  The current generation of precision radial velocity (RV) spectrographs are
seeing-limited instruments. In order to achieve high spectral resolution on 8m
class telescopes, these spectrographs require large optics and in turn, large
instrument volumes. Achieving milli-Kelvin thermal stability for these systems
is challenging but is vital in order to obtain a single measurement RV
precision of better than 1m/s. This precision is crucial to study Earth-like
exoplanets within the habitable zone. iLocater is a next generation RV
instrument being developed for the Large Binocular Telescope. Unlike
seeing-limited RV instruments, iLocater uses adaptive optics (AO) to inject a
diffraction-limited beam into single-mode fibers. These fibers illuminate the
instrument spectrograph, facilitating a diffraction-limited design and a small
instrument volume compared to present-day instruments. This enables intrinsic
instrument stability and facilitates precision thermal control. We present the
current design of the iLocater cryostat which houses the instrument
spectrograph and the strategy for its thermal control. The spectrograph is
situated within a pair of radiation shields mounted inside an MLI lined vacuum
chamber. The outer radiation shield is actively controlled to maintain
instrument stability at the sub-mK level and minimize effects of thermal
changes from the external environment. An inner shield passively dampens any
residual temperature fluctuations and is radiatively coupled to the optical
board. To provide intrinsic stability, the optical board and optic mounts will
be made from Invar and cooled to 58K to benefit from a zero coefficient of
thermal expansion (CTE) value at this temperature. Combined, the small
footprint of the instrument spectrograph, the use of Invar, and precision
thermal control will allow long-term sub-milliKelvin stability to facilitate
precision RV measurements.
