Thang Trinh

Palomar adaptive optics project: status and performance

We describe the current performance of the Palomar 200 inch (5 m) adaptive optics system, which in December of 1998 achieved its first high order (241 actuators) lock on a natural guide star. In the K band (2.2 micrometer), the system has achieved Strehl ratios as high as 50% in the presence of 1.0 arcsecond seeing (0.5 micrometer). Predictions of the systems performance based on the analysis of real-time wavefront sensor telemetry data and an analysis based on a fitted Kolmogorov atmospheric model are shown to both agree with the observed science image performance. Performance predictions for various seeing conditions are presented and an analysis of the error budget is used to show which subsystems limit the performance of the AO system under various atmospheric conditions.

The Gemini Planet Imager Calibration Wavefront Sensor Instrument

The Gemini Planet Imager is an extreme adaptive optics system that will employ an apodized-pupil coronagraph to make direct detections of faint companions of nearby stars to a contrast level of the 10-7 within a few lambda/D of the parent star. Such high contrasts from the ground require exquisite wavefront sensing and control both for the AO system as well as for the coronagraph. Un-sensed non-common path phase and amplitude errors after the wavefront sensor dichroic but before the coronagraph would lead to speckles which would ultimately limit the contrast. The calibration wavefront system for GPI will measure the complex wavefront at the system pupil before the apodizer and provide slow phase corrections to the AO system to mitigate errors that would cause a loss in contrast. The calibration wavefront sensor instrument for GPI has been built. We will describe the instrument and its performance.

A CLOSE COMPANION SEARCH AROUND L DWARFS USING APERTURE MASKING INTERFEROMETRY AND PALOMAR LASER GUIDE STAR ADAPTIVE OPTICS

We present a close companion search around 16 known early L dwarfs using aperture masking interferometry with Palomar laser guide star adaptive optics (LGS AO). The use of aperture masking allows the detection of close binaries, corresponding to projected physical separations of 0.6-10.0 AU for the targets of our survey. This survey achieved median contrast limits of ΔK ~ 2.3 for separations between 1.2λ/D-4λ/D and ΔK ~ 1.4 at 2/3λ/D. We present four candidate binaries detected with moderate-to-high confidence (90%-98%). Two have projected physical separations less than 1.5 AU. This may indicate that tight-separation binaries contribute more significantly to the binary fraction than currently assumed, consistent with spectroscopic and photometric overluminosity studies. Ten targets of this survey have previously been observed with the Hubble Space Telescope as part of companion searches. We use the increased resolution of aperture masking to search for close or dim companions that would be obscured by full aperture imaging, finding two candidate binaries. This survey is the first application of aperture masking with LGS AO at Palomar. Several new techniques for the analysis of aperture masking data in the low signal-to-noise regime are explored.

Tuning and tailoring of broadband quantum-well infrared photodetector responsivity spectrum

The spectral response of quantum-well infrared photodetectors (QWIPs) based on the III-V material system are tailorable to narrow or broad bandwidths within mid- and long-wavelength infrared bands. Typical broad-band QWIPs show considerable spectral shape change with bias voltage, particularly near the cut-off wavelength region. Two alternatives to the typical broadband QWIP design have been demonstrated. These designs consist of two multiquantum-well (QW) stacks or alternatively placed QWs and produce nearly fixed spectrums within the operating bias voltages. Flexibility in many design parameters of these detectors allows for tuning and tailoring the spectral shape according to application requirements, specifically for spectral imaging instruments.

The PALM-3000 high-order adaptive optics system for Palomar Observatory

Deployed as a multi-user shared facility on the 5.1 meter Hale Telescope at Palomar Observatory, the PALM-3000 highorder upgrade to the successful Palomar Adaptive Optics System will deliver extreme AO correction in the near-infrared, and diffraction-limited images down to visible wavelengths, using both natural and sodium laser guide stars. Wavefront control will be provided by two deformable mirrors, a 3368 active actuator woofer and 349 active actuator tweeter, controlled at up to 3 kHz using an innovative wavefront processor based on a cluster of 17 graphics processing units. A Shack-Hartmann wavefront sensor with selectable pupil sampling will provide high-order wavefront sensing, while an infrared tip/tilt sensor and visible truth wavefront sensor will provide low-order LGS control. Four back-end instruments are planned at first light: the PHARO near-infrared camera/spectrograph, the SWIFT visible light integral field spectrograph, Project 1640, a near-infrared coronagraphic integral field spectrograph, and 888Cam, a high-resolution visible light imager.

