Colby A. Jurgenson

State of the Field: Extreme Precision Radial Velocities*

The Second Workshop on Extreme Precision Radial Velocities defined circa 2015 the state of the art Doppler precision and identified the critical path challenges for reaching 10 cm s^(−1) measurement precision. The presentations and discussion of key issues for instrumentation and data analysis and the workshop recommendations for achieving this bold precision are summarized here. Beginning with the High Accuracy Radial Velocity Planet Searcher spectrograph, technological advances for precision radial velocity (RV) measurements have focused on building extremely stable instruments. To reach still higher precision, future spectrometers will need to improve upon the state of the art, producing even higher fidelity spectra. This should be possible with improved environmental control, greater stability in the illumination of the spectrometer optics, better detectors, more precise wavelength calibration, and broader bandwidth spectra. Key data analysis challenges for the precision RV community include distinguishing center of mass (COM) Keplerian motion from photospheric velocities (time correlated noise) and the proper treatment of telluric contamination. Success here is coupled to the instrument design, but also requires the implementation of robust statistical and modeling techniques. COM velocities produce Doppler shifts that affect every line identically, while photospheric velocities produce line profile asymmetries with wavelength and temporal dependencies that are different from Keplerian signals. Exoplanets are an important subfield of astronomy and there has been an impressive rate of discovery over the past two decades. However, higher precision RV measurements are required to serve as a discovery technique for potentially habitable worlds, to confirm and characterize detections from transit missions, and to provide mass measurements for other space-based missions. The future of exoplanet science has very different trajectories depending on the precision that can ultimately be achieved with Doppler measurements.

Magdalena Ridge Observatory Interferometer: Status Update

The Magdalena Ridge Observatory Interferometer (MROI) is a ten element optical and near-infrared imaging interferometer being built in the Magdalena mountains west of Socorro, NM at an altitude of 3230 m. The interferometer is being designed and built by a collaboration which includes the New Mexico Institute of Mining and Technology (NMT) as the prime contractor and center for the technical team, and the University of Cambridge, Physics Department at the Cavendish Laboratory, which participates in the design and executes work packages under contract with NMT. This manuscript serves as a status update on MROI, and will present progress and milestones toward the observatorys first fringes in 2008.

Magdalena Ridge Observatory Interferometer automated alignment system

Here is presented the current outline and progress of MROIs automated alignment system design. Depending on the location of each of MROIs unit telescopes (UT), light can travel distances ranging from 460 to 660 meters via several reflections that redirect the beams path through the beam relay system (BRS), delay line system (DLS), beam compressing telescope (BCR), switchyards and finally to the beam combiners (BC). All of these sub-systems comprise three major optical axes of the MROI which must be coaligned on a nightly basis by the AAS. The AAS consists of four subsystems: the primary fiducial-for beam injection, the UT tilt and shear measurement components (TASM), the BC TASM components, and the secondary fiducial-for quick alignment checks. All of these subsystems contribute to the unique design of the AAS which will allow for simultaneous measurements from the visible to the near-IR wavelengths, full automation, the capability to perform optical path difference (OPD) alignment and spectral calibration, making it cost effective and saving on realty in the beam combining area (BCA). The AAS is nearing completion and assembly of the various subsystems is expected to commence soon. The latest results on all of the following are reviewed here.

Mechanical design of the Magdalena Ridge Observatory Interferometer

We report on the mechanical design currently performed at the Magdalena Ridge Observatory Interferometer (MROI) and how the construction, assembly, integration and verification are planned towards commissioning. Novel features were added to the mechanical design, and high level of automation and reliability are being devised, which allows the number of reflections to be kept down to a minimum possible. This includes unit telescope and associated enclosure and transporter, fast tip-tilt system, beam relay system, delay line system, beam compressor, automated alignment system, beam turning mirror, switchyard, fringe tracker and vacuum system.

Fiber Scrambling for High-Resolution Spectrographs. II. A Double Fiber Scrambler for Keck Observatory

We have designed a fiber scrambler as a prototype for the Keck HIRES spectrograph, using double scrambling to stabilize illumination of the spectrometer and a pupil slicer to increase spectral resolution to R = 70,000 with minimal slit losses. We find that the spectral line spread function (SLSF) for the double scrambler observations is 18 times more stable than the SLSF for comparable slit observations and 9 times more stable than the SLSF for a single fiber scrambler that we tested in 2010. For the double scrambler test data, we further reduced the radial velocity scatter from an average of 2.1 m/s to 1.5 m/s after adopting a median description of the stabilized SLSF in our Doppler model. This demonstrates that inaccuracies in modeling the SLSF contribute to the velocity RMS. Imperfect knowledge of the SLSF, rather than stellar jitter, sets the precision floor for chromospherically quiet stars analyzed with the iodine technique using Keck HIRES and other slit-fed spectrometers. It is increasingly common practice for astronomers to scale stellar noise in quadrature with formal errors such that their Keplerian model yields a chi-squared fit of 1.0. When this is done, errors from inaccurate modeling of the SLSF (and perhaps from other sources) are attributed to the star and the floor of the stellar noise is overestimated.

