Scott A. Severson

Demonstrating sub-nm closed loop MEMS flattening

Ground based high-contrast imaging (e.g. extrasolar giant planet detection) has demanding wavefront control requirements two orders of magnitude more precise than standard adaptive optics systems. We demonstrate that these requirements can be achieved with a 1024-Micro-Electrical-Mechanical-Systems (MEMS) deformable mirror having an actuator spacing of 340 microm and a stroke of approximately 1 microm, over an active aperture 27 actuators across. We have flattened the mirror to a residual wavefront error of 0.54 nm rms within the range of controllable spatial frequencies. Individual contributors to final wavefront quality, such as voltage response and uniformity, have been identified and characterized.

THE SOUTH POLE NEAR INFRARED SKY BRIGHTNESS

We report our finding that the South Pole is the darkest known Earth-based site for near infrared astronomical observations. For this reason it has great potentail for the most sensitive surveys of distant or faint objects. We find that the south polar sky background is substantially darker in the standard near infrared J, H, and K filters, and in an optimized KDARK filter centered at 2.36 microns. In particular, the KDARK background at the South Pole is only 162 ± 67 mu-Jy arcsec-2 at the zenith. This is consistent with the results described in an accompanying paper by Ashley et al. 1996, and is comparable to the sky brightness measured by high altitude balloon in the 2.4 micron (Matsumoto et al. 1994).

IRCAL: the infrared camera for adaptive optics at Lick Observatory

We describe the design, characterization and performance of the IR Camera for Adaptive Optics at Lick (IRCAL). IRCAL is a 1-2.5 micron camera optimized for use with the LLNL Lick adaptive optics system on the Shane 3 m telescope. Using diamond-turned gold-coated optics, the camera provides high efficiency diffraction limited imaging throughout the near- IR. IRCAL incorporates optimizations for obtaining high dynamic range images afforded by adaptive optics, coronagraphic masks, and a cross-dispersed silicon grism for high resolution spectroscopy.

Extreme adaptive optics testbed: performance and characterization of a 1024-MEMS deformable mirror

We have demonstrated that a microelectrical mechanical systems (MEMS) deformable mirror can be flattened to < 1 nm RMS within controllable spatial frequencies over a 9.2-mm aperture making it a viable option for high-contrast adaptive optics systems (also known as Extreme Adaptive Optics). The Extreme Adaptive Optics Testbed at UC Santa Cruz is being used to investigate and develop technologies for high-contrast imaging, especially wavefront control. A phase shifting diffraction interferometer (PSDI) measures wavefront errors with sub-nm precision and accuracy for metrology and wavefront control. Consistent flattening, required testing and characterization of the individual actuator response, including the effects of dead and low-response actuators. Stability and repeatability of the MEMS devices was also tested. An error budget for MEMS closed loop performance will summarize MEMS characterization.

Recent science and engineering results with the laser guidestar adaptive optic system at Lick Observatory

The Lick Observatory laser guide star adaptive optics system has undergone continual improvement and testing as it is being integrated as a facility science instrument on the Shane 3 meter telescope. Both Natural Guide Star (NGS) and Laser Guide Star (LGS) modes are now used in science observing programs. We report on system performance results as derived from data taken on both science and engineering nights and also describe the newly developed on-line techniques for seeing and system performance characterization. We also describe the future enhancements to the Lick system that will enable additional science goals such as long-exposure spectroscopy.

Wavefront control for the Gemini Planet Imager

The wavefront control strategy for the proposed Gemini Planet Imager, an extreme adaptive optics coronagraph for planet detection, is presented. Two key parts of this strategy are experimentally verified in a testbed at the Laboratory for Adaptive Optics, which features a 32 × 32 MEMS device. Detailed analytic models and algorithms for Shack-Hartmann wavefront sensor alignment and calibration are presented. It is demonstrated that with these procedures, the spatially filtered WFS and the Fourier Transform reconstructor can be used to flatten to the MEMS to 1 nm RMS in the controllable band. Performance is further improved using the technique of modifying the reference slopes using a measurement of the static wavefront error in the science leg.

