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.
In this concept study, we are targeting to build a new instrument to sequentially observe exoplanet atmospheres and their parent’s stellar spectra over a significant time in NUV and FUV. The Compact Homodyne Astrophysics Spectrometer for Exoplanets (CHASE) offers integrated spectra over a wide field-of-view (FOV~40arcsec) in high spectral resolution (R>105) in a miniaturized architecture using no (or a small < 1m) primary mirror. CHASE’s wide FOV is compatible with the relaxed pointing requirements of current CubeSats and SmallSats which makes it readily qualifiable for space in a compact format and have the potential to enable major scientific breakthroughs.
The Yale Exoplanet Laboratory is under contract to design, build, and deliver a high-resolution (R = 60,000) echelle
spectrograph for the Moletai Astronomical Observatory 1.65-meter telescope at the Vilnius University. We present a
fiber-fed, white-pupil architecture that will operate from 400 to 880nm. The optomechanical design implements a
modular approach for stability and ease of alignment that can be reproduced for other telescopes. It will utilize highperformance
off-the-shelf optical components with a custom designed refractive camera for high throughput and good
Pushing the RV technique to the precision required to detect Earth-like planets around Solar-type stars requires extreme stability in the wavelength calibrator. We are developing a wavelength calibration technique based on a Fabry-Perot interferometer locked to a stabilized laser. This approach offers advantages over other methods: it produces a broadband, emission comb output from 380-790 nm that is difficult to achieve with a laser frequency comb; by injecting into the science fibers before and after observations, weak signals from velocities in the stellar photosphere that would be masked by iodine reference lines can be now be identified; and by locking the laser to an atomic transition, the spectrum will be stabilized to better than 1 part in 10 e-11, corresponding to a wavelength solution that is known to better than 1 cms-1.
The detection of Earth analogs with radial velocity requires extreme Doppler precision and long term stability.
Variations in the illumination of the slit and of the spectrograph optics occur on time scales of seconds and
minutes, primarily because of guiding, seeing and focusing. These variations yield differences in the instrumental
profile (IP). In order to stabilize the IP, we designed a fiber feed for the Hamilton spectrograph at Lick and for
HIRES at Keck. Here, we report all results obtained with these fiber scramblers. We also present the design of
a new double scrambler/pupil slicer for HIRES at Keck.
CHIRON is a fiber-fed Echelle spectrograph with observing modes for resolutions from 28,000 to 120,000, built
primarily for measuring precise radial velocities (RVs). We present the instrument performance as determined during
integration and commissioning. We discuss the PSF, the effect of glass inhomogeneity on the cross-dispersion prism,
temperature stabilization, stability of the spectrum on the CCD, and detector characteristics. The RV precision is
characterized, with an iodine cell or a ThAr lamp as the wavelength reference. Including all losses from the sky to the
detector, the overall efficiency is about 6%; the dominant limitation is coupling losses into the fiber due to poor guiding.
The detection of earth-like exoplanets with the Doppler technique requires extreme precision spectrographs stable over
timescales of years. The precision requirement of 10 cm/s is equivalent to a relative uncertainty of 3x10-10, and, with the typical dispersion of the Echelle spectrographs used for this purpose, translates to a shift of a few nanometers of the spectrum on the detector. Consequently, the instrument must be well understood and optimized in every component and detail. We describe the Yale Doppler diagnostic facility (YDDF), a dedicated bench mounted Echelle spectrograph in
our lab at Yale University, which will be used to systematically study the influence of different components at this
precision level. The spectrograph bench allows for a flexible optical configuration, high resolution and sampling, and
wide spectral coverage. Further, we incorporated a turbulence and guiding simulator to realistically reproduce the
situation at the telescope, enabling end-to-end tests of important parameters.
