There is an offset imposed by the requirement that the gaps in the CCDs caused by butting them do not overlap. Both coarse and fine focus must be provided for. The bench must be of sufficient stiffness to enable the focus and alignment of the telescopes to be maintained over periods of not less than 30 days. 6.2.3.3 CCD sensitivity vs. time 7. 9.5 Focal plane must be capable of following a feature at the solar equator for exposures between 1-200 seconds.
Weight reduction is a primary consideration.