The focus of IUE's telescope is thermally controlled. Heaters near the primary mirror and on the camera deck are used to maintain optimum focus values. The focus setting is calibrated in units of focus "step", where -6 < "step" < -1 is the best range of focus (Cassatella et al. 1985; Pérez et al. 1991). The focus is normally kept near optimum values by the manipulation of the primary-mirror heaters. However, at low Betas (less than 35 degrees) it is not possible to maintain thermal stability and thus the focus deteriorates with time. Power considerations may restrict the use of heaters at low and high Betas (Beta < 32 and Beta > 112 degrees). Without use of the heaters, the focus may degrade even more rapidly.