  We present here a small-scale liquid Helium (LHe) immersion cryostat with an
innovative optical setup suitable to work in long wavelength radiation ranges
and under applied magnetic field. The cryostat is a multi stage device with
several shielding in addition to several optical stages. The system has been
designed with an external liquid Nitrogen boiler to reduce the liquid bubbling.
The optical and mechanical properties of the optical elements were calculated
and optimized for the designed configuration while the optical layout has been
simulated and optimized among different configurations based on the geometry of
the device. The final design has been optimized for low noise radiation
measurements of proximity junction arrays under applied magnetic field in the
wavelength range $\lambda$=250-2500 $\mu$m.
