Extremely precise radial velocity (EPRV) measurements are critical for characterizing nearby terrestrial worlds. EPRV instrument precisions of σRV = 1−10 cm/s are required to study Earth-analog systems, imposing stringent, sub-mK, thermo-mechanical stability requirements on Doppler spectrograph designs. iLocater is a new, high resolution (R = 190, 500 median) near infrared (NIR) EPRV spectrograph under construction for the dual 8.4 m diameter Large Binocular Telescope (LBT). The instrument is one of the first to operate in the diffraction-limited regime enabled by the use of adaptive optics and single-mode fibers. This facilitates affordable optomechanical fabrication of the spectrograph using intrinsically stable materials. We present the final design and performance of the iLocater cryostat and thermal control system which houses the instrument spectrograph. The spectrograph is situated inside an actively temperature-controlled radiation shield mounted inside a multi-layer-insulation (MLI) lined vacuum chamber. The radiation shield provides sub-mK thermal stability, building on the existing heritage of the Habitable-zone Planet Finder (HPF) and NEID instruments. The instrument operating temperature (T = 80−100 K) is driven by the requirement to minimize detector background and instantaneous coefficient of thermal expansion (CTE) of the materials used for spectrograph fabrication. This combination allows for a reduced thermomechanical impact on measurement precision, improving the scientific capabilities of the instrument.
Several external hardware upgrades have been developed for the APOGEE Spectrographs as part of the Sloan Digital Sky Survey-V (SDSS-V) to improve their radial velocity (RV) precision from a floor of 100-200 m/sec to approx. 30 m/sec. The upgrades include: (1) Back Pressure Regulator (BPR) systems to stabilize the internal instrument LN2 tank boil-off pressure, lessening induced movement of the APOGEE optical bench; (2) Fabry-Perot Interferometer (FPI) calibration sources to improve wavelength calibration; and (3), the use of octagonal core fiber segments in the fiber train to improve radial scrambling. We discuss the fabrication, commissioning, and early performance of these upgrades.
Two key areas of emphasis in contemporary experimental exoplanet science are the detailed characterization of transiting terrestrial planets and the search for Earth analog planets to be targeted by future imaging missions. Both of these pursuits are dependent on an order-of-magnitude improvement in the measurement of stellar radial velocities (RV), setting a requirement on single-measurement instrumental uncertainty of order 10 cm / s. Achieving such extraordinary precision on a high-resolution spectrometer requires thermomechanically stabilizing the instrument to unprecedented levels. We describe the environment control system (ECS) of the NEID spectrometer, which will be commissioned on the 3.5-m WIYN Telescope at Kitt Peak National Observatory in 2019, and has a performance specification of on-sky RV precision <50 cm / s. Because NEID’s optical table and mounts are made from aluminum, which has a high coefficient of thermal expansion, sub-milliKelvin temperature control is especially critical. NEID inherits its ECS from that of the Habitable-Zone Planet Finder (HPF), but with modifications for improved performance and operation near room temperature. Our full-system stability test shows the NEID system exceeds the already impressive performance of HPF, maintaining vacuum pressures below 10 − 6 Torr and a root mean square (RMS) temperature stability better than 0.4 mK over 30 days. Our ECS design is fully open-source; the design of our temperature-controlled vacuum chamber has already been made public, and here we release the electrical schematics for our custom temperature monitoring and control system.
KEYWORDS: Spectrographs, Control systems, Doppler effect, Precision optics, Velocimetry, Control systems design, Optical benches, Exoplanets, Temperature metrology, Electronics
We present preliminary results for the environmental control system from NEID, our instrument concept for NASA's Extreme Precision Doppler Spectrograph, which is now in development. Exquisite temperature control is a requirement for Doppler spectrographs, as small temperature shifts induce systematic Doppler shifts far exceeding the instrumental specifications. Our system is adapted from that of the Habitable Zone Planet Finder instrument, which operates at a temperature of 180K.We discuss system modifications for operation at T ~ 300K, and show data demonstrating sub-mK stability over two weeks from a full-scale system test.
