Modern precise radial velocity spectrometers are designed to infer the existence of planets orbiting other stars by measuring few-nm shifts in the positions of stellar spectral lines recorded at high spectral resolution on a large-area digital detector. While the spectrometer may be highly stabilized in terms of temperature, the detector itself may undergo changes in temperature during readout that are an order of magnitude or more larger than the other optomechanical components within the instrument. These variations in detector temperature can translate directly into systematic measurement errors. We explore a technique for reducing the amplitude of CCD temperature variations by shuffling charge within a pixel in the parallel direction during integration. We find that this “dither clocking” mode greatly reduces temperature variations in the CCDs being tested for the NEID spectrometer. We investigate several potential negative effects this clocking scheme could have on the underlying spectral data.
The SwRI Detector Characterization Lab (SDCL) was established in order to facilitate the rapid calibration of large numbers of detector arrays for upcoming ground and space missions. The SDCL is equipped with a McPherson monochromator with exchangeable gratings and light sources enabling wavelength coverage from 0.3 to 5.0 micron at sub nanometer resolution. The SDCL also has cryostats capable of maintaining thermal control of detector subassemblies and transfer optics to a precision of 0.1K at 77K and 0.01K at 4K. Using this calibration system, we have calibrated the EEM and ETU detector for read noise, dark current, modulation transfer function, quantum efficiency, cross talk, and total system throughput. The data were collected using standard Photon Transfer Curve techniques at the various wavelengths corresponding to the MVIC filter bandpasses. Here, we will present the data for the engineering unit, the methodology used to perform the calibration, and the steps forward for calibration of the flight unit.
Las Cumbres Observatory Global Telescope Network (LCOGT) has built the Network of Robotic Echelle Spectrographs (NRES), consisting of four identical, high-resolution optical spectrographs, each fiber-fed simultaneously by up to two 1-meter telescopes and a calibration source. Two units have been installed and are currently executing scientific observations. A third unit has been installed and is presently in commissioning. A fourth unit has been shipped to site and will be installed in mid 2018. Operating on four separate continents in both the Northern and Southern hemispheres, these instruments comprise a globally-distributed, autonomous spectrograph facility for stellar classification and high-precision radial velocity of bright stars. Simulations suggest we will achieve long-term radial velocity precision of 3 m/s in less than an hour for stars with V < 12. Radial velocity precision of 75 m/s has already been demonstrated with our automatic data-processing pipeline across multiple sites. Work is ongoing to improve several NRES system components including telescope control (robotic source acquisition in particular) and the data-processing pipeline. In this document we briefly overview the NRES design, its purpose and goals, results achieved to date in the field, and the ongoing development effort to improve instrument performance.
Las Cumbres Observatory Global Network (LCOGT) is building the Network of Robotic Echelle Spectrographs (NRES), which will consist of six identical, optical (390 - 860 nm) high-precision spectrographs, each fiber-fed simultaneously by up to two 1-meter telescopes and a thorium argon calibration source. We plan to install one at up to 6 observatory sites in the Northern and Southern hemispheres, creating a single, globally-distributed, autonomous spectrograph facility using up to twelve 1-meter telescopes. Simulations suggest we will achieve long-term radial velocity precision of 3 m/s in less than an hour for stars brighter than V = 12. We have been funded with NSF MRI and ATI grants, and expect our first spectrograph to be deployed in fall 2016, with the full network operation of 5 or 6 units beginning in 2017. We will briefly overview the NRES design, goals, robotic operation, and status. In addition, we will discuss early results from our prototype spectrograph, the laboratory and on-sky performance of our first production unit, and the ongoing software development effort to bring this resource online.
Las Cumbres Observatory Global Network (LCOGT) is building the Network of Robotic Echelle Spectrographs (NRES), which will consist of six identical, optical (390 - 860 nm) high-precision spectrographs, each fiber-fed simultaneously by two 1 meter telescopes and a thorium argon calibration source, one at each of our observatory sites in the Northern and Southern hemispheres. Thus, NRES will be a single, globally-distributed, autonomous observing facility using twelve 1-m telescopes. Simulations suggest we will achieve long-term precision of better than 3 m/s in less than an hour for stars brighter than V = 12. We have been fully funded with an NSF MRI grant, and expect our first spectrograph to be deployed in Spring of 2015, with the full network operation of all 6 units beginning in Spring of 2016. We discuss the NRES design, goals, and robotic operation, as well as the early results from our prototype spectrograph.
