Aerial reconnaissance optical systems are often subjected to extremes in thermal environment. Designers have been able to engineer systems which maintain stable performance over a wide range of steady-state temperatures. But how long does it take a reconnaissance system to reach steady state after being subjected to a thermal shock (or step function)? This paper reports on experimental measurements made on several long focal length reconnaissance lenses exposed to thermal step functions. The most significant finding showed that the thermal recovery time is on the order of hours .... a period during which the reconnaissance mission may already have been completed.