Miguel Snyder

Experimental results of ground-layer and tomographic wavefront reconstruction from multiple laser guide stars

We describe results from the first multi-laser wavefront sensing system designed to support tomographic modes of adaptive optics (AO). The system, now operating at the 6.5 m MMT telescope in Arizona, creates five beacons by Rayleigh scattering of laser beams at 532 nm integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics. The return light is analyzed by a Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate. A separate high-order Shack-Hartmann sensor records simultaneous measurements of wavefronts from a natural star. From open-loop measurements, we find the average beacon wavefront gives a good estimate of ground layer aberration. We present results of full tomographic wavefront analysis, enabled by supplementing the laser data with simultaneous fast image motion measurements from three stars in the field. We describe plans for an early demonstration at the MMT of closed-loop ground layer AO, and later tomographic AO.

Scientific goals for the MMT's multi-laser-guided adaptive optics

The MMTs five Rayleigh laser guide star system has successfully demonstrated open loop wavefront sensing for both ground-layer and laser tomography adaptive optics (AO). Closed loop correction is expected for the first time in the autumn of 2006. The program is moving into its second phase: construction of a permanent facility to feed AO instruments now used with the telescopes existing natural star AO system. The new facility will preserve the thermal cleanliness afforded by the systems adaptive secondary mirror. With the present laser power of 4 W in each of the Rayleigh beacons, we will first offer ground-layer correction over a 2 arcmin field in J, H, and K bands, with expected image quality routinely 0.2 arcsec or better. Later, we will also offer imaging and spectroscopy from 1.5 to 4.8 μm with a tomographically corrected diffraction limited beam. The development of these techniques will lead to a facility all-sky capability at the MMT for both ground-layer and diffraction-limited imaging, and will be a critical advance in the tools necessary for extremely large telescopes of the future, particularly the Giant Magellan Telescope. We describe the present state of system development, planned progress to completion, and highlight the early scientific applications.

Astronomical imaging using ground-layer adaptive optics

Over the past several years, experiments in adaptive optics involving multiple natural and Rayleigh laser guide stars have been carried out by our group at the 1.5 m Kuiper telescope and the 6.5 m MMT telescope. From open-loop data we have calculated the performance gains anticipated from ground-layer adaptive optics (GLAO) and laser tomography adaptive optics corrections. In July 2007, the GLAO control loop was closed around the focus signal from all five laser guide stars at the MMT, leading to a reduction in the measured focus mode on the laser wavefront sensor by 60%. For the first time, we expect to close the full high order GLAO control loop around the five laser beacons and a tilt star at the MMT in October 2007, where we predict image quality of < 0.2 arc seconds FWHM in K band (λ = 2.2 μm) over a 2 arc minute field. We intend to explore the image quality, stability and sensitivity of GLAO correction as a function of waveband with the science instrument PISCES. PISCES is a 1-2.5 µm imager with a field of view of 110 arc seconds, at a scale of 0.11 arc seconds per pixel. This is well matched to the expected FWHM performance of the GLAO corrected field and will be able to examine PSF non-uniformity and temporal stability across a wide field. FGD.

Tomographic reconstruction of stellar wavefronts from multiple laser guide stars

Experiments have been carried out at the MMT telescope in June 2005 and again in April 2006 to validate open loop tomographic wavefront reconstruction using five dynamically refocused Rayleigh laser beacons (RLGS) and multiple tilt natural guide stars (NGS). Wavefront sensing in this manner is recognized as a critical precursor to the development of adaptive optics for Extremely Large Telescopes. At the MMT, wavefronts from the laser beacons are recorded by five 60-element Shack-Hartmann sensors implemented on a single CCD. A wide-field camera measures image motion from multiple field stars to calculate global tilt and distinguish effects of contributions to second order aberrations from low and high altitude turbulence. Together, the signals from these sensors are used to estimate the first 45 Zernike modes in the wavefront of a star within the LGS constellation. The reconstruction is compared off line to simultaneous wavefront measurements made of the star with a separate Shack-Hartmann sensor. We will present the results in this paper and quantify the wavefront improvement expected from tomographic adaptive optics correction.

