  We have designed and built a scanning tunneling microscope (STM) setup for
operation at millikelvin temperatures in ultra high vacuum. A compact cryostat
with an integrated dilution refrigerator has been built, that allows
measurements at a base temperature of 25 mK in magnetic field up to 7.5 T with
low mechanical and electronic noise. The cryostat is not larger than
conventional helium bath cryostats (23 and 13 liters of nitrogen and helium
respectively), so that the setup does not require a large experimental hall and
fits easily into a standard lab space. Mechanical vibrations with running
dilution circulation were kept below 300 fm/$\sqrt{\mathrm{Hz}}$ by
mechanically decoupling the STM from the cryostat and the pumping system. All
electronic input lines were low-pass filtered, reducing the electronic
temperature to below 100 mK, as deduced from the quasiparticle peaks of
superconducting aluminium. The microscope is optically accessible in the parked
position, making sample and tip exchange fast and user-friendly. For
measurement the STM is lowered 60 mm down so that the sample ends in the middle
of a wet superconducting magnetic coil.