Effect of wavefront error on 10^-^7 contrast measurements

Received October 11, 2005; accepted November 10, 2005; posted December 2, 2005 (Doc. ID 65234) We have measured a contrast of 6.5 x 10(-8) from 10 to 25 lambda/D in visible light on the Extreme Adaptive Optics testbed, using a shaped pupil for diffraction suppression. The testbed was designed with a minimal number of high-quality optics to ensure low wavefront error and uses a phase-shifting diffraction interferometer for metrology. This level of contrast is within the regime needed for imaging young Jupiter-like planets, a primary application of high-contrast imaging. We have concluded that wavefront error, not pupil quality, is the limiting error source for improved contrast in our system.

The extreme adaptive optics testbed at UCSC: current results and coronagraphic upgrade

We present a summary of our current results from the Extreme Adaptive Optics (ExAO) Testbed and the design and status of its coronagraphic upgrade. The ExAO Testbed at the Laboratory for Adaptive Optics at UCO/Lick Observatory is optimized for ultra-high contrast applications requiring high-order wavefront control. It is being used to investigate and develop technologies for the Gemini Planet Imager (GPI). The testbed is equipped with a phase shifting diffraction interferometer (PSDI), which measures the wavefront with sub-nm precision and accuracy. The testbed also includes a 1024-actuator Micro Electro Mechanical Systems (MEMS) deformable mirror manufactured by Boston Micromachines. We present a summary of the current results with the testbed encompassing MEMS flattening via PSDI, MEMS flattening via a Shack-Hartmann wavefront sensor (with and without spatial filtering), the introduction of Kolmogorov phase screens, and contrast in the far-field. Upgrades in progress include adding additional focal and pupil planes to better control scattered light and allow alternative coronagraph architectures, the introduction and testing of high-quality reflecting optics, and a variety of input phase aberrations. Ultimately, the system will serve as a full prototype for GPI.

Experimental demonstration of phase correction with a 32x32 microelectromechanical systems mirror and a spatially filtered wavefront sensor

A 32 x 32 microelectromechanical systems deformable mirror is controlled in closed loop with a spatially filtered Shack-Hartmann wavefront sensor and a Fourier-transform wavefront reconstruction algorithm. A phase plate based on atmospheric turbulence statistics is used to generate a 1 microm peak-valley static phase aberration. Far-field images and direct phase measurements of the residual are used to compare performance with and without the spatial filter. Use of the spatial filter reduces error in the controllable band from 20 to 6 nm rms. Residual phase power is reduced by more than a factor of 5 for all spatial frequencies up to 0.85 x 1/2d, with a maximum attenuation factor of 37.

Near-Infrared Halo Emission in the Edge-on Spiral Galaxy ESO 240-G11

In an extremely deep Kdark band (2.27-2.43 μm) image of the southern edge-on spiral galaxy ESO 240-G11, we detect halo emission extending to between 10 and 15 h−169 kpc away from the disk in vertical cuts near the nucleus. In vertical cuts taken well away from the nucleus, no halo emission is detected. To our detection limit, these data are well modeled by a spherically symmetric component having an exponential radial surface brightness profile plus a sech z disk. The exponential radial surface brightness profile suggests an unusually faint and extended spiral bulge. A ρ ∝ r-3.5 spheroid plus sech z disk is nearly as good a fit. It is also possible to fit these data with a ρ ∝ r-2 component tracing the massive halo. However, this requires a larger error in setting the sky level than appears likely. These data, which were taken with a 60 cm infrared optimized telescope at the South Pole, permit surface photometry reaching 25 mag arcsec-2. ESO 240-G11 is dynamically, environmentally, and morphologically similar to NGC 5907. In NGC 5907, several authors have detected red and near-infrared halo emission at comparable imaging depths that appears to trace the dark matter halo.