LCOGT are currently building and deploying a worldwide network of at least fifteen 1-meter
and twenty-four 0.4-meter telescopes to three sites in each hemisphere, enabling
extended, redundant and optimally continuous coverage of time variable or transient
sources. Each site will support two or more 1m telescopes and four or more 0.4m
All telescope classes provide a full range of optical narrow-band and broad-band UBVRI
and ugriZY imaging filters. All telescopes are being equipped with a moving light-bar flatfielding
system called Lambert.
The 1m network is intended primarily for science observing while the 0.4m network
additionally provides educational opportunities to participating schools and institutes. The
global network is designed to accommodate multiple science, educational and rapid
For LCOGT, the network IS the telescope.
Scientific performance specifications, a necessity for ease of commissioning and minimal maintenance, and a desire for
automated sensing and remote collimation have led to novel designs and features in LCOGT's one-meter Optical Tube
Assembly (OTA). We discuss the design and performance of the quasi-RC optical system with 18 point whiffletree and
radial hub mount. Position probes and IR temperature sensors on the primary and secondary mirrors give feedback for
active collimation and thermal control. A carbon fiber/epoxy composite truss, with unique spherical node connections,
mounts to parallel and offset Invar vanes. A flexure based, closed loop, 3-DOF secondary mirror mechanism is used for
tip/tilt collimation. The optics and deflections of the OTA components were iteratively designed for passive collimation
with a changing gravity vector. We present the FEA predictions, measured deflections, and measured hysteresis for
many of the components. Vibration modes, amplitudes, and damping of the system are presented with an FFT frequency
analysis. Thermal CTE effects on loading and focal position are quantified. All of these system effects are then related to
the overall scientific performance of the 1.0 m telescope.
The Las Cumbres Observatory Global Telescope Network (LCOGT) is an ambitious project to build and operate,
within 5 years, a worldwide robotic network of 50 0.4, 1, and 2 m telescopes sharing identical instrumentation and
optimized for precision photometry of time-varying sources. The telescopes, instrumentation, and software are all
developed in house with two 2 m telescopes already installed. The LCOGT Imaging Lab is responsible for assembly
and characterization of the network's cameras and instrumentation. In addition to a fully equipped CNC machine
shop, two electronics labs, and a future optics lab, the Imaging Lab is designed from the ground up to be a superb
environment for bare detectors, precision filters, and assembled instruments. At the heart of the lab is an ISO class 5
cleanroom with full ionization. Surrounding this, the class 7 main lab houses equipment for detector
characterization including QE and CTE, and equipment for measuring transmission and reflection of optics.
Although the first science cameras installed, two TEC cooled e2v 42-40 deep depletion based units and two
CryoTiger cooled Fairchild Imaging CCD486-BI based units, are from outside manufacturers, their 18 position filter
wheels and the remainder of the network's science cameras, controllers, and instrumentation will be built in house.
Currently being designed, the first generation LCOGT cameras for the network's 1 m telescopes use existing
CCD486-BI devices and an in-house controller. Additionally, the controller uses digital signal processing to
optimize readout noise vs. speed, and all instrumentation uses embedded microprocessors for communication over
We present the design and performance of the prototype Visible Integral-field Replicable Unit Spectrograph
(VIRUS-P) camera. Commissioned in 2007, VIRUS-P is the prototype for 150+ identical fiber-fed integral field
spectrographs for the Hobby-Eberly Telescope Dark Energy Experiment. With minimal complexity, the gimbal
mounted, double-Schmidt design achieves high on-sky throughput, image quality, contrast, and stability with novel
optics, coatings, baffling, and minimization of obscuration. The system corrector working for both the collimator
and f / 1.33 vacuum Schmidt camera serves as the cryostat window while a 49 mm square aspheric field flattener sets
the central obscuration. The mount, electronics, and cooling of the 2k × 2k, Fairchild Imaging CCD3041-BI fit in
the field-flattener footprint. Ultra-black knife edge baffles at the corrector, spider, and adjustable mirror, and a
detector mask, match the optical footprints at each location and help maximize the 94% contrast between 245
spectra. An optimally stiff and light symmetric four vane stainless steel spider supports the CCD which is thermally
isolated with an equally stiff Ultem-1000 structure. The detector/field flattener spacing is maintained to 1 μm for all
camera orientations and repeatably reassembled to 12 μm. Invar rods in tension hold the camera focus to ±4 μm
over a -5-25 °C temperature range. Delivering a read noise of 4.2 e- RMS, sCTE of 1-10-5 , and pCTE of
100 kpix/s, the McDonald V2 controller also helps to achieve a 38 hr hold time with 3 L of LN2 while maintaining
the detector temperature setpoint to 150 μK (5σ RMS).
