The Secondary Guider images a 105 x 105 arc-sec field of view at 0.10 arc-sec/pixel, re-binned 2 x 2 for 0.2 arc-sec/pixel. It is used to provide feedback to the active optical system for the primary mirror support. Gross image motion from the Shack-Hartman reference is also used to provide feedback for derotation correction of the instrument on the Nasmyth rotator bearing.
Like the principal guider, the Secondary Guider uses a pick-off mirror
to sample the field of view, and reflect the reference source into the
camera optics. Also like the principal guider, the SH camera is
mounted such that it can traverse a 45 degree sector of the field of
view. Unlike the Principal Guider, the Secondary Guider can position
radially up to one field diameter, so that the acquired star can be
centered on the aperture of the Shack-Hartman sensor. The mode of
operation of the Secondary Guider is to integrate for 
30-60
seconds in order to collect enough light for a low order correction of
the mirror figure. This is sufficiently frequent to update the
rotation as well.