STRAY has been developed to quickly but roughly model the amount of stray radiation reaching the focal plane of a well-baffled telescope. This radiation arises from the scattering and diffraction of the light of bright off-axis sources, such as the Sun, Earth and Moon, as well as the thermal emission of the telescope itself. A series of specular reflections, scatters and diffractions propagates this energy to the focal plane. The sum of the various paths yields the focal plane irradiance. The telescope may be of Cassegrain, Gregorian or Herschelian design, as well as prime focus. A two-section conical aperture shade is included, which may be sliced off at an angle to the optical axis. Data on Class 100, 300 and 500 mirror contaminations, as well as a number of clean mirrors, are available for BRDF calculations. The stray radiation signal accumulated after a series of telescope nods and/or chops against a time-varying background may also be evaluated. The user enters the nodding and chopping frequencies, the chopping amplitude and duty cycle, and the time rate-of-change of the off-axis angle or temperature.