We describe the design, construction, and performance of VIRUS-P (Visible Integral-field Replicable Unit
Spectrograph - Prototype), the prototype for 150+ identical fiber-fed integral field spectrographs for the Hobby-Eberly
Telescope Dark Energy Experiment (HETDEX). VIRUS-P was commissioned in 2007, is in regular service on the
McDonald Observatory 2.7 m Smith telescope, and offers the largest field of any integral field spectrograph. The 246-fiber IFU uses a densepak-type fiber bundle with a 1/3 fill factor. It is fed at f/3.65 through a telecentric, two-group
dioptric focal reducer. The spectrograph's double-Schmidt optical design uses a volume phase holographic grating at
the pupil between the articulating f/3.32 folded collimator and the f/1.33 cryogenic prime focus camera. High on-sky
throughput is achieved with this catadioptric system by the use of high reflectivity dielectric coatings, which set the
340-670 nm bandwidth. VIRUS-P is gimbal-mounted on the telescope to allow short fibers for high UV throughput,
while maintaining high mechanical stability. The instrument software and the 18 square arcmin field, fixed-offset guider
provide rapid acquisition, guiding, and precision dithering to fill in the IFU field. Custom software yields Poisson noise
limited, sky subtracted spectra. The design characteristics are described that achieved uniformly high image quality with
low scattered light and fiber-to-fiber cross talk. System throughput exceeds requirements and peaks at 40%. The
observing procedures are described, and example observations are given.
Traditional dome flat fielding methods typically have difficulties providing spatially uniform illumination and adequate
flux over a telescopic instrument's entire spectral range. Traditional flat fielding screens, with an illumination source at
least the size of the primary, can be difficult or impractical to mount and uniformly illuminate. The Las Cumbres
Observatory Global Telescope Network (LCOGTN) will consist of approximately 50 robotic telescopes of 0.4 m, 1.0 m,
and 2.0 m apertures with instrument bandwidth ranging from 350 - 1800 nm. The network requires a robust flat-field
solution to fit in compact enclosures.
A scanning illuminated flat fielding bar, Lambert, was developed to meet these requirements. Illumination is from a
linear arrangement of sources that are spatially dispersed by a narrow holographic or glass diffuser equal in length to the
primary's diameter. We have investigated a linearly scanning, enclosure mounted, deployable unit, and a rotary scanning,
telescope mounted unit. For complete visible-light bandwidth, a set of different color LEDs is used. The source density,
scan speed, and variable intensity tunes the flux to the instrument wavelength and bandwidth. The Lambert flat fields in
comparison to sky flats match pixel to pixel variations better than 0.5%; large scale illumination differences, which are
stable and repeatable, are ~1%.
We present the design of, and the science drivers for, the Visible Integral-field Replicable Unit Spectrograph (VIRUS). This instrument is made up of 145 individually small and simple spectrographs, each fed by a fiber integral field unit. The total VIRUS-145 instrument covers ~30 sq. arcminutes per observation, providing integral field spectroscopy from 340 to 570 nm, simultaneously, of 35,670 spatial elements, each 1 sq. arcsecond on the sky. This corresponds to 15 million resolution elements per exposure. VIRUS-145 will be mounted on the Hobby-Eberly Telescope and fed by a new wide-field corrector with 22 arcminutes diameter field of view. VIRUS represents a new approach to spectrograph design, offering the science multiplex advantage of huge sky coverage for an integral field spectrograph, coupled with the engineering multiplex advantage of >100 spectrographs making up a whole. VIRUS is designed for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) which will use baryonic acoustic oscillations imprinted on the large-scale distribution of Lyman-α emitting galaxies to provide unique constraints on the expansion history of the universe that can constrain the properties of dark energy.
LRS-J is a 142 mm f/1 near infrared (J-Band) camera designed as a drop in replacement for the optical camera installed on the Hobby-Eberly Telescope (HET) multi-object low resolution spectrograph (LRS). The Hawaii-1RG (H1RG) based instrument is liquid nitrogen cooled, but it mates to the warm LRS making use of the existing longslit and multi-object (MOS) units as well as the existing optical collimator. LRS-J utilizes a molecular beam epitaxy (MBE) based Hawaii-1RG (H1RG) detector. The design is a fully cryogenic catadioptric Maksutov-type, with the detector at the internal focus. This configuration produces excellent images, but presents particular challenges in the mounting of the detector. This is the first time that such an arrangement has been used in an astronomical instrument with an infrared detector. By replacing the conventional optical grisms with two 170 mm diameter near-IR VPH grisms, LRS-J covers the 0.9-1.3 μm bandpass with R ~ 1750-2000. We present the opto-mechanical design of LRS-J including the thermally self-compensating corrector doublet mount, and 100 μm/turn cryogenic mirror adjusters, FEA optimized vacuum housing, and custom Dewar. We also characterize the electrical and thermal connections necessary to mount the detector head in this unusually small inverted arrangement.
