We have designed, calibrated, and deployed an electron spectrograph to measure the effect of the free‐electron laser interaction on the relativistic electron beam in an FEL. The spectrograph spans a wide energy range (&Dgr;E/E∼1) and is designed for electron energies from 2 to 4 MeV. Calibration was achieved using a 2–6‐keV proton beam, and the measured dispersion is in good agreement with the value predicted from ray tracing. The spectrograph has been used to estimate the extraction efficiency, and the results agree within 25% with the observed microwave power levels.