GRAVITY is a second generation near-infrared VLTI instrument that will combine the light of the four unit or four auxiliary telescopes of the ESO Paranal observatory in Chile. The major science goals are the observation of objects in close orbit around, or spiraling into the black hole in the Galactic center with unrivaled sensitivity and angular resolution as well as studies of young stellar objects and evolved stars. In order to cancel out the effect of atmospheric turbulence and to be able to see beyond dusty layers, it needs infrared wave-front sensors when operating with the unit telescopes. Therefore GRAVITY consists of the Beam Combiner Instrument (BCI) located in the VLTI laboratory and a wave-front sensor in each unit telescope Coudé room, thus aptly named Coudé Infrared Adaptive Optics (CIAO). This paper describes the CIAO design, assembly, integration and verification at the Paranal observatory.
GRAVITY is a near-infrared interferometric instrument that allows astronomers to combine the light of the four unit or four auxiliary telescopes of the ESO Very Large Telescope in Paranal, Chile. GRAVITY will deliver extremely precise relative astrometry and spatially resolved spectra. In order to study objects in regions of high extinction (e.g. the Galactic Center, or star forming regions), GRAVITY will use infrared wavefront sensors. The suite of four wavefront sensors located in the Coudé room of each of the unit telescopes are known as the Coudé Integrated Adaptive Optics (CIAO). The CIAO wavefront sensors are being constructed by the Max Planck Institute for Astronomy (MPIA) and are being installed and commissioned at Paranal between February and September of 2016. This presentation will focus on system tests performed in the MPIA adaptive optics laboratory in Heidelberg, Germany in preparation for shipment to Paranal, as well as on-sky data from the commissioning of the first instrument. We will discuss the CIAO instruments, control strategy, optimizations, and performance at the telescope.
GRAVITY, a second generation instrument for the Very Large Telescope Interferometer (VLTI), will provide an astrometric precision of order 10 micro-arcseconds, an imaging resolution of 4 milli-arcseconds, and low/medium resolution spectro-interferometry. These improvements to the VLTI represent a major upgrade to its current infrared interferometric capabilities, allowing detailed study of obscured environments (e.g. the Galactic Center, young dusty planet-forming disks, dense stellar cores, AGN, etc...). Crucial to the final performance of GRAVITY, the Coudé IR Adaptive Optics (CIAO) system will correct for the effects of the atmosphere at each of the VLT Unit Telescopes. CIAO consists of four new infrared Shack-Hartmann wavefront sensors (WFS) and associated real-time computers/software which will provide infrared wavefront sensing from 1.45-2.45 microns, allowing AO corrections even in regions where optically bright reference sources are scarce. We present here the latest progress on the GRAVITY wavefront sensors. We describe the adaptation and testing of a light-weight version of the ESO Standard Platform for Adaptive optics Real Time Applications (SPARTA-Light) software architecture to the needs of GRAVITY. We also describe the latest integration and test milestones for construction of the initial wave front sensor.
We report here on the software Hack Day organised at the 2014 SPIE conference on Astronomical Telescopes and Instrumentation in Montréal. The first ever Hack Day to take place at an SPIE event, the aim of the day was to bring together developers to collaborate on innovative solutions to problems of their choice. Such events have proliferated in the technology community, providing opportunities to showcase, share and learn skills. In academic environments, these events are often also instrumental in building community beyond the limits of national borders, institutions and projects. We show examples of projects the participants worked on, and provide some lessons learned for future events.
Large flat mirrors can be characterized using a standard interferometer coupled with stitching the subaperture
measurement data. Such systems can measure the global full map of the optical surface by minimizing the inconsistency
of data in the adjacent regions. We present a stitching technique that makes use of a commercial phase-shifting Twyman-
Green interferometer in combination with an iterative optimized stitching algorithm. The proposed method has been
applied to determine the surface errors of planar mirrors with an accuracy of a few nanometers. Moreover, the effect of
reference wavefront error is explored. The feasibility and the performance of the proposed system are also demonstrated,
along with a detailed error analysis and experimental results.
