  We present the design and calibration measurements of a scanning tunneling
microscope setup in a 3He ultrahigh-vacuum cryostat operating at 400 mK with a
hold time of 10 days. With 2.70 m in height and 4.70 m free space needed for
assembly, the cryostat fits in a one-story lab building. The microscope
features optical access, an xy table, in situ tip and sample exchange, and
enough contacts to facilitate atomic force microscopy in tuning fork operation
and simultaneous magneto-transport measurements on the sample. Hence, it
enables scanning tunneling spectroscopy on microstructured samples which are
tuned into preselected transport regimes. A superconducting magnet provides a
perpendicular field of up to 14 T. The vertical noise of the scanning tunneling
microscope amounts to 1 pmrms within a 700 Hz bandwidth. Tunneling spectroscopy
using one superconducting electrode revealed an energy resolution of 120 mueV.
Data on tip-sample Josephson contacts yield an even smaller feature size of 60
mueV, implying that the system operates close to the physical noise limit.
