  We developed a scan mirror mechanism (SMM) that enable a slit-based
spectrometer or spectropolarimeter to precisely and quickly map an astronomical
object. The SMM, designed to be installed in the optical path preceding the
entrance slit, tilts a folding mirror and then moves the reflected image
laterally on the slit plane, thereby feeding a different one-dimensional image
to be dispersed by the spectroscopic equipment. In general, the SMM is required
to scan quickly and broadly while precisely placing the slit position across
the field-of-view (FOV). These performances are highly in demand for
near-future observations, such as studies on the magnetohydrodynamics of the
photosphere and the chromosphere. Our SMM implements a closed-loop control
system by installing electromagnetic actuators and gap-based capacitance
sensors. Our optical test measurements confirmed that the SMM fulfils the
following performance criteria: i) supreme scan-step uniformity (linearity of
0.08%) across the wide scan range (${\pm}$1005 arcsec), ii) high stability
(3${\sigma}$ = 0.1 arcsec), where the angles are expressed in mechanical angle,
and iii) fast stepping speed (26 ms). The excellent capability of the SMM will
be demonstrated soon in actual use by installing the mechanism for a
near-infrared spectropolarimeter onboard the balloon-borne solar observatory
for the third launch, Sunrise III.
