On caveat here: since the ISI was floating we couldn't attach the corner cube to get an accurate longitudinal measurement. The total station can use a "Sheet Refelctor" mode to measure the distance to the optic from the HR face directly, but this introduces more error in the measurement. For the purpose of calculating the lateral and vertical errors a few mm of error in the longitudinal measurement means almost nothing at this distance (>33 m; a 5 mm shift in the longitudinal direction only imparts a change of <1 µm in the lateral/vertical error calculation), but I would not use this longitudinal measurement as a basis for the actual optic position (and adjust HEPI when it might not be necessary). When the ISI is locked down we will attach the corner cube and get an accurate measurement of the longitudinal position error of the ITMx (and make any decisions about HEPI adjustments at that point).