The Yale Exoplanet Lab is building a high resolution EXtreme PREcision Spectrometer (EXPRES) for the 4.3-m Lowell Observatory Discovery Channel Telescope (DCT) through funding by the NSF MRI program under co-PI's D. Fischer and C. Jurgenson. The spectrograph design is nearly complete and many components are currently being fabricated.
This will be the third high-resolution spectrometer built by the Yale team. However, this time when we say high-resolution, we're not messing around; the spectrometer will have a spectral resolution of R = 150,000. Finding analogs of our Earth is so hard that it hasn't been done before. A small planet like Earth only induces a 10 cm/s reflex velocity in its host star. We need to detect that 10 cm/s velocity variation if we are to find Earth analogs around other stars; however, the biggest challenge we face is that the surfaces of stars are boiling cauldrons of gas with velocities of hundreds of meters per second. If we didn't know better, we might guess that it would be impossible to measure Doppler velocities of 1 m/s in the presence of this stellar noise source -- yet 1 m/s precision is the current state of the art.
- It is a fact that the velocities from the surface of the star are imprinted in the spectrum differently than the Doppler shifts (which arise from velocity of the entire star).
- It is a fact that current spectrometers cannot distinguish between the surface velocities and the Doppler velocity because they lack sufficient resolution.
EXPRES is an experiment to try to separate out the stellar photospheric noise from the Doppler shifts. We are trying to resolve the "noise" from the surface of the star so that we can distinguish it from the dynamical velocity.
There are two ways that EXPRES will be different from other planet-detecting spectrometers. First, it will have extraordinary resolution and wavelength coverage. Second, we will use a novel frequency comb with an inherent precision in the wavelength solution of 1 cm/s. We think that these technical advances will dramatically improve our measurement precision and we will use this instrument for the 100 Earths and 100 Suns Project.
The major assemblies that will make up EXPRES are shown in Figure 1. The instrument has two major subsystems; the Front-End Module (FEM), mounted on a telescope instrument port (see Figure 2), and the Back-End Module (BEM), located in an environmentally stable room on the ground floor of the observatory. The BEM includes three major assemblies; the vacuum enclosed spectrograph, the double scrambler/pupil slicer, and the calibration unit. Fiber optics couple light between the various components of EXPRES. These major systems are described in the following sections.
Figure 1: The architecture of EXPRES showing the major components, locations, and fiber interconnections.
Figure 2: The Cassegrain instrument cube on the Discovery Channel Telescope. The instrument cube at the center of the image provides ports for five instruments. The EXPRES front-end module will mount on the port facing up in the image.
The function of the FEM is to direct starlight from the telescope into a fiber that feeds the spectrograph. The FEM corrects for atmospheric dispersion, measures and corrects for tip-tilt image motion, keeps the object centered on the fiber, and injects reference sources for data calibration. Figure 3 shows the optical design of the FEM. The optical design and coatings are optimized for the wavelength range of 380 to 680 nm. The blueward wavelength range is selected to include the Ca II K and H lines (393.37 and 396.85nm), which are strong indicators of stellar chromospheric activity.
Figure 3: The optical layout of the EXPRES front-end module.
The FEM transmissive optical elements are currently being fabricated by Nikon. There are 14 total elements using six different materials. Nikon will measure the material index of refraction of the actual lenses prior to polishing to allow the curvatures to be optimized. Nikon will then apply specialized AR coatings that are matched to the material index of refraction to optimize performance over our instrument bandpass.
The FEM mounts at one of the small instrument ports of the DCT Cassegrain instrument cube as shown in Figure 4. The mechanical design incorporates the FEM optics, ADC rotation mechanisms, fast tip-tilt mirror, calibration injection mechanism and FTT EMCCD camera into a compact and stiff housing. A view of the FEM components with the housing hidden is shown in Figure 5.
Figure 4: The EXPRES front-end module mounted on the DCT Cassegrain instrument cube. The FEM electronics are mounted within the instrument port cavity as seen in the upper left corner.
Figure 5: The components of the EXPRES FEM located inside the opto-mechanical housing assembly.
The architecture for the FEM control system is shown in Figure 6. An electronics enclosure, mounted at the instrument port as shown in Figure 4, contains a National Instruments CompactRIO device controller that interfaces to the fast tip-tilt mirror, ADC motion stages, calibration injection mechanism, environmental cover, and thermocouple sensors. A Windows-based control computer is located in the DCT facility computer room and communicates with the CompactRIO controller over TCP/IP Ethernet. An Andor EMCCD camera acquires images of the spillover light on the fiber face at a rate of 600 Hz and sends them to the control computer using USB extended over Ethernet fiber. The control computer interfaces to the DCT control system to share telemetry information and initiate telescope functions, such as re-pointing to new targets, and commencing focus sequences using the telescope secondary mirror. Figure 7 shows the completed FEM electronics enclosure undergoing bench testing.
Figure 6: The control architecture for the FEM functions. A Windows-based computer is located in the Observatory computer room and communicates with the National Instruments CRIO controller using TCP/IP and acquires FTT camera images over extended USB.