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.
We present recent long-term stability test results of the cryogenic Environmental Control System (ECS) for the Habitable zone Planet Finder (HPF), a near infrared ultra-stable spectrograph operating at 180 Kelvin. Exquisite temperature and pressure stability is required for high precision radial velocity (< 1m=s) instruments, as temperature and pressure variations can easily induce instrumental drifts of several tens-to-hundreds of meters per second. Here we present the results from long-term stability tests performed at the 180K operating temperature of HPF, demonstrating that the HPF ECS is stable at the 0:6mK level over 15-days, and <10-7 Torr over months.
We describe the Instrument Control Software (ICS) package that we have built for The Habitable-Zone Planet Finder (HPF) spectrometer. The ICS controls and monitors instrument subsystems, facilitates communication with the Hobby-Eberly Telescope facility, and provides user interfaces for observers and telescope operators. The backend is built around the asynchronous network software stack provided by the Python Twisted engine, and is linked to a suite of custom hardware communication protocols. This backend is accessed through Python-based command-line and PyQt graphical frontends. In this paper we describe several of the customized subsystem communication protocols that provide access to and help maintain the hardware systems that comprise HPF, and show how asynchronous communication benefits the numerous hardware components. We also discuss our Detector Control Subsystem, built as a set of custom Python wrappers around a C-library that provides native Linux access to the SIDECAR ASIC and Hawaii-2RG detector system used by HPF. HPF will be one of the first astronomical instruments on sky to utilize this native Linux capability through the SIDECAR Acquisition Module (SAM) electronics. The ICS we have created is very flexible, and we are adapting it for NEID, NASA's Extreme Precision Doppler Spectrometer for the WIYN telescope; we will describe this adaptation, and describe the potential for use in other astronomical instruments.
We are developing a stable and precise spectrograph for the Large Binocular Telescope (LBT) named “iLocater.” The instrument comprises three principal components: a cross-dispersed echelle spectrograph that operates in the YJ-bands (0.97-1.30 μm), a fiber-injection acquisition camera system, and a wavelength calibration unit. iLocater will deliver high spectral resolution (R~150,000-240,000) measurements that permit novel studies of stellar and substellar objects in the solar neighborhood including extrasolar planets. Unlike previous planet-finding instruments, which are seeing-limited, iLocater operates at the diffraction limit and uses single mode fibers to eliminate the effects of modal noise entirely. By receiving starlight from two 8.4m diameter telescopes that each use “extreme” adaptive optics (AO), iLocater shows promise to overcome the limitations that prevent existing instruments from generating sub-meter-per-second radial velocity (RV) precision. Although optimized for the characterization of low-mass planets using the Doppler technique, iLocater will also advance areas of research that involve crowded fields, line-blanketing, and weak absorption lines.
Integral field spectrographs are an important technology for exoplanet imaging, due to their ability to take spectra in a high-contrast environment, and improve planet detection sensitivity through spectral differential imaging. ALES is the first integral field spectrograph capable of imaging exoplanets from 3-5 μm, and will extend our ability to characterize self-luminous exoplanets into a wavelength range where they peak in brightness. ALES is installed inside LBTI/LMIRcam on the Large Binocular Telescope, taking advantage of existing AO systems, camera optics, and a HAWAII-2RG detector. The new optics that comprise ALES are a Keplerian magnifier, a silicon lenslet array with diffraction suppressing pinholes, a direct vision prism, and calibration optics. All of these components are installed in filter wheels making ALES a completely modular design. ALES saw first light at the LBT in June 2015.
