Silicon immersion gratings and grisms enable compact, near-infrared spectrographs with high throughput. These instruments find use in ground-based efforts to characterize stellar and exoplanet atmospheres, and in space-based observatories. Our grating fabrication technique uses x-ray crystallography to orient silicon parts prior to cutting, followed by lithography and wet chemical etching to produce the blaze. This process takes advantage of the crystal structure and relative difference in etching rates between the (100) and (111) planes such that we can produce parts that have surface errors <4 . Previous measurements indicate that chemical etching can yield a final etched blaze that slightly differs from the orientation of the (111) plane. This difference can be corrected by the mechanical mount in the case of the immersion gratings, but doing so may compromise grating throughput. In the case of the grisms, failure to take the actual blaze into account will alter the wavelength of the undeviated array. We report on multiple techniques to precisely measure the blaze of our in-house fabricated immersion gratings. The first method uses a scanning electron microscope to image the blaze profile, which yields a measurement precision of 0.5 degrees. The second method is an optical method of measuring the angle between blaze faces using a rotation stage, which yields a measurement precision of 0.2 degrees. Finally, we describe a theoretical blaze function modeling method, which we expect to yield a measurement precision of 0.1 degrees. With these methods, we can quantify the accuracy with which the wet etching produces the required blaze and further optimize grating and grism efficiencies.
The Immersion Grating Infrared Spectrometer (IGRINS) is a revolutionary instrument that exploits broad spectral coverage at high-resolution in the near-infrared. IGRINS employs a silicon immersion grating as the primary disperser, and volume-phase holographic gratings cross-disperse the H and K bands onto Teledyne Hawaii-2RG arrays. The use of an immersion grating facilitates a compact cryostat while providing simultaneous wavelength coverage from 1.45 - 2.5 μm. There are no cryogenic mechanisms in IGRINS and its high-throughput design maximizes sensitivity. IGRINS on the 2.7 meter Harlan J. Smith Telescope at McDonald Observatory is nearly as sensitive as CRIRES at the 8 meter Very Large Telescope. However, IGRINS at R≈45,000 has more than 30 times the spectral grasp of CRIRES* in a single exposure. Here we summarize the performance of IGRINS from the first 300 nights of science since commissioning in summer 2014. IGRINS observers have targeted solar system objects like Pluto and Ceres, comets, nearby young stars, star forming regions like Taurus and Ophiuchus, the interstellar medium, photo dissociation regions, the Galactic Center, planetary nebulae, galaxy cores and super novae. The rich near-infrared spectra of these objects motivate unique science cases, and provide information on instrument performance. There are more than ten submitted IGRINS papers and dozens more in preparation. With IGRINS on a 2.7m telescope we realize signal-to-noise ratios greater than 100 for K=10.3 magnitude sources in one hour of exposure time. Although IGRINS is Cassegrain mounted, instrument flexure is sub-pixel thanks to the compact design. Detector characteristics and stability have been tested regularly, allowing us to adjust the instrument operation and improve science quality. A wide variety of science programs motivate new tools for analyzing high-resolution spectra including multiplexed spectral extraction, atmospheric model fitting, rotation and radial velocity, unique line identification, and circumstellar disk modeling. Here we discuss details of instrument performance, summarize early science results, and show the characteristics of IGRINS as a versatile near-infrared spectrograph and forerunner of future silicon immersion grating spectrographs like iSHELL2 and GMTNIRS.3
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.
Silicon immersion gratings and grisms offer significant advantages in compactness and performance over frontsurface gratings and over grisms made from lower-index materials. At the same time, the high refractive index of Si (3.4) leads to very stringent constraints on the allowable groove position errors, typically rms < 20 nm over 100 mm and repetitive error of <5 nm amplitude. For both types of devices, we produce grooves in silicon using photolithography, plasma etching, and wet etching. To date, producers have used contact photolithography to pattern UV sensitive photoresist as the initial processing step, then transferred this pattern to a layer of silicon nitride that, in turn, serves as a hard mask during the wet etching of grooves into silicon.