Figure 7: The FEM control electronics being bench tested.
In many spectrographs, starlight from the telescope is coupled directly to the instrument using a slit. The slit illumination can vary rapidly due to changes in seeing, focus, and guiding errors. Since spectral lines are direct images of the slit, a varying illumination will change the shape of those lines on the detector. In order to measure spectral shifts that correspond to one ten-thousandth of a pixel on the detector, coupling between the telescope and spectrograph is extremely important in our ability to characterize the PSF during data analysis.
Light is coupled between the various components of EXPRES using fibers to stabilize the illumination and to allow the spectrograph itself to be located off the telescope in gravity invariant and environmentally stabilized environment. The fiber architecture for EXPRES is shown in Figure 8.
Figure 8: EXPRES fiber architecture. Science light is coupled to the spectrograph using an octagonal fiber that is sliced at a double scrambler and injected into the spectrograph as a rectangular fiber. A splitter in the fore-optics of the spectrograph allows 2% of the light to be directed to an exposure meter, as well as an oversized flat-field fiber to inject light for an extended flat. The calibration unit sources are sent to the FEM to be coupled through the science fiber, or injected directly for simultaneous laser frequency comb calibrations or extended flats.
Science light is coupled to the spectrograph using an octagonal fiber, which provides spatial scrambling due to asymmetries it creates in the light propagation through the fiber. Prior to entering the spectrograph, the science light is coupled through a very efficient “double scrambler”, which inverts the near and far fields of the fiber to achieve high spatial scrambling gain, and slices the image to increase the spectral resolution. The far field image of the double scrambler is injected into a rectangular fiber that connects to the vacuum stabilized spectrograph.
The “double scrambler” slicer design, developed by Fraunhaufer IOF in Germany, is shown in Figure 9. It is a reflective, modified Bowen-Walraven design that is very compact and stable with no post fabrication alignment necessary. The slicer mirrors are diamond machined to an accuracy of < 2 μm, and then coated with a high reflective silver coating. The lenses will have their mounts post machined via an alignment turning process to align the system optical axis to < 2 μm. The spectrograph shutter is located in the dead space of this device, before the slicer, as shown in Figure 10.
Figure 9: The Fraunhaufer double scrambler and pupil slicer design with an octagonal fiber input and a rectangular fiber output (bottom). The design uses a Bowen-Walraven two slice reflective mirror setup which tilts mirror B in two axes relative to mirror A (top right) to slice the image onto a rectangular fiber as illustrated (top left).
Figure 10: Opto-mechanical design of the Fraunhaufer double scrambler/pupil slicer. A solenoid shutter, for starting and ending exposures, will be located in the gap after the pupil imaging lenses.
A fiber is used to couple light from the spectrograph to an “exposure meter” for monitoring the exposure signal. A splitter in the fore-optics of the spectrograph allows 2% of the light to be directed to an exposure meter. Corrections for the velocity of the Earth must be applied in order to recover precise radial velocity measurements of stars for exoplanet detection. This is accomplished by monitoring the flux of starlight into the spectrograph at regular 1-10 second intervals over the exposure duration. To correct for the acceleration term in the barycentric correction, the barycentric velocity must be calculated for each sampled flux measurement and then weighted by the photon counts. An error of 1 second in time introduces a semi-amplitude error of about 3 cm/s; to reach an error of less than 1 cm/s, the midpoint time must be accurate to about 250 milli-seconds. It is also important to sample photon counts as a function of wavelength, which will be an add-on for EXPRES.
The EXPRES fiber architecture also includes fibers for coupling calibration unit sources into the spectrograph. The calibration sources are injected in the FEM so that they are coupled through the science fiber to precisely reproduce the science light path. In some cases, though, the calibration sources are coupled directly into the spectrograph; 1) a fiber from the laser frequency comb can be injected through a fiber that is adjacent to the science fiber to allow a simultaneous calibration spectrum to be obtained to track instrument stability during exposure, or 2) a larger diameter fiber is injected at the fore-optics splitter to produce an “extended flat” that extends beyond the edges of the science fiber in the cross-dispersion direction for optimal pre-extraction, 2-dimensional flat fielding, which is crucial for reaching photon-limited precision.
Vacuum Enclosed Spectrograph
Light exits the second fiber from the double-scrambler and immediately enters the spectrograph vacuum chamber. The optical layout in Figure 11 uses the evenly illuminated rectangular fiber core, reimaged at f/8, as the input “slit image”. Light from the re-imaged fiber core diverges to the main collimator before being dispersed by a Newport R4 mosaic echelle grating (see Figure 12). The echelle is slightly rotated so that upon second pass to the main collimator, its image is displaced slightly from that of the slit image. A Mangin transfer mirror serves as the field flattener in the white pupil design and directs the beam to the transfer collimator. The camera is comprised of six spherical elements with the last lens serving as the cryostat window.
Figure 11: The white pupil optical design for EXPRES. In addition to optimizing for sampling, the camera is also optimized to be insensitive to pupil instabilities. This provides an additional level of scrambling, and requires a specialized merit function during the optimization process.