The Large Binocular Telescope (LBT) houses two 8.4-meter mirrors separated by 14.4 meters on a common mount. Coherent combination of these two AO-corrected apertures via the LBT Interferometer (LBTI) produces Fizeau interferometric images with a spatial resolution equivalent to that of a 22.8-meter telescope and the light- gathering power of single 11.8-meter mirror. Capitalizing on these unique capabilities, we used LBTI/LMIRcam to image thermal radiation from volcanic activity on the surface of Io at M-Band (4.8 μm) over a range of parallactic angles. At the distance of Io, the M-Band resolution of the interferometric baseline corresponds to a physical distance of ~135 km, enabling high-resolution monitoring of Io volcanism such as ares and outbursts inaccessible from other ground-based telescopes operating in this wavelength regime. Two deconvolution routines are used to recover the full spatial resolution of the combined images, resolving at least sixteen known volcanic hot spots. Coupling these observations with advanced image reconstruction algorithms demonstrates the versatility of Fizeau interferometry and realizes the LBT as the first in a series of extremely large telescopes.
The Habitable-Zone Planet Finder is a stabilized, fiber-fed, NIR spectrograph being built for the 10m Hobby- Eberly telescope (HET) that will be capable of discovering low mass planets around M dwarfs. The optical design of the HPF is a white pupil spectrograph layout in a vacuum cryostat cooled to 180 K. The spectrograph uses gold-coated mirrors, a mosaic echelle grating, and a single Teledyne Hawaii-2RG (H2RG) NIR detector with a 1.7-micron cutoff covering parts of the information rich z, Y and J NIR bands at a spectral resolution of R∼50,000. The unique design of the HET requires attention to both near and far-field fiber scrambling, which we accomplish with double scramblers and octagonal fibers. In this paper we discuss and summarize the main requirements and challenges of precision RV measurements in the NIR with HPF and how we are overcoming these issues with technology, hardware and algorithm developments to achieve high RV precision and address stellar activity.
HPF is an ultra-stable, precision radial velocity near infrared spectrograph with a unique environmental
control scheme. The spectrograph will operate at a mid-range temperature of 180K, approximately half
way between room temperature and liquid nitrogen temperature; it will be stable to sub -milli-Kelvin(mK)
levels over a calibration cycle and a few mK over months to years. HPF‟s sensor is a 1.7 micron H2RG
device by Teledyne. The environmental control boundary is a 9 m2 thermal enclosure that completely
surrounds the optical train and produces a near blackbody cavity for all components. A large, pressure -
stabilized liquid nitrogen tank provides the heat sink for the system via thermal straps while a multichannel
resistive heater control system provides the stabilizing heat source. High efficiency multi-layer
insulation blanketing provides the outermost boundary of the thermal enclosure to largely isolate the
environmental system from ambient conditions. The cryostat, a stainless steel shell derived from the
APOGEE design, surrounds the thermal enclosure and provides a stable, high quality vacuum environment.
The full instrument will be housed in a passive „meat -locker‟ enclosure to add a degree of additional
thermal stability and as well as protect the instrument. Effectiveness of this approach is being empirically
demonstrated via long duration scale model testing. The full scale cryostat and environmental control
system are being constructed for a 2016 delivery of the instrument to the Hobby-Eberly Telescope. This
report describes the configuration of the hardware and the scale-model test results as well as projections for
performance of the full system.
We present the scientific motivation and conceptual design for the recently funded Habitable-zone Planet Finder (HPF), a stabilized fiber-fed near-infrared (NIR) spectrograph for the 10 meter class Hobby-Eberly Telescope (HET) that will be capable of discovering low mass planets around M dwarfs. The HPF will cover the NIR Y and J bands to enable precise radial velocities to be obtained on mid M dwarfs, and enable the detection of low mass planets around these stars. The conceptual design is comprised of a cryostat cooled to 200K, a dual fiber-feed with a science and calibration fiber, a gold coated mosaic echelle grating, and a Teledyne Hawaii-2RG (H2RG) *NIR detector with a 1.7μm cutoff. A uranium-neon hollow-cathode lamp is the baseline wavelength calibration source, and we are actively testing laser frequency combs to enable even higher radial velocity precision. We will present the overall instrument system design and integration with the HET, and discuss major system challenges, key choices, and ongoing research and development projects to mitigate risk. We also discuss the ongoing process of target selection for the HPF survey.
