both seek to maximize sensitivity. Mathematical modeling of stray light is unlike other engineering analyses, and the differences often lead to unfamiliarity and subsequent underrating of its importance. Fortunately, the JWST Project undertook these analyses early enough to guide important aspects of the optical and thermal control designs. Software tools of unprecedented power continue in use to model the stray light performance of the James Webb Space Telescope (JWST). This paper describes how one such tool is used by NASA's Goddard Space Flight Center (GSFC) to provide cross-checks of analyses performed by JWST's industry partners. The methods described for JWST are broadly applicable to other astronomical instrumentation.