GRAVITY is a second generation instrument for the VLT Interferometer, designed to enhance the near-infrared
astrometric and spectro-imaging capabilities of VLTI. Combining beams from four telescopes, GRAVITY will
provide an astrometric precision of order 10 micro-arcseconds, imaging resolution of 4 milli-arcseconds, and low
and medium resolution spectro-interferometry, pushing its performance far beyond current infrared interferometric
capabilities. To maximise the performance of GRAVITY, adaptive optics correction will be implemented
at each of the VLT Unit Telescopes to correct for the e_ects of atmospheric turbulence. To achieve this, the
GRAVITY project includes a development programme for four new wavefront sensors (WFS) and NIR-optimized
real time control system. These devices will enable closed-loop adaptive correction at the four Unit Telescopes
in the range 1.4-2.4 μm. This is crucially important for an e_cient adaptive optics implementation in regions
where optically bright references sources are scarce, such as the Galactic Centre. We present here the design of
the GRAVITY wavefront sensors and give an overview of the expected adaptive optics performance under typical
observing conditions. Bene_ting from newly developed SELEX/ESO SAPHIRA electron avalanche photodiode
(eAPD) detectors providing fast readout with low noise in the near-infrared, the AO systems are expected to
achieve residual wavefront errors of 400 nm at an operating frequency of 500 Hz.≤
Silicon immersion gratings offer size and cost savings for high-resolution near-infrared spectrographs. The
IGRINS instrument at McDonald Observatory will employ a high-performance silicon immersion echelle grating to achieve spectral resolution R = λ/Δλ40,000 simultaneously over H and K near-infrared band atmospheric
windows (1.5-2.5 μm). We chronicle the metrology of an R3 silicon immersion echelle grating for IGRINS. The grating is 30x80 mm, etched into a monolithic silicon prism. Optical interferometry of the grating surface in
reflection indicates high phase coherence (<λ/6 peak to valley surface error over a 25 mm beam at λ= 632 nm).
Optical interferometry shows small periodic position errors of the grating grooves. These periodic errors manifest
as spectroscopic ghosts. High dynamic range monochromatic spectral purity measurements reveal ghost levels
relative to the main diffraction peak at 1.6x10<sup>-3</sup> at λ = 632 nm in reflection, consistent with the interferometric
results Improved grating surfaces demonstrate reflection-measured ghosts at negligible levels of 10<sup>-4</sup> of the main
diffraction peak. Relative on-blaze efficiency is ~75%. We investigate the immersion grating blaze efficiency
performance over the entire operational bandwidth 1500 <λ(nm) < 2500 at room temperature. The projected
performance at operational cryogenic temperatures meets the design specifications.
The GRAVITY instrument’s adaptive optics system consists of a novel cryogenic near-infrared wavefront sensor to be
installed at each of the four unit telescopes of the VLT. Feeding the GRAVITY wavefront sensor with light in the 1.4 -
2.4 micrometer band, while suppressing laser light originating from the GRAVITY metrology system, custom-built
optical components are required. Here we report on optical and near-infrared testing of the silicon entrance windows of the wavefront sensor cryostat and other reflective optics used in the warm feeding optics.
We have designed and fabricated a suite of grisms for use in FORCAST, a mid-infrared camera scheduled as a
first-light instrument on SOFIA. The grism suite gives SOFIA a new capability: low resolution spectroscopy from
5 to 38 microns at resolving powers from R=200 to R=1200, without the addition of a new instrument. We have
developed an IDL based spectral data reduction and quick-look software package, in anticipation of FORCAST
grism spectroscopy becoming a facility observing mode on the SOFIA telescope. The package allows users to
quickly view their data by extracting single-order and cross-dispersed spectra immediately after acquiring them
in flight. We have optimized the quick-look software to reduce the number of steps required to turn a set of
observations into a fully reduced extracted spectrum. We present a description of the philosophy of the data
reduction software, supplemented with screen shots and examples in hopes of garnering feedback and critiques
from potential end users, software developers, and instrument builders.
We have implemented and tested a suite of grisms that will enable a moderate-resolution mid-infrared spectroscopic
mode in FORCAST, the facility mid-infrared camera on SOFIA. We have tested the hardware for the spectral modes
extensively in the laboratory with grisms installed in the FORCAST filter wheels. The grisms perform as designed,
consistently producing spectra at resolving powers in the 200-1200 range at wavelengths from 5 to 38 microns. In
anticipation of offering this capability as a SOFIA general observer mode, we are developing software for reduction and
analysis of FORCAST spectra, a spectrophotometric calibration plan, and detailed plans for in-flight tests prior to
commissioning the modes. We present a brief summary of the FORCAST grism spectroscopic system and a status report.
We have recently completed a set of silicon grisms for JWST-NIRCam. These devices have exquisite optical
characteristics: phase surfaces flat to λ/100 peak to valley at the blaze wavlength, diffraction-limited PSFs down
to 10<sup>-5</sup> of the peak, low scattered light levels, and large resolving-power slit-width products for their width and
thickness. The one possible drawback to these devices is the large Fresnel loss caused by the large refractive
index of Si. We report here on throughput and phase-surface measurements for a sample grating with a high
performance antireflection coating on both the flat and grooved surfaces. These results indicate that we can
achieve very high on-blaze efficiencies. The high throughput should make Si grisms an attractive dispersive
element for moderate resolution IR spectroscopy in both ground and space based instruments throughout the
1.2-8 μm spectral region.