John Wilson, F. Hearty, M. Skrutskie, S. Majewski, R. Schiavon, D. Eisenstein, J. Gunn, J. Holtzman, D. Nidever, B. Gillespie, D. Weinberg, B. Blank, C. Henderson, S. Smee, R. Barkhouser, A. Harding, S. Hope, G. Fitzgerald, T. Stolberg, J. Arns, M. Nelson, S. Brunner, A. Burton, E. Walker, C. Lam, P. Maseman, J. Barr, F. Leger, L. Carey, N. MacDonald, G. Ebelke, S. Beland, T. Horne, E. Young, G. Rieke, M. Rieke, T. O'Brien, J. Crane, M. Carr, C. Harrison, R. Stoll, M. Vernieri, M. Shetrone, C. Allende-Prieto, J. Johnson, P. Frinchaboy, G. Zasowski, A. Garcia Perez, D. Bizyaev, K. Cunha, V. Smith, Sz. Meszaros, B. Zhao, M. Hayden, S. D. Chojnowski, B. Andrews, C. Loomis, R. Owen, M. Klaene, J. Brinkmann, F. Stauffer, D. Long, W. Jordan, D. Holder, F. Cope, T. Naugle, B. Pfaffenberger, D. Schlegel, M. Blanton, D. Muna, B. Weaver, S. Snedden, K. Pan, H. Brewington, E. Malanushenko, V. Malanushenko, A. Simmons, D. Oravetz, S. Mahadevan, S. Halverson
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) uses a dedicated 300-fiber, narrow-band
near-infrared (1.51-1.7 μm), high resolution (R~22,500) spectrograph to survey approximately 100,000 giant stars across
the Milky Way. This three-year survey, in operation since late-summer 2011 as part of the Sloan Digital Sky Survey III
(SDSS III), will revolutionize our understanding of the kinematical and chemical enrichment histories of all Galactic
stellar populations. We present the performance of the instrument from its first year in operation. The instrument is
housed in a separate building adjacent to the 2.5-m SDSS telescope and fed light via approximately 45-meter fiber runs
from the telescope. The instrument design includes numerous innovations including a gang connector that allows
simultaneous connection of all fibers with a single plug to a telescope cartridge that positions the fibers on the sky,
numerous places in the fiber train in which focal ratio degradation had to be minimized, a large mosaic-VPH (290 mm x
475 mm elliptically-shaped recorded area), an f/1.4 six-element refractive camera featuring silicon and fused silica
elements with diameters as large as 393 mm, three near-infrared detectors mounted in a 1 x 3 mosaic with sub-pixel
translation capability, and all of these components housed within a custom, LN2-cooled, stainless steel vacuum cryostat
with dimensions 1.4-m x 2.3-m x 1.3-m.
The L/M-band (3−5 μm) InfraRed Camera (LMIRcam) sits at the combined focal plane of the Large Binocular
Telescope Interferometer (LBTI), ultimately imaging the coherently combined focus of the LBT’s two 8.4-meter
mirrors. LMIRcam achieved first light at the LBT in May 2011 using a single AO-enabled 8.4-meter aperture.
With the delivery of LBT’s final adaptive secondary mirror in Fall of 2011, dual-aperture AO-corrected interferometric
fringes were realized in April 2012. We report on the performance of these configurations and characterize
the noise performance of LMIRcam’s HAWAII-2RG 5.3-μm cutoff array paired with Cornell FORCAST readout
electronics. In addition, we describe recent science highlights and discuss future improvements to the LMIRcam
hardware.
We report the first phased images using adaptive optics correction from the Large Binocular Telescope Interferometer.
LBTI achieved first fringes in late 2010, with seeing-limited operation. Initial tests verified the feasibility of the setup
and allowed us to characterize the phase variations from both the atmosphere and mechanical vibrations. Integration of
the secondary-base AO systems was carried out in spring 2011 and spring 2012 for the right and left side respectively.
