  We describe the design, construction, and performance of an ultra-high vacuum
(UHV) scanning tunneling microscope (STM) capable of imaging at
dilution-refrigerator temperatures and equipped with a vector magnet. The
primary objective of our design is to achieve a high level of modularity by
partitioning the STM system into a set of easily separable, interchangeable
components. This naturally segregates the UHV needs of STM instrumentation from
the typically non-UHV construction of a dilution refrigerator, facilitating the
usage of non-UHV materials while maintaining a fully bakeable UHV chamber that
houses the STM. The modular design also permits speedy removal of the
microscope head from the rest of the system, allowing for repairs,
modifications, and even replacement of the entire microscope head to be made at
any time without warming the cryostat or compromising the vacuum. Without using
cryogenic filters, we measured an electron temperature of 184 mK on a
superconducting Al(100) single crystal.