The effective focal length of the camera is optimized to maximize the spectral sampling, and the cross-dispersion is set such that there is enough order spacing to accommodate the use of sky fibers. The camera is also optimized to be insensitive to pupil instabilities. This provides an additional level of scrambling, and requires a specialized merit function during the optimization process. The requirement of 0.01 angstroms per pixel is placed on the longest wavelength (680 nm), resulting in greater sampling at the shorter wavelengths. For a typical linewidth of 0.2 angstroms, this provides a minimum of 20 pixels samples per line for analyzing stellar activity. All the optics in this design will be in vacuum at a pressure of 0.01mbar to eliminate convection effects, and held at a nightly temperature stable to +/-0.001K to eliminate thermal expansion effects.
Figure 12: The Newport R4 Echelle grating for EXPRES. This is a mosaic grating with dimensions of 84 cm long x 21 cm wide x 13 cm thick. The gap visible between the mosaic grating surfaces is 15 mm.
Figure 13 is a ray trace through the spectrograph showing a subset of dispersed orders on the detector. Five wavelengths in all 72 orders between 380 and 680 nm were modeled during the optical optimization and a subset of these orders is used to illustrate the dispersion pattern. All 72 orders fit on a single STA 10Kx10K CCD with 9 μm pixels.
Figure 13: Optical ray trace of the dispersion patterns. Five wavelengths within a subset of 72 orders are shown. The central wavelength of subset of orders is listed. The camera focal length is optimized for 4-pixel sampling with an average of about 0.01 Angstroms per pixel.
The optical materials and anti-reflection coatings have been selected to optimize the instrument throughput over the full 380-680 nm bandpass. The expected throughput for the spectrograph itself and the total system, including the FEM and fibers, is shown in Figure 14. The total system efficiency ranges from 8.5% at 380 nm to a peak of around 22%.
Figure 14: Throughput estimations for EXPRES. The red line is the expected throughput of the spectrograph; the blue line is the expected throughput of the instrument, including the FEM and fibers.
The vacuum enclosure, depicted in Figure 15, provides a stabilized thermal, pressure and vibration environment for the spectrograph using a novel design that minimizes the footprint of the spectrograph and provides convenient access for maintenance. The vacuum enclosure is located in an environmentally controlled room on the ground level of the DCT facility. A concrete isolation slab provides a stable foundation for the spectrograph to minimize the transmission of mechanical vibrations into the spectrograph from unpredictable noise sources, such as wind or dome rotation.
Figure 15: The EXPRES spectrograph structure. The outer vacuum enclosure maintains the spectrograph in a vacuum at better than 0.01 mBar, and using radiation shields and insulation maintains the temperature to better than 1 mK. The optics support structure visible inside the chamber maintains precise alignment of the optics and is mounted on a suspension to isolate it from mechanical vibrations. Removable panels allow easy access to the spectrograph for maintenance.
The calibration unit provides a selection of light sources which can be coupled to the spectrograph in various ways to allow thorough correction of instrument errors and effective removal of instrument signatures, which are needed to achieve photon-limited precision. The architecture of the calibration unit is illustrated in Figure 16. There are four sources of calibration light including; a laser frequency comb for broadband wavelength calibration, a ThAr lamp for a secondary wavelength reference, an LED source tuned to produce a flat “white” spectrum for flat-field calibration, and a solar telescope for Telluric line calibration. Any of the four different light sources can be injected into the science fiber in the FEM. A flat mirror on a rotation stage is used to direct the light from each of the sources to a calibration fiber that feeds up to the FEM where it can be injected into the science light path.
Figure 16: The EXPRES calibration unit architecture. The calibration unit provides a selection of light sources that are coupled to the spectrograph in various ways to allow thorough correction of instrument errors and effective removal of instrument signatures, which are needed to achieve photon-limited precision.
The Menlo Systems laser frequency comb (LFC) will have a free spectral range of 14 GHz, which will produce a comb line every 10 pixels from 450-680 nm. To obtain LFC spectra we will have the option of interleaving calibration observations through the science fiber to match the science light path, or using a second calibration fiber to obtain simultaneous lines adjacent to the science orders for tracking wavelengths during exposures. An integrating sphere is implemented to couple the single mode LFC fiber to a multi-mode injection fiber and scramble the light. In addition, mechanical agitation of the injection fibers will be implemented to scramble modal noise (see Figure 8 for location of fiber agitation). A shutter is incorporated in the LFC light path so that the LFC can be left on for stability while interleaving calibration exposures with science exposures without contamination of the science exposures. A neutral density filter wheel is provided to allow the intensity of the simultaneous calibration spectra to match the science target signal level.
Observing Support Software
An instrument is only as good as its data reduction pipeline and our team brings significant experience in extraction techniques, Doppler analysis pipelines, methods for rejecting stellar noise and removing telluric contamination, and a legacy of reaching a nightly precision of 0.5 m/s with the CHIRON spectrograph at CTIO. EXPRES will inherit web interfaces from our past projects that allow observers to schedule observations with required calibrations, and retrieve their wavelength calibrated extracted spectra and raw data files.