The detection of Earth analogs with radial velocity requires long-term precision of 10 cm/s. One of the factors
limiting precision is variation in instrumental profile from observation to observation due to changes in the
illumination of the slit and spectrograph optics. Fiber optics are naturally efficient scramblers. Our research is
focused on understanding the scrambling properties of fibers with different geometries. We have characterized
circular and octagonal fibers in terms of focal ratio degradation, near-field and far-field distributions. We have
characterized these fibers using a bench-mounted high-resolution spectrograph: the Yale Doppler Diagnostics
Exoplanets can be detected from a time series of stellar spectra by looking for small, periodic shifts in the absorption
features that are consistent with Doppler shifts caused by the presence of an exoplanet, or multiple exoplanets, in the
system. While hundreds of large exoplanets have already been discovered with the Doppler technique (also called radial
velocity), our goal is to improve the measurement precision so that many Earth-like planets can be detected. The smaller
mass and longer period of true Earth analogues require the ability to detect a reflex velocity of ~10 cm/s over long time
periods. Currently, typical astronomical spectrographs calibrate using either Iodine absorptive cells or Thorium Argon
lamps and achieve ~10 m/s precision, with the most stable spectrographs pushing down to ~2 m/s. High velocity
precision is currently achieved at HARPS by controlling the thermal and pressure environment of the spectrograph.
These environmental controls increase the cost of the spectrograph, and it is not feasible to simply retrofit existing
spectrometers. We propose a fiber-fed high precision spectrograph design that combines the existing ~5000-6000 A
Iodine calibration system with a high-precision Laser Frequency Comb (LFC) system from ~6000-7000 A that just
meets the redward side of the Iodine lines. The scientific motivation for such a system includes: a 1000 A span in the red
is currently achievable with LFC systems, combining the two calibration methods increases the wavelength range by a
factor of two, and moving redward decreases the "noise" from starspots. The proposed LFC system design employs a
fiber laser, tunable serial Fabry-Perot cavity filters to match the resolution of the LFC system to that of standard
astronomical spectrographs, and terminal ultrasonic vibration of the multimode fiber for a stable point spread function.
Small telescopes coupled to high resolution spectrometers are powerful tools for Doppler planet searches. They allow for
high cadence observations and flexible scheduling; yet there are few such facilities. We present an innovative and
inexpensive design for CHIRON, a high resolution (R~80.000) Echelle spectrometer for the 1.5m telescope at CTIO.
Performance and throughput are very good, over the whole spectral range from 410 to 870nm, with a peak efficiency of
15% in the iodine absorption region. The spectrograph will be fibre-fed, and use an iodine cell for wavelength
calibration. An image slicer permits a moderate beam size. We use commercially available, high performance optical
components, which is key for quick and efficient implementation. We discuss the optical design, opto-mechanical
tolerances and resulting image quality.
The detection of Earth analogues requires extreme Doppler precision and long term stability in order to measure tiny reflex velocities in the host star. The PSF from the spectrometer should be slowly varying with temperature and pressure changes. However, variations in the illumination of the slit and of the spectrograph optics occur on time scales of seconds, primarily because of guiding errors, but also on timescales of minutes, because of changes in the focus or seeing. These variations yield differences in the PSF from observation to observation, which are currently limiting the Doppler precision. Here, we present the design of a low cost fiber optic feed, FINDS, used to stabilize the PSF of the Hamilton spectrograph of Lick observatory along with the first measurements that show dramatic improvement in stability.
Wide angle astrometry with the Space Interferometry Mission needs a set of several thousand grid stars distributed uniformly over the sky. The requirements for candidate grid stars are quite stringent: the photocenters of these stars have to be astrometrically stable to within a few microarcseconds, which makes binary stars unacceptable as grid stars. We search the most precise and comprehensive astrometric catalogs available today--the Hipparcos, Tycho-1 and Tycho-2 Catalogues--for possible grid stars, and discuss the properties of samples of K giants derived from these catalogs. Furthermore, we present results of a precise radial velocity study of a small proxy sample of Hipparcos K giants. We demonstrate that it is possible to find K giants with radial velocity variations smaller than a few tens of meters per second on timescales of several months. It is thus possible to detect stellar companions in samples of candidate grid stars by means of a radial velocity survey. We discuss the results of Monte Carlo simulations that address the consequences of the measurement accuracy of a radial velocity survey on the portion of undetected binary systems.