Single aperture, diffraction-limited, operation has been commissioned and is used as a productive mode of the LBTI with
the LMIRCam subsystem. We describe the initial observation for dual aperture observations and coherent imaging
results.
We report on the testing of a set of InAs/GaSb multicolor strained-layer superlattice photodetectors and Dotin-
Well detectors grown with InAs dots in InGaAs/GaAs wells fabricated by the Center for High Technology
Materials at the University of New Mexico. These devices are 2-color devices sensitive to near-IR and mid-IR
wavelengths. The wavelength sensitivities of these devices are a function of the applied forward and reverse bias.
We present measurements of the dark current and relative spectral response of these photodetectors measured
at both cryogenic and room temperatures.
We report on the final design and the fabrication status of LMIRcam - a mid-infrared imager/spectrograph that will
operate behind the Large Binocular Telescope Interferometer (LBTI) primarily at wavelengths between 3 and 5um (the
astronomical L- and M-bands). Within LMIRcam a pair of diamond-turned biconic mirrors re-images a ten arcsecond
square field onto a 1024x1024 HAWAII-1RG 5.1um cutoff array. The re-imaging optics provide two pupil planes for
the placement of filters and grisms as well as an intermediate image plane. Flexible readout electronics enable operating
modes ranging from high frame rate broadband imaging at the longest wavelengths to low background R=400
spectroscopy at shorter wavelengths. The LBTI will provide LMIRcam with a diffraction limited two-mirror PSF with
first null dictated by the 14.4 meter separation of the two LBT mirror centers (22.8 meter baseline from edge to edge).
We present the integration of a low dark current extended wavelength (2.3μm cutoff) InGaAs array into the
Cornell-Massachusetts Slit Spectrograph (CorMASS) spectrograph. The InGaAs array was fabricated onto a SB-
206 512×512 readout integrated circuit (ROIC) by Goodrich/Sensors Unlimited and subsequently went through a
series of laboratory characterization tests at the University of Virginia demonstrating dark current performance
of better than 10 e-/s. The InGaAs array is adapted for use with the CorMASS to verify its performance in a
proven astronomical instrument, and for eventual deployment to a telescope to test stability and performance.
KEYWORDS: Sensors, Electronics, Clocks, Infrared cameras, Telescopes, Data acquisition, Field programmable gate arrays, Space telescopes, Point spread functions, Human-machine interfaces
The L/M-band Infrared Camera (LMIRcam) is a first-generation imager being constructed for the Large Binocular
Telescope Interferometer, operating at 3-5 μm. Given the high sky background at these wavelengths, an
FPGA-based controller provides high-speed, flexible data acquisition. Originally designed for FORCAST, a mid-
IR camera/spectrograph built by Cornell University, the controller was modified to interface with LMIRcam's
Teledyne HAWAII-1RG 1024×1024 array. In order to facilitate the different operating modes and increased array
size, we have developed a modified version of the FORCAST device driver, reconfigured the FPGAs, altered the
control software, and plan to implement a window mode.
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) will use a dedicated 300-fiber, narrow-band
(1.5-1.7 micron), high resolution (R~30,000), near-infrared spectrograph to survey approximately 100,000 giant stars
across the Milky Way. This survey, conducted as part of the Sloan Digital Sky Survey III (SDSS III), will revolutionize
our understanding of kinematical and chemical enrichment histories of all Galactic stellar populations. The instrument,
currently in fabrication, will be housed in a separate building adjacent to the 2.5 m SDSS telescope and fed light via
approximately 45-meter fiber runs from the telescope. The instrument design includes numerous technological
challenges and innovations including a gang connector that allows simultaneous connection of all fibers with a single
plug to a telescope cartridge that positions the fibers on the sky, numerous places in the fiber train in which focal ratio
degradation must be minimized, a large (290 mm x 475 mm elliptically-shaped recorded area) mosaic-VPH, an f/1.4 sixelement
refractive camera featuring silicon and fused silica elements with diameters as large as 393 mm, three near-within a custom, LN2-cooled, stainless steel vacuum cryostat with dimensions 1.4 m x 2.3 m x 1.3 m.
