The next generation of large ground based telescopes use segmented mirrors. Sensors mounted on the edge of
the segments measure the relative heights of the segments. The segments are actively controlled in height by
three actuators per segment, but lateral motion is only passively constrained. Thus there will be some small
change in the gap and shear between segments as changing telescope orientation and temperature make small
distortions in the telescope structure. These "in-plane" motions place an additional performance burden on the
edge sensors, and on the precision with which they must be mounted relative to the optical surface. In addition,
both the scaling and offset of sensor edge height readings also vary with changes in gap.
Our method for correcting the in-plane motion induced errors in the primary mirror has three parts.
First, the edge sensors are modified to report segment-to-segment gap, as well as the height difference, to make
the in-plane segment positions observable. Second, the mirror segments are phased optically, at a set of zenith
angles and temperatures, to give a set of "measured sensor readings". Finally, during observing, a calibration
procedure combines these data into "desired sensor readings" which are optimal for the current telescope state.
We have included this calibration process in a control loop model of the Thirty Meter Telescope primary mirror
control system. We present our calibration method and the model.