Thermal Design for a Diffraction-Limited Doppler Spectrometer (iLocater)

The Univ. of Notre Dame is building a new high-resolution spectrometer named “iLocater” to achieve unprecedented radial velocity (RV) precision for stellar Doppler measurements. iLocater will provide essential follow-up observations for NASA’s TESS space mission and identify high-priority exoplanets for transit spectroscopy with JWST. We request 0.6 FTE’s of GSFC engineering support to design iLocater’s opto-mechanical mounts and perform thermal simulations of the cryostat and radiation shield to optimize Doppler performance.

The goal of this project is to design and build the most sensitive RV instrument ever constructed. “iLocater” will receive a well-corrected beam of starlight from an “extreme” adaptive optics (AO) system at the Large Binocular Telescope (LBT) and become the world's first diffraction-limited Doppler spectrometer (Fig. 1). With input images that have 30 times higher spatial resolution than “seeing-limited” designs (i.e., all radial velocity predecessors), iLocater will simultaneously achieve high spectral resolution (R=110,000), high throughput, and a compact optical design at low cost. By operating in a new wavelength range, the infrared, and simultaneously correcting for image-blurring effects introduced by Earth’s turbulent atmosphere, iLocater will generate the first Doppler measurements with sufficient precision (20 cm/s) to detect the gravitational reflex motion caused by Earth-like planets orbiting the nearest stars. iLocater will directly support three NASA space missions through dedicated RV observations (TESS, JWST, WFIRST) .

The first goal of this proposal is to complete the design of iLocater’s cryostat. The spectrograph is a cross-dispersed echelle that operates in the Y-band from 0.95-1.12 microns. The spectrograph will be evacuated (UHV 10^-9 Torr goal), surrounded by a precision temperature-controlled radiation shield, and enclosed in an outer chamber made of stainless steel.  Trade studies between cryogen-fed and mechanical cryocooler designs will be performed.  The second goal is to design customized designs for the mounting of the spectrograph optics.  Proposal tasks include: fully incorporating the iLocater cryostat design into Solidworks and Comsol and initializing the thermal model by specifying conduction paths, thermal feedback channels, and ambient temperature fluctuations; designing mechanical mounts; incorporating mounts into the thermal model, and refining the radiation shield and mechanical design by minimizing thermal gradients.  Deliverables are designs for the optical element mounts (including the detector and fiber feeds), plus designs for the cryostat and temperature control system.