![](Fig2.gif)
Figure 2. This figure shows the amount of secondary piston
motion required to correct for defocus owing to sag in the secondary
spider rods. During the acquisition of these measurements the
telescope collimation process was running, therefore, the defocus
shown here is the residual sag which is not being corrected for. The
solid line shows the fit of to these data and the ledgend shows the
functional form of the fit used. The coefficients m1 through m3
should be added to the SecPistCoefs which were in use at the time of
these measurements. The telescope temperature was quite constant
during the time these measurements were taken. If this were not the
case, then some of these variations could be attributable to
temperature changes in the truss.