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.
Space-based observation of gravitational waves promises to enable the study of a rich variety of high energy astrophysical sources in the 0.0001 to 1 Hz band using signals complementary to traditional electromagnetic waves. Gravitational waves represent the first new tool for studying the sky since gamma ray telescopes debuted in the 1970s, and we expect compelling science to be the result. The fundamental measurement is to monitor the path length difference between pairs of freely falling test masses with laser interferometry to a precision of picometers over gigameter baselines. The test masses are arranged in an equilateral triangle to allow simultaneous measurement of both gravitational wave polarizations. The heliocentric orbital space environment enables the test masses to be shielded from large ground motions at low frequencies, and allows the construction of long measurement baselines that are well matched to the signal wavelengths. Optical telescopes play an important role in the measurement because they deliver laser light efficiently from one spacecraft to another. The telescopes are directly in the measurement path, so there are additional performance requirements to support precision metrology beyond the usual requirements for good image formation.
The Climate Absolute Radiance and Refractivity Observatory (CLARREO) program objectives are recommended by the
NRC as a Tier-1 mission in its January 15, 2007 Earth Science Decadal Survey to be the key component of a future
decade-scale, global climate change observing system. The purpose of CLARREO is to make SI-traceable absolute
observations sensitive to the most critical, but least understood climate forcing phenomena, responses, and feedbacks.
NASA / LaRC is the mission lead as well as the Infrared (IR) instrument suite development lead. The Reflected Solar
(RS) instrument lead center has been assigned to GSFC where engineering risk reduction and science calibration
demonstration studies are being conducted on flight-like ETUs in anticipation of entry into Phase A.
The RS instrument suite (SOLARIS) is composed of multiple all-aluminum, slit-based, push-broom imaging spectroradiometers
of nearly identical construction. Each 'box' will be optimized to provide better than 8nm spectral resolution
(using multiple detector elements) over a specific spectral band covering the 320-2300nm total range with significant
overlaps to aid calibration. Optical design, fabrication, and alignment will provide for 500m nadir resolutions over a full
slit field of 100km from an approximately 600km polar orbit greater than 90% of the time. SNRs are likewise required to
exceed 33 for λ < 900nm and 25 for λ < 900nm. The maximum radiometric sensitivity to any naturally-occurring
polarized scene elements is expected to be between 0.5% - 0.75% for λ < 1000nm and λ <1000nm respectively. The RS
suite system will be capable of demonstrating a long-term, spectrally- & spatially-averaged, systematic radiometric error
of less than 0.3% (k=2).
Coupled with measurements from on-board GPS radio occultation receivers and inherent inter-calibration compatibility
with existing and future Earth science and operational missions, these measurements will provide a long-term
benchmarking data record for the detection, projection, and attribution of changes to our planet's climate system. The
CLARREO Project team successfully completed its Mission Concept Review (MCR) on November 17, 2010 at LaRC
with high marks and remains dedicated to the mission and its instruments. However, the launch readiness date (LRD) is
yet to be determined pending budget directive updates from the White House along with review of the IR and RS
calibration demonstration efforts (extended pre-Phase A).
Each mirror produced by this NASA developed process is a monolithic structure from a single crystal of silicon. Due to single crystal silicon's extraordinary homogeneity and lack of internal stress, we light weight after optical polishing. Mirrors produced by our original process were about 1/4th the mass of an equivalent quartz mirror and were typically 1/50th wave or better. We have recently revised our process, replacing the isogrid structures with ones optimized to minimize distortion due to mounting errors. We have also switched from ultrasonic machining to CNC grinding to enable the production of larger mirrors. We report results to date for mirrors produced by the revised process and cryogenic test results for an ultrasonically light weighted mirror.
In measuring the figure error of an aspheric optic using a null lens, the wavefront contribution from the null lens must be
independently and accurately characterized in order to isolate the optical performance of the aspheric optic alone.
Various techniques can be used to characterize such a null lens, including interferometry, profilometry and image-based
methods. Only image-based methods, such as phase retrieval, can measure the null-lens wavefront in situ – in single-pass,
and at the same conjugates and in the same alignment state in which the null lens will ultimately be used – with no
additional optical components. Due to the intended purpose of a null lens (e.g., to null a large aspheric wavefront with a
near-equal-but-opposite spherical wavefront), characterizing a null-lens wavefront presents several challenges to image-based
phase retrieval: Large wavefront slopes and high-dynamic-range data decrease the capture range of phase-retrieval
algorithms, increase the requirements on the fidelity of the forward model of the optical system, and make it difficult to
extract diagnostic information (e.g., the system F/#) from the image data. In this paper, we present a study of these
effects on phase-retrieval algorithms in the context of a null lens used in component development for the Climate
Absolute Radiance and Refractivity Observatory (CLARREO) mission. Approaches for mitigation are also discussed.
Presented is the design of a nulling interferometer testbed which is capable of maintaining the suppression of a
broadband, infrared source in the presence of external perturbations. Pathlength stability is accomplished by
introducing a dispersive phase shift which allows light at a SWIR band to be used as a wavefront sensor to stabilize the
nulled output of a broadband MWIR channel. Since both channels are common path, fluctuations in OPD observed
with the wavefront sensor directly correlate to fluctuations of the nulling passband. Results obtained from the testbed
will be useful to future nulling interferometers such as the Large Binocular Telescope Interferometer and the Terrestrial
Planet Finder Interferometer which are currently being designed to aid in the search for earth-like planets outside our