For each step of the groove production, we have used new and sensitive techniques to determine the contribution of that step to the phase non-uniformity. Armed with an understanding of the errors and their origins, we could then implement process controls for each step. The plasma uniformity was improved for the silicon nitride mask etch process and the phase contribution of the plasma etch step was measured. We then used grayscale lithography, a technique in which the photoresist is deliberately underexposed, to measure large-scale nonuniformities in the UV exposure system to an accuracy of 3-5%, allowing us to make corrections to the optical alignment. Additionally, we used a new multiple-exposure technique combined with laser interferometry to measure the relationship between UV exposure dose and line edge shift. From these data we predict the contribution of the etching and photolithographic steps to phase error of the grating surface. These measurements indicate that the errors introduced during the exposure step dominate the contributions of all the other processing steps. This paper presents the techniques used to quantify individual process contributions to phase errors and steps that were taken to improve overall phase uniformity.
GMTNIRS, the Giant Magellan Telescope near-infrared spectrograph, is a first-generation instrument for the GMT that
will provide detailed spectroscopic information about young stellar objects, exoplanets, and cool and/or obscured stars.
The optical and mechanical design GMTNIRS presented at a conceptual design review in October 2011 covered all
accessible parts of the spectrum from 1.12 to 5.3 microns at R=50,000 (1.12-2.5 microns) and R=100,000 (3-5.3
microns). GMTNIRS uses the GMT adaptive-optics system and has a single 85 milliarcsecond slit. The instrument
includes five separate spectrographs for the different atmospheric windows. By use of dichroics that divide the incident
light between five separate spectrographs, it observes its entire spectral grasp in a single exposure while having only one
cryogenic moving part, a rotating pupil stop.
Large, highly accurate silicon immersion gratings are critical to GMTNIRS, since they both permit a design within the
allowable instrument volume and enable continuous wavelength coverage on existing detectors. We describe the effort
during the preliminary design phase to refine the design of the spectrograph to meet the science goals while minimizing
the cost and risk involved in the grating production. We discuss different design options for the individual spectrographs
at R=50,000, 67,000, 75,000, and 100,000 and their impact on science return.
Infrared spectrographs employing silicon immersion gratings can be significantly more compact than spectro-
graphs using front-surface gratings. The Si gratings can also offer continuous wavelength coverage at high
spectral resolution. The grooves in Si gratings are made with semiconductor lithography techniques, to date
almost entirely using contact mask photolithography. Planned near-infrared astronomical spectrographs require
either finer groove pitches or higher positional accuracy than standard UV contact mask photolithography can
reach. A collaboration between the University of Texas at Austin Silicon Diffractive Optics Group and the Jet
Propulsion Laboratory Microdevices Laboratory has experimented with direct writing silicon immersion grating
grooves with electron beam lithography. The patterning process involves depositing positive e-beam resist on
1 to 30 mm thick, 100 mm diameter monolithic crystalline silicon substrates. We then use the facility JEOL
9300FS e-beam writer at JPL to produce the linear pattern that defines the gratings.
There are three key challenges to produce high-performance e-beam written silicon immersion gratings. (1) E-
beam field and subfield stitching boundaries cause periodic cross-hatch structures along the grating grooves. The
structures manifest themselves as spectral and spatial dimension ghosts in the diffraction limited point spread
function (PSF) of the diffraction grating. In this paper, we show that the effects of e-beam field boundaries must
be mitigated. We have significantly reduced ghost power with only minor increases in write time by using four or
more field sizes of less than 500 μm. (2) The finite e-beam stage drift and run-out error cause large-scale structure
in the wavefront error. We deal with this problem by applying a mark detection loop to check for and correct out
minuscule stage drifts. We measure the level and direction of stage drift and show that mark detection reduces
peak-to-valley wavefront error by a factor of 5. (3) The serial write process for typical gratings yields write times
of about 24 hours- this makes prototyping costly. We discuss work with negative e-beam resist to reduce the
fill factor of exposure, and therefore limit the exposure time. We also discuss the tradeoffs of long write-time
serial write processes like e-beam with UV photomask lithography. We show the results of experiments on small
pattern size prototypes on silicon wafers. Current prototypes now exceed 30 dB of suppression on spectral and
spatial dimension ghosts compared to monochromatic spectral purity measurements of the backside of Si echelle
gratings in reflection at 632 nm. We perform interferometry at 632 nm in reflection with a 25 mm circular beam
on a grating with a blaze angle of 71.6°. The measured wavefront error is 0.09 waves peak to valley.