Silicon immersion gratings have been a promising future technology for high resolution infrared spectroscopy for
over 15 years. We report here on our current immersion grating research, including extensive measurements of
the performance of micromachined silicon devices. We are currently producing gratings for two high resolution
spectrometers: iSHELL at the University of Hawaii and IGRINS at the University of Texas and the Korea
Astronomy and Space Science Institute. The gratings are R3 devices with total lengths of ~95 mm. The use of a
high index material like silicon permits the spectrometers to have high resolving powers (40,000-70,000) at
decent slit sizes with very small (25mm) collimated beams. The lithographic production of coarse grooves allows
for instrument designs with continuous wavelength coverage over broad spectral ranges. We discuss the science
requirements for grating quality and efficiency and the measurements we have made to verify that the gratings
meet these requirements. The measurements include optical interferometry and measurements of the
monochromatic point spread function in reflection.
We are designing a sensitive high resolution (R=60,000-100,000) spectrograph for the Giant Magellan Telescope
(GMTNIRS, the GMT Near-Infrared Spectrograph). Using large-format IR arrays and silicon immersion gratings, this
instrument will cover all of the J (longer than 1.1 μm), H, and K atmospheric windows or all of the L and M windows in
a single exposure. GMTNIRS makes use of the GMT adaptive optics system for all bands. The small slits will offer the
possibility of spatially resolved spectroscopy as well as superior sensitivity and wavelength coverage. The GMTNIRS
team is composed of scientists and engineers at the University of Texas, the Korea Astronomy and Space Science
Institute, and Kyung Hee University. In this paper, we describe the optical and mechanical design of the instrument. The
principal innovative feature of the design is the use of silicon immersion gratings which are now being produced by our
team with sufficient quality to permit designs with high resolving power and broad instantaneous wavelength coverage
across the near-IR.
The Korea Astronomy and Space Science Institute (KASI) and the Department of Astronomy at the University of Texas
at Austin (UT) are developing a near infrared wide-band high resolution spectrograph, IGRINS. IGRINS can observe all
of the H- and K-band atmospheric windows with a resolving power of 40,000 in a single exposure. The spectrograph
uses a white pupil cross-dispersed layout and includes a dichroic to divide the light between separate H and K cameras,
each provided with a 2kx2k HgCdTe detector. A silicon immersion grating serves as the primary disperser and a pair of
volume phased holographic gratings serve as cross dispersers, allowing the high resolution echelle spectrograph to be
very compact. IGRINS is designed to be compatible with telescopes ranging in diameter from 2.7m (the Harlan J. Smith
telescope; HJST) to 4 - 8 m telescopes. Commissioning and initial operation will be on the 2.7m telescope at McDonald
Observatory from 2013.
We report on the design, fabrication, and evaluation of a set of silicon grisms for the NIRCam instrument on
NASA's James Webb Space Telescope. The primary purpose of these devices is to aid in the alignment of JWST's
deployable primary mirror. The grisms will also offer opportunities for slitless astronomical spectroscopy. The
design of the grisms was driven by a need to fit into a constrained space, by a need for high resolving power
across a broad spectral band, and by the need to survive the cosmic ray dosage to which the instrument will
be subjected. The University of Texas Silicon Grating Laboratory is fabricating four identical grisms to cover
2.5-5 μm with a resolving power of 1770 at the blaze wavelength. There will be two grisms, with dispersion axes
oriented at 90°, in each arm of the NIRCam long-wavelength camera. We pattern the gratings lithographically
onto high resistivity float-zone silicon prisms following the recipe developed for the recently completed grism
suite for the FORCAST camera on SOFIA. We discuss the design and production of the NIRCam devices and
present the results of the optical testing of the grating surfaces showing that the devices will likely exceed their
We have designed and fabricated a suite of grisms for use in FORCAST, a mid-infrared camera scheduled as a
first-light instrument on SOFIA. The grism suite gives SOFIA a new capability: low and moderate resolution
spectroscopy from 5μm to 37μm, without the addition of a new instrument. One feature of the optical design
is that it includes a mode using pairs of cross-dispersed grisms, providing continuous wavelength coverage over
a broad range at higher resolving power. We fabricated four silicon (n = 3.44) grisms using photolithographic
techniques and purchased two additional mechanically ruled KRS-5 (n = 2.3) grisms. One pair of silicon grisms
permits observations of the 5 - 8μm band with a long slit at R~ 200 or, in a cross-dispersed mode, at resolving
powers up to 1500. In the 8 - 14μm region, where silicon absorbs heavily, the KRS-5 grisms produce resolving
powers of 300 and 800 in long-slit and cross-dispersed mode, respectively. The remaining two silicon grisms cover
17 - 37μm at resolving powers of 140 and 250. We have thoroughly tested the silicon grisms in the laboratory,
measuring efficiencies in transmission at 1.4 - 1.8μm. We report on these measurements as well as on cryogenic
performance tests of the silicon and KRS-5 devices after installation in FORCAST.