Figure 7. The temperatures of the air and the telescope truss are shown as a function of time in this figure. The inset shows the parameters to a least-squares fit of an exponential to the truss data. On this night the time of the enclosure opening was well known and is given as a constant in the fit. The parameter m3 in the fit gives the inverse time constant of the fit in the units 1/hr. The fit time constant is therefore 1.41 hr. The air temperature was also decreasing at the time of these measurements which makes the estimate of the time constant for truss cooling given above an upper limit. However, unlike the estimate of the rod cooling time shown in Figure 8, the longer time constant of the truss and the smaller concurrent changes in the air temperature make this a better estimate of the truss cooling time constant.