Facilitizing the Palomar AO laser guide star system

We describe the work that has gone into taking the sodium Laser Guide Star (LGS) program on the Palomar AO system from a successful experiment to a facility instrument. In particular, we describe the operation of the system, the BTO (beam transfer optics) system which controls the path of the laser in the dome, the aircraft safety systems and the optical systems which allow us to take advantage of the unique properties of the macro/micro pulse laser. In addition we present on sky performance results that demonstrate K-band Strehl ratios of up to 48%

Real-time wavefront processors for the next generation of adaptive optics systems: a design and analysis

Adaptive optics (AO) systems currently under investigation will require at least two orders of magitude increase in the number of actuators, which in turn translates to effectively a 104 increase in compute latency. Since the performance of an AO system invariably improves as the compute latency decreases, it is important to study how todays computer systems will scale to address this expected increase in actuator utilization. This paper answers this question by characterizing the performance of a single deformable mirror (DM) Shack-Hartmann natural guide star AO system implemented on the present-generation digital signal processor (DSP) TMS320C6701 from Texas Instruments. We derive the compute latency of such a system in terms of a few basic parameters, such as the number of DM actuators, the number of data channels used to read out the camera pixels, the number of DSPs, the available memory bandwidth, as well as the inter-processor communication (IPC) bandwidth and the pixel transfer rate. We show how the results would scale for future systems that utilizes multiple DMs and guide stars. We demonstrate that the principal performance bottleneck of such a system is the available memory bandwidth of the processors and to lesser extent the IPC bandwidth. This paper concludes with suggestions for mitigating this bottleneck.

Quantum well infrared photodetectors for low background applications

Abstract Quantum well infrared photodetectors (QWIPs) afford greater flexibility than the usual extrinsically doped semiconductor IR detectors. The wavelength of the peak response and cutoff can be continuously tailored over any wavelength range between 6–20 μm. The spectral band width of these detectors can be tuned from narrow ( Δλ / λ ∼10%) to wide ( Δλ / λ ∼40%) allowing various applications. Also, QWIP device parameters can be optimized to achieve extremely high performances at high back ground at lower operating temperatures (∼30 K). However, for low-background irradiance levels at low operating temperatures, high resistivity of the active region could leads to non-linear responsivity behavior. A new structure with a photosensitive superlattice and a thick blocking barrier has been tested, and is expected to avoid anomalous behavior under low backgrounds at low temperatures.

Large-format dual-broadband QWIP focal plane array imaging interferometers

The Jet Propulsion Laboratory (JPL) is developing a 512x640-format, dual broadband, Quanum Well Infrared Photodetector (QWIP) focal plane array (FPA) for an imaging interferometer. This is a new type of imaging interferometer which is based on special Fourier-transform spectroscopy, scans interferograms digitally without moving any optical components. It is stable enough to measure fluctuating target signatures from unstable platforms, making it ideal for detecting chemical agents from a remotely piloted aircraft. These static interferometers require large-format FPAs with high uniformity and operability. QWIP technology is ideal for this instrument because it has achieved remarkable success in advancing highly uniform, highly-operability, and large-format multicolor FPAs. The FPA used in the interferometer covers the wavelength from 6-10 μm and 10-15 μm in alternative rows.

Solar system science with subarcsecond slit spectroscopy

During its first year of shared-risk observations, the PALAO/PHARO adaptive optics system has been employed to obtain near-infrared R approximately 1000 spectra of solar system targets at spectroscopic slit widths of 0.5 and 0.1 arcsec, and corresponding spatial resolution along the slit as fine as 0.08 arcsec. Phenomena undergoing initial investigation include condensate formation in the atmospheres of Neptune, and the Saturnian moon, Titan. We present the results of this AO spectroscopy campaign and discuss AO specific considerations in the reduction and interpretation of this data.