The Immersion Grating Infrared Spectrometer (IGRINS) is a compact high-resolution near-infrared cross-dispersed
spectrograph whose primary disperser is a silicon immersion grating. IGRINS covers the entire portion of the
wavelength range between 1.45 and 2.45μm that is accessible from the ground and does so in a single exposure with a
resolving power of 40,000. Individual volume phase holographic (VPH) gratings serve as cross-dispersing elements for
separate spectrograph arms covering the H and K bands. On the 2.7m Harlan J. Smith telescope at the McDonald
Observatory, the slit size is 1ʺ x 15ʺ and the plate scale is 0.27ʺ pixel. The spectrograph employs two 2048 x 2048
pixel Teledyne Scientific and Imaging HAWAII-2RG detectors with SIDECAR ASIC cryogenic controllers. The
instrument includes four subsystems; a calibration unit, an input relay optics module, a slit-viewing camera, and nearly
identical H and K spectrograph modules. The use of a silicon immersion grating and a compact white pupil design allows
the spectrograph collimated beam size to be only 25mm, which permits a moderately sized (0.96m x 0.6m x 0.38m)
rectangular cryostat to contain the entire spectrograph. The fabrication and assembly of the optical and mechanical
components were completed in 2013. We describe the major design characteristics of the instrument including the
system requirements and the technical strategy to meet them. We also present early performance test results obtained
from the commissioning runs at the McDonald Observatory.
IGRINS, the Immersion GRating INfrared Spectrometer includes an immersion grating made of silicon and observes
both H-band (1.49~1.80 μm) and K-band (1.96~2.46 μm), simultaneously. In order to align such an infrared optical
system, the compensator in its optical components has been adjusted within tolerances at room temperature without
vacuum environment. However, such a system will ultimately operate at low temperature and vacuum with no
adjustment mechanism. Therefore a reasonable relationship between different environmental variations such as room and
low temperature might provide useful knowledge to align the system properly. We are attempting to develop a new
process to predict the Wave Front Error (WFE), and to produce correct mechanical control values when the optical
system is perturbed by moving the lens at room temperature. The purpose is to provide adequate optical performance
without making changes at operating temperature. In other words, WFE was measured at operating temperature without
any modification but a compensator was altered correctly at room temperature to meet target performance. The ‘no
adjustment’ philosophy was achieved by deterministic mechanical adjustment at room temperature from a simulation
that we developed. In this study, an achromatic doublet lens was used to substitute for the H and K band camera of
IGRINS. This novel process exhibits accuracy predictability of about 0.002 λ rms WFE and can be applied to a cooled
infrared optical systems.
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.
The Near Infrared Camera (NIRCam) is one of the four science instruments of the James Webb Space Telescope
(JWST). Its high sensitivity, high spatial resolution images over the 0.6 - 5 μm wavelength region will be
essential for making significant findings in many science areas as well as for aligning the JWST primary mirror
segments and telescope. The NIRCam engineering test unit was recently assembled and has undergone successful
cryogenic testing. The NIRCam collimator and camera optics and their mountings are also progressing, with a
brass-board system demonstrating relatively low wavefront error across a wide field of view. The flight model's
long-wavelength Si grisms have been fabricated, and its coronagraph masks are now being made. Both the short
(0.6 - 2.3 μm) and long (2.4 - 5.0 μm) wavelength flight detectors show good performance and are undergoing
final assembly and testing. The flight model subsystems should all be completed later this year through early
2011, and NIRCam will be cryogenically tested in the first half of 2011 before delivery to the JWST integrated
science instrument module (ISIM).
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.
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.