The L/M-band mid-InfraRed Camera (LMIRcam) will use a mid-wave (5.1 μm cut-off) Teledyne Imaging Systems
HgCdTe HAWAII 1-RG array to image the coherently combined (Fizeau) focus of the Large Binocular Telescope's
twin 8.4-meter primary mirrors generated by the University of Arizona's beam combiner - the Large Binocular
Telescope Interferometer (LBTI). The 1024x1024 array will have a pixel scale of 10.9 milliarcsec (mas) per
pixel and a field of view of 10"x10". The highest achievable angular resolution will be 26mas (34mas) for
3.6 μm (4.8 μm). LMIRcam will operate in parallel with the Nulling Infrared Camera (NIC), sharing the same
Dewar. In addition to a suite of broad and narrow-band filters, LMIRcam will contain grisms for low-resolution
spectroscopy, and serve as a test-bed for novel pupil masks to enable high-contrast imaging. The opto-mechanical
design, anticipated performance, and a sample of potential science applications are presented. LMIRcam is funded
by the National Science Foundation and the University of Virginia.
Recent development of low dark current 2.34 μm-cutoff InGaAs material has resulted in the successful construction of a hybrid focal plane array built on the SB-206 512×512 format astronomical quality Read Out Integrated Circuit(ROIC). This contribution reports on the verification of the quality of the InGaAs material as well as the essential characteristics and performance of the hybrid focal plane array. The results of the investigation indicate that the dark current levels surpass the requirements for ground-based broadband and narrowband imaging as
well as for low resolution spectroscopy in the astronomical H and Ks bands.
We report the performance of Triplespec from commissioning observations on the 200-inch Hale Telescope
at Palomar Observatory. Triplespec is one of a set of three near-infrared, cross-dispersed spectrographs
covering wavelengths from 1 - 2.4 microns simultaneously at a resolution of ~2700. At Palomar, Triplespec
uses a 1×30 arcsecond slit. Triplespec will be used for a variety of scientific observations, including
moderate to high redshift galaxies, star formation, and low mass stars and brown dwarfs. When used in
conjunction with an externally dispersed interferometer, Triplespec will also detect and characterize
extrasolar planets.
We report the results of a program to mitigate defect induced (tunneling) dark current which arises from lattice
mismatch between In0.82Ga0.18As 'extended wavelength' detector material and the InP substrate upon which
it is grown. Our goal is to produce material suitable for ground-based broadband astronomical observation by
achieving a dark current level in individual 25x25μm array pixels which is less than the atmospheric airglow
and telescope thermal emission in the astronomical H (1.50-1.80 μm) and Ks (2.00-2.32 μm) bands. We have
cryogenically tested multiple growths of candidate materials, packaged as both individual diodes and focal plane
arrays, supplied by Sensors Unlimited, Inc. (SU). Results indicate dark current levels, in the current generation
of array materials, surpassing the requirements for broadband imaging, and with the potential to be used for
narrow band imaging and low-resolution spectroscopy.
We describe the optical and mechanical design of a simple hand-held near infrared spectrograph constructed
to produce observations of the spectrum of scrambled light from the Earth from aboard the International Space
Station. Observing the Earth in this manner simulates the changing perspective on an extra-solar terrestrial
planet observed as a point source by the Terrestrial Planet Finder. A Sensors Unlimited, Inc. SU320-M
InGaAs(0.86 - 1.72μm) camera detects the dispersed spectrum and outputs NTSC video to be recorded and
also permits frame grabbing. One of the three copies of the instrument is currently aboard the International
Space Station. The optical and mechanical design was conceived and executed by graduate and undergraduate
students at the University of Virginia.
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