  Motivated by the ever increasing pursuit of science with the transient sky
(dubbed Time Domain Astronomy or TDA), we are fabricating and will commission a
new deployable tertiary mirror for the Keck I telescope (K1DM3) at the W.M.
Keck Observatory. This paper presents the detailed design of K1DM3 with
emphasis on the opto-mechanics. This project has presented several design
challenges. Foremost are the competing requirements to avoid vignetting the
light path when retracted against a sufficiently rigid system for
high-precision and repeatable pointing. The design utilizes an actuated swing
arm to retract the mirror or deploy it into a kinematic coupling. The K1DM3
project has also required the design and development of custom connections to
provide power, communications, and compressed air to the system. This NSF-MRI
funded project is planned to be commissioned in Spring 2017.