The Hobby-Eberly Telescope (HET) imposes unique constraints on the design of a spectral calibration system. Its 9.2 m aperture and queue scheduled operation make traditional dome screens impractical. Furthermore, the changing pupil of the HET's tilted Aricebo design is far more drastic than the simple rotation of traditional alt-azimuth telescopes. Given these constraints we elected to build an internal spectral calibration system (SCS) common to all instruments.
The SCS can feed all HET instruments from a uniformly illuminated Lambertian screen located within the spherical abberation corrector (SAC) at the telescope's second pupil. A moving baffle installed at the third pupil will reproduce, during calibration, the actual HET pupil seen in a science exposure. We eliminated all heat sources at the SAC by locating the lamps in the basement below the telescope and coupling source to screen through 12 600 μm diameter 35 m long fibers.
The first second-generation instrument for the Hobby-Eberly telescope is a novel J band camera (LRS-J) which mates to the existing low resolution spectrograph (LRS). This camera uses the existing LRS longslit and mutltiobject units as well as the LRS five element collimator but uses its own J optimized volume holographic grisms, f/1 cryogenically cooled camera, and readout electronics built around a Rockwell HAWAII-1 array.
We minimized the development time of the controller by reusing as much of the existing framework as possible. The modular design of the existing LRS CCD controller allows us to modify only the clock-driver and penthouse (pre-amplifier) modules. Furthermore, we were able to use existing multilayer circuit boards already fabricated for these two modules. Thus, the LRS-J controller required only the substitution of components on two modules and the design of a new header (dewar) board to fit the HAWAII-1 socket. With these modifications, based on its perfomance with CCDs, we predict a noise and crosstalk performance at the most competitive level.
The Texas-Oxford One Thousand (TOOT) radio source redshift survey is aimed at understanding the evolution of the radio source population down to flux density of S151MHz = 100 mJy. This low frequency survey has a depth equivalent to about five times the NVSS limit, but does not contain the population of star forming galaxies detected in the NVSS survey. In addition, the survey reaches a high enough surface density of sources on the sky to probe large-scale structure at z ~ 1. Radio sources inhabit massive elliptical galaxies, and as such provide excellent sparse tracers of large-scale structure that are both easily identified and easily observed with spectrographs to map out their space distribution. We review the properties of the TOOT survey and its current status.
We also report the discovery, using radio sources, of a huge structure at z=0.27 traced by radio sources and galaxy clusters. Such superstructures are aggregates of clusters seeded by rare (> 3σ) peaks in the power spectrum at recombination. The radio sources and galaxy clusters are shown to trace the same matter distribution. This is the first demonstration of radio AGN as direct sparse tracers of the underlying dark matter distribution. The extent of the superstructure is of order 100 h-1 Mpc in three dimensions, making it the largest structure known, and indicating a mass similar to that of the Great Attractor. We report the properties of the superstructure, and consider the implications of its existence.
Highly efficient Volume phase holographic (VPH) gratings do not lend themselves to use in existing spectrographs except for grism spectrographs where VPH grisms can be designed that disperse but do not deviate the light. We discuss our program to outfit existing spectrographs [the Imaging grism instrument (IGI) on the McDonald Observatory Smith Reflector, and the Hobby-Eberly Telescope Marcario Low Resolution Spectrograph (LRS)] with efficient VPH grisms. We present test data on sample gratings from Ralcon Development Lab, and compare them to theoretical predictions. We have created a simple test bench for efficiency measurements of VPH gratings, which we describe. Finally we present first results from the use of VPH grisms in IGI and the LRS, the latter being the largest grism ever deployed in an astronomical spectrograph. We also look forward to using VPH grisms in the LRS infrared extension, which covers the wavelength range from 0.9 to 1.3 microns.
This paper presents the design of a near IR camera for the 9.2 m Hobby-Eberly Telescope (HET) Low Resolution Spectrograph (LRS), which will cover the wavelength range 0.85 to 1.35 micrometers . The LRS-J, an upgrade to the existing LRS, replaces the optical camera with an f/1 camera optimized for the J-band. The instrument design is strongly motivated by the desire to observe galaxies at 1 < z < 2, where the principal strong spectral features used to measure redshifts are shifted into the J-band. Since we are primarily interested in wavelengths up to 1.35 micrometers , mating the cryogenically cooled camera to the warm LRS spectrograph does not result in enough thermal background emission to compromise its performance. LRS-J represents a rapid and cost-effective way to enable multi-object near-IR spectroscopy on a very large telescope.