Thomas Stalcup

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.

Design and testing of a dynamic refocus system for Rayleigh laser beacons

We present the design, laboratory tests and preliminary field tests of a dynamic refocus system for 351nm Rayleigh beacon laser guide stars. The purpose of dynamic refocus is to increase the beacon signal from a pulsed laser, by maintaining focus in a fixed plane while the laser pulse travels through the atmosphere over an extended height range. The focusing element in our system is a moving concave mirror. The optics have been designed and built to focus on a ring of 5 beacons at 1 arc minute radius at the 6.5 m MMT, covering the range 18 through 40 km. Laboratory tests of image quality resulted in 0.5 arcsec refocused images corresponding to the height range 22 through 28 km, free from spherical aberration. Preliminary field tests were performed on the Mt. Bigelow Observatory 1.5 m telescope, with a frequency tripled, Q switched YLF laser beam projected from a 25 cm telescope. To simulate an off axis sub aperture of the MMT, the laser and telescope axes were set 3 m apart and reimaging optics were placed ahead of the refocus unit to image at the same plate scale as the MMT (500 μm/arcsec). Returns from different heights were selected by gating the detector with a Pockels cell. Returns over a 10 km height range from 8km to 18km were brought into focus for a total mirror motion measured to be 900 μm. The system is now ready for testing dynamic refocus, which will be accomplished by attaching the mirror to a metal resonator tuned to the laser pulse frequency. The range from 23 to 35 km to be used will require a motion of 500 μm.

Dish-based high concentration PV system with Köhler optics

We present work at the Steward Observatory Solar Lab on a high concentration photovoltaic system in which sunlight focused by a single large paraboloidal mirror powers many small triple-junction cells. The optical system is of the XRX-Köhler type, comprising the primary reflector (X) and a ball lens (R) at the focus that reimages the primary reflector onto an array of small reflectors (X) that apportion the light to the cells. We present a design methodology that provides generous tolerance to mis-pointing, uniform illumination across individual cells, minimal optical loss and even distribution between cells, for efficient series connection. An operational prototype has been constructed with a 3.3m x 3.3m square primary reflector of 2m focal length powering 36 actively cooled triple-junction cells at 1200x concentration (geometric). The measured end-to-end system conversion efficiency is 28%, including the parasitic loss of the active cooling system. Efficiency ~32% is projected for the next system.

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.

Commissioning the MMT ground-layer and laser tomography adaptive optics systems

A multi-laser adaptive optics system, at the 6.5 m MMT telescope, has been undergoing commissioning in preparation for wide-field, partially corrected as well as narrow-field, diffraction limited science observations in the thermal and near infrared. After several delays due to bad weather, we have successfully closed the full high order ground-layer adaptive optics (GLAO) control loop for the first time in February 2008 using five Rayleigh laser guide stars and a single tilt star. Characterization and automated correction of static aberrations such as non-common path errors were addressed in May 2008. Calibration measurements in preparation for laser tomography adaptive optics (LTAO) operation are planned for the fall of 2008 along with the start of shared-risk GLAO science observations. We present the results of GLAO observations with the PISCES imager, a 1 - 2.5 &mgr;m camera with a field of view of 110 arc seconds. The status of the remaining GLAO commissioning work is also reviewed. Finally, we present plans for commissioning work to implement the LTAO operating mode of the system.

Field tests of dynamic refocus of Rayleigh laser beacons

Dynamically refocusing the Rayleigh backscatter of a modestly powered laser beacon is a concept for increasing LGS brightness by 10 times. Dynamic refocus will allow for high photon return from multiple Rayleigh beacons enabling MCAO for wide field correction of the MMT and Magellan telescopes. In a system without dynamic refocus, light from a beacon integrated from 20 to 30km is blurred to a length of 14arcsecs. In a system with dynamic refocus, the bow tie is restored to a spot limited only by atmospheric seeing. The dynamic refocus system has been designed to deliver images with <3/4arcsec of induced aberration. This paper reports on field tests performed on the Mt. Bigelow Observatory 61” telescope, optically configured to appear as an off-axis sub-aperture of the 6.5m MMT. In these tests the Rayleigh backscatter from pulses of a Q-switched doubled Nd:YAG operating at 5kHZ was dynamically refocused. These preliminary tests present an uncorrected 7 by 3arcsec beacon image. The 7arcsec length is a result of using a field stop as the range-gating mechanism and the 3arcsec limit is due to double pass imaging (projecting and imaging) through the atmosphere in less than ideal seeing conditions. Upon correction, this 7x3arcsec image is dynamically refocused to a 3arcsec FWHM diameter spot.

On-sun performance of an improved dish-based HCPV system

The University of Arizona has developed a new dish-based High Concentration Photovoltaic (HCPV) system which is in the process of being commercialized by REhnu, Inc. The basic unit uses a paraboloidal glass reflector 3.1 m x 3.1 m square to bring sunlight to a high power point focus at a concentration of ~20,000x. A unique optical system at the focus reformats the concentrated sunlight so as to uniformly illuminate 36 triple junction cells at 1200x geometric concentration1. The relay optics and cells are integrated with an active cooling system in a self-contained Power Conversion Unit (PCU) suspended above the dish reflector. Only electrical connections are made to the PCU as the active cooling system within is completely sealed. Eight of these reflector/PCU units can be mounted on a single two axis tracking structure2. Our 1st generation prototype reflector/PCU unit consistently generated 2.2 kW of power normalized to 1kW/m2 DNI in over 200 hours of on-sun testing in 20113. Here, we present on-sun performance results for our 2nd generation prototype reflector/PCU unit, which has been in operation since June 2012. This improved system consistently generates 2.7 kW of power normalized to 1kW/m2 DNI and has logged over 100 hours of on-sun testing. This system is currently operating at28% DC net system efficiency with an operating cell temperature of only 20°C above ambient. Having proven this system concept, work on our 3rd generation prototype is underway with a focus on manufacturability, lower cost, and DC efficiency target of 32% or better.

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.

Shaping solar concentrator mirrors by radiative heating

Here we report a newly developed method for gravity sag molding of large glass solar reflectors, 1.65 m x 1.65 m square, with either line or point focus, and short focal length. The method is designed for high volume manufacture when incorporated into a production line with separate pre-heating and cooling. The tests reported here have been made in a custom batch furnace, with high power radiative heating to soften the glass for slumping. The mold surface is machined to the required shape as grooves which intersect the glass at cusps, reducing the mold contact area to <1%. Optical metrology of replicas made with the system has been carried out with a novel test using a linear array of coaligned lasers translated in a perpendicular direction across the reflector while the deviation of each beam from perfect focus is measured. Slopes measured over an array of 4000 points show an absolute accuracy of <0.3 mrad rms in sx and sy. The most accurate replicas we have made are from a 2.6 m2 point focus mold, showing slope errors in x and y of 1.0 mrad rms. The slump cycle, starting with rigid flat glass at 500C, uses a 350 kW burst of radiative heating for 200 seconds, followed by radiative and convective cooling.