EXPRES: A Next Generation RV Spectrograph in the Search for Earth-like Worlds

The EXtreme PREcision Spectrograph (EXPRES) is an optical fiber fed echelle instrument being designed and built at the Yale Exoplanet Laboratory to be installed on the 4.3-meter Discovery Channel Telescope operated by Lowell Observatory. The primary science driver for EXPRES is to detect Earth-like worlds around Sun-like stars. With this in mind, we are designing the spectrograph to have an instrumental precision of 15 cm/s so that the on-sky measurement precision (that includes modeling for RV noise from the star) can reach to better than 30 cm/s. This goal places challenging requirements on every aspect of the instrument development, including optomechanical design, environmental control, image stabilization, wavelength calibration, and data analysis. In this paper we describe our error budget, and instrument optomechanical design.

Final mechanical and opto-mechanical design of the Magdalena Ridge Observatory interferometer

Most subsystems of the Magdalena Ridge Observatory Interferometer (MROI) have progressed towards final mechanical design, construction and testing since the last SPIE meeting in San Diego - CA. The first 1.4-meter telescope has successfully passed factory acceptance test, and construction of telescopes #2 and #3 has started. The beam relay system has been prototyped on site, and full construction is awaiting funding. A complete 100-meter length delay line system, which includes its laser metrology unit, has been installed and tested on site, and the first delay line trolley has successfully passed factory acceptance testing. A fully operational fringe tracker is integrated with a prototyped version of the automated alignment system for a closed looping fringe tracking experiment. In this paper, we present details of the final mechanical and opto-mechanical design for these MROI subsystems and report their status on fabrication, assembly, integration and testing.

Custom beamsplitter and AR coatings for interferometry

We report on final fabrication tests for the dielectric coatings for the Magdalena Ridge Observatory Interferometer (MROI) fringe tracking beam combiner. The broadband anti-reflection (1.1 μm to 2.4 μm) and beamsplitter (1.49 μm to 2.31 μm) coatings required have been designed with both optical and mechanical constraints in mind. Not only do these coatings have very low optical losses, but they induce minimal bending of their substrates, thereby giving very low fringe contrast reductions. Performance tests on the deposited coatings at our manufacturers (Optical Surface Technologies, Albuquerque, NM) demonstrate reflection losses of less than 0.5% over the full bandpass of the AR coating, and deviations from the desired 50:50 intensity ratio of the beamsplitter coating of no more than 2%. When combined with the measured wavefront perturbations these results imply that the total fringe visibility losses induced by imperfect coating quality will be no more than 2% for all outputs of the MROI fringe tracking beam combiner.

MROI's automated alignment system

The Magdalena Ridge Observatory Interferometer (MROI) will be a reconfigurable (7.5-345 meter baselines) 10 element optical/near-infrared imaging interferometer. Depending on the location of each unit telescope (UT), light can travel distances ranging from 460 to 660 meters via several reflections that redirect the beams path through the beam relay trains, delay lines (DL), beam reducing telescope (BCR), switchyards and finally to the beam combiners (BC). All of these sub-systems comprise three major optical axes of the MROI to be coaligned on a nightly basis by the alignment system. One major obstacle in designing the automated alignment system (AAS) is the required simultaneous measurements from the visible through near-IR wavelengths. Another difficulty is making it fully automated, which has not been accomplished at other optical/near-IR interferometers. The conceptual design of this system has been completed and is currently in its preliminary design phase. Prototyping has also commenced with designs of some hardware near completion. Here is presented the current outline and progress of MROIs automated alignment system design and some results of the prototyping.

The Magdalena Ridge Observatory interferometer: first light and deployment of the first telescope on the array

The Magdalena Ridge Observatory Interferometer (MROI) has been under development for almost two decades. Initial funding for the facility started before the year 2000 under the Army and then Navy, and continues today through the Air Force Research Laboratory. With a projected total cost of substantially less than