The iLocater cryostat: design and thermal control strategy for precision radial velocity measurements
The iLocater cryostat: design and thermal control strategy for precision radial velocity measurements
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iLocater 低温恒温器：用于精确径向速度测量的设计和热控制策略
作者：
J. Crass;L. Fantano;F. Hearty;J. Crepp;M. Nelson;S. Wall;D. Cavalieri;C. Koca;D. King;R. Reynolds;K. Stapelfeldt
当前一代的精密径向速度光谱仪是视界受限的仪器。为了在8M级望远镜上实现高光谱分辨率，这些光谱仪需要较大的光学元件，进而需要较大的仪器体积。实现这些系统的毫开尔文热稳定性是具有挑战性的，但对于获得优于1m/S的单次测量RV精度来说是至关重要的。这种精度对于研究宜居带内的类地系外行星至关重要。ILocater是正在为大型双筒望远镜(LBT)开发的新一代RV仪器。与SeeingLimited RV仪器不同，iLocater使用自适应光学(AO)将衍射受限光束注入单模光纤。这些光纤照亮了仪器光谱仪，便于衍射限制设计，与目前的仪器相比体积更小。这实现了仪器固有的稳定性，并促进了精确的热控制。我们介绍了装有光谱仪的iLocater低温恒温器的当前设计及其热控制策略。该光谱仪位于一对安装在MLI衬里真空室内的辐射屏蔽物内。外部辐射屏蔽受到主动控制，以保持仪器在亚MK级别的稳定性，并最大限度地减少外部环境热变化的影响。内屏被动地抑制任何残余温度波动，并辐射耦合到光板。为了提供固有的稳定性，光路板和光学支架将由因瓦制成，并冷却到58K，以受益于在此温度下的零热膨胀系数(CTE)值。相结合，仪器光谱仪占地面积小，INVAR的使用，以及精确的热控制将允许长期亚毫米级开尔文稳定性，以促进精确的RV测量。
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 (LBT). Unlike seeinglimited 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.