The spatial infrared imaging telescope (SPIRIT III) Fourier transform spectrometer, a Michelson interferometer, contains six IR detectors having independent fields of view and spectral responsivities. The Space Dynamics Laboratory at Utah State University (SDL/USU) designed, built, and calibrated the instrument for the Midcourse Space Experiment (MSX) sponsored by the Ballistic Missile Defense Organization (BMDO). The spectrometer uses a HeNe laser to record the optical path difference introduced by moving one mirror in the spectrometer. Spectral line position errors in the spectrometer were expected as a result of slight deviations in the optical axes of each detector and the reference laser detector relative to the optical axis of the instrument. These spectral line position errors were measured for the spectrometer by comparing measurements of earthlimb radiance to published line position values from the HITRAN database. These errors were fit to a model of the expected optical axis deviations to generate correction factors for the SPIRIT III spectrometer and to infer the approximate effective focal plane location of the reference laser detector relative to the focal plane location of each detector.