The UnISIS adaptive optics system is now completed and ready for science observations. We describe the experience we have gained in building and using the system, and we give a preview of one new science goal: the use of Gaussian aperture pupil masks for high-contrast imaging of companion objects near bright stars. A key aspect of the UnISIS design is the simultaneous use of two wavefront sensors, one for natural stars and the other for the laser guide star. We demonstrate the performance of this calibration system with results from on-the-bench tests. We describe several practical aspects of observing at Mt. Wilson including our ability to predict the nights of best seeing with weather data available on-line. We also show how the laser guide star return signal is enhanced by observing at large zenith angles and compare this to Rayleigh scattering models.
We describe an "Origins Survey" that will provide a comprehensive picture of the era of galaxy formation and assembly. The survey data will allow us to develop and test models of when and how the first condensed objects in the universe are formed. We propose to do this by accumulating enough redshifts to have 10,000 galaxies of each of 20 types (defined empirically by the real state of galaxies) in each of 10 time zones of duration 1.5 Gyr each. Discounting the first two such zones which will be covered by the SDSS, the 2DF, and other surveys, our plan is to obtain redshifts for a total of 2 million galaxies. The hardware design is driven