A stray light model of the Apache Point 2.5 meter telescope has been constructed to predict its performance from a stray light perspective. We conducted extensive modelling of the baffle and vane surfaces and of the surface scatter characteristics of reflective and refractive optical elements in order to characterize the overall scattering characteristics of the telescope system. Baffle and vane surfaces were modelled two ways. The first model used the specular surface properties and the second used the Bidirectional Reflectance Distribution Function (BRDF) of the surfaces. The lens and mirror surfaces were modelled using scatter characteristics of surfaces that are cleaned on a regular basis. A scatter analysis yielded the irradiances at the focal plane for off-axis sources from in-field to 65 degrees. An analysis of the scatter paths contributing to irradiance levels has identified the most important scatter paths. Most of the scatter paths which involve baffle surfaces can be blocked or eliminated. These options are not discussed here, but will be discussed in a second paper.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.