Progress towards tomographic wavefront reconstruction using dynamically refocused Rayleigh laser beacons

Simultaneous wavefront measurements are planned at the 6.5 m MMT telescope of five dynamically refocused Rayleigh laser beacons (RLGS) and a bright natural star to demonstrate tomographic wavefront reconstruction. In this paper, we summarize preliminary data recorded from the five laser beacons during the first telescope run at the MMT in June 2004. Beam projection is from behind the secondary of the MMT to form a regular pentagon of beacons on the sky with a radius of 60 arcseconds around the natural star. Beacon images are recorded over a range gate from 20 to 30 km, with dynamic refocus optics in the focal plane to remove perspective elongation (Stalcup, et. al., these proceedings). Separate externally synchronized Shack-Hartmann sensors record wavefront measurements of the beacons and the star, which will yield the first 33 Zernike modes from each wavefront measurement. A linear tomographic reconstructor, implemented as a matrix multiplication of the combined Zernike modal amplitudes from all five RLGS, has been computed to estimate contributions to the atmospheric aberration in two layers at 0 and 6 km. To validate the tomographic approach, the wavefront of the natural star will be predicted by computing the sum of the aberration in the direction of the star, and the prediction compared to simultaneous measurements recorded from the star directly.

Manufacture and use of a Shack-Hartmann sensor with a multifaceted prism for simultaneous sensing of multiple wavefronts

A new requirement for astronomical adaptive optics is the simultaneous measurement of wavefronts of multiple natural or laser guide stars. We have devised a new implementation of the Shack-Hartmann method to image multiple spot patterns on a single imaging array. An image of the telescope pupil is formed on a multifaceted prism with rings of subapertures. All beacons in the field are then imaged by a camera lens to form the same spot pattern repeated over the detector format. The facets are fly-cut in polycarbonate, tangent to a convex surface. In order to minimize scattering and aid manufacturing, the prism angles are exaggerated, and an index-matching fluid is used to reduce the refracted angles by a factor of 15. Results from lab and telescope tests are presented.

Optical design study for the 1-5 μm spectral band

The increased desire for multispectral infrared optical systems in compact packages significantly complicates the optical design of such systems. Add in the fact that multiple spectral bands are now imaging on the same detector, and the optical designs become quite challenging. The availability of materials over the desired multiple spectral bands that transmit well with the desired dispersion properties further complicates the design. Designing optics for good performance in the SWIR (1.0 - 2.0μm) and the MWIR (3.5 - 5.0μm) bands is an example where this challenge can be significant. Many of the preferred optical materials in the MWIR start to cut off prior to reaching 1μm, or have dispersions that are very difficult to control over this broad of a spectral band. Reflective designs are often limited because of packaging limitations. In this paper, multiple approaches are designed and examined to find the best balance between risk, performance, and size. The analyzed design studies include the use of traditional MWIR materials, harmonic diffractive lenses, and alternative materials that will require further development to be used in a tactical environment.

Compositional dependence of properties and lens performance of As-Se chalcogenide glass

The market for thermal imaging sensors and cameras has been increasingly focused on higher volumes and lower costs. Precision glass molding (PGM) is a high volume, low cost method which has been utilized for decades to produce lenses from oxide glasses. Due to the recent development of high quality precision-molded chalcogenide glasses, which are transparent at critical thermal imaging wavelengths, PGM has emerged as the enabling technology for low cost infrared optics. Since the price of germanium is high and volatile, it plays a large role in the high price of chalcogenide glasses that contain it. As40Se60 has previously been investigated as a lower-cost alternative to germanium-containing chalcogenide glasses and was found suitable for the PGM process. This paper investigates the composition-dependence of PGM-relevant properties for As38Se62 and standard As40Se60 and presents a comparison of molding behavior and lens performance.

Front Matter: Volume 9822

This PDF file contains the front matter associated with SPIE Proceedings Volume 9822, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.