  This paper presents the design and technical progress of a precision X-Y
stage for detector dithering and flexure compensation. The stage is being
developed for use in the Magellan InfraRed Multi-Object Spectrograph, MIRMOS.
MIRMOS is a very large Nasmyth mounted spectrograph containing a combination of
refractive, reflective and diffractive optics mounted on a long cryogenic
optical bench. The instrument utilizes five science cameras, each having a
custom x-y stage to control the in-plane detector position within each camera,
providing both dithering capability for improved sampling, and flexure
compensation to correct for image motion that results from the gravity variant
operation of the instrument. Designed to operate at 120~K, the stage will
accurately control detector position in two orthogonal degrees of freedom, and
have manual fine adjustment features to set detector tip, tilt and piston. The
piezo-driven flexure stage provides high-resolution backlash-free motion of the
detector and is very compact along the optical path, keeping camera length to a
minimum. A magnetoresistive bridge provides position feedback in each degree of
freedom, greatly reducing hysteresis, which is common in piezoelectric
actuators. The system is designed to operate in open loop using a lookup table
keyed to the Nasmyth rotator angle for flexure control. Here, the
optomechanical design of the stage, electrical control system, and current
performance results from early prototype efforts are presented and discussed.
