Casey P. Deen

Preliminary design of IGRINS (Immersion GRating INfrared Spectrograph)

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.

A silicon and KRS-5 grism suite for FORCAST on SOFIA

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.

GRAVITY Coudé Infrared Adaptive Optics (CIAO) system for the VLT Interferometer

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.≤

GMTNIRS (Giant Magellan Telescope near-infrared spectrograph): design concept

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.

Near-infrared metrology of high-performance silicon immersion gratings

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 1.6x10-3 at λ = 632 nm in reflection, consistent with the interferometric results Improved grating surfaces demonstrate reflection-measured ghosts at negligible levels of 10-4 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.

Optimizing the transmission of the GRAVITY/VLTI near-infrared wavefront sensor

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.

Modification Of The MOOG Spectral Synthesis Codes To Account For Zeeman Broadening Of Spectral Lines

In an attempt to widen access to the study of magnetic fields in stellar astronomy, I present MOOGStokes, a version of the MOOG one-dimensional LTE radiative transfer code, overhauled to incorporate a Stokes vector treatment of polarized radiation through a magnetic medium. MOOGStokes is a suite of three complementary programs, which together can synthesize the disk-averaged emergent spectrum of a star with a magnetic field. The first element (a pre-processing script called CounterPoint) calculates for a given magnetic field strength, wavelength shifts and polarizations for the components of Zeeman sensitive lines. The second element (a MOOG driver called SynStokes derived from the existing MOOG driver Synth) uses the list of Zeeman shifted absorption lines together with the existing machinery of MOOG to synthesize the emergent spectrum at numerous locations across the stellar disk, accounting for stellar and magnetic field geometry. The third and final element (a post-processing script called DiskoBall) calculates the disk-averaged spectrum by weighting the individual emergent spectra by limb darkening and projected area, and applying the effects of Doppler broadening. All together, the MOOGStokes package allows users to synthesize emergent spectra of stars with magnetic fields in a familiar computational framework. MOOGStokes produces disk-averaged spectra for all Stokes vectors (I, Q, U, and V), normalized by the continuum. MOOGStokes agrees well with the predictions of INVERS10 a polarized radiative transfer code with a long history of use in the study of stellar magnetic fields. In the non-magnetic limit, MOOGStokes also agrees with the predictions of the scalar version of MOOG.

Characterization of the transmitted near-infrared wavefront error for the GRAVITY/VLTI Coudé Infrared Adaptive Optics System

The adaptive optics system for the second-generation VLT-interferometer (VLTI) instrument GRAVITY consists of a novel cryogenic near-infrared wavefront sensor to be installed at each of the four unit telescopes of the Very Large Telescope (VLT). Feeding the GRAVITY wavefront sensor with light in the 1.4 to 2.4 micrometer band, while suppressing laser light originating from the GRAVITY metrology system, custom-built optical components are required. In this paper, we present the development of a quantitative near-infrared point diffraction interferometric characterization technique, which allows measuring the transmitted wavefront error of the silicon entrance windows of the wavefront sensor cryostat. The technique can be readily applied to quantitative phase measurements in the near-infrared regime. Moreover, by employing a slightly off-axis optical setup, the proposed method can optimize the required spatial resolution and enable real time measurement capabilities. The feasibility of the proposed setup is demonstrated, followed by theoretical analysis and experimental results. Our experimental results show that the phase error repeatability in the nanometer regime can be achieved.

High-performance silicon grisms for 1.2-8.0 μm: detailed results from the JWST-NIRCam devices

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 10-5 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.

Manufacturing of silicon immersion gratings for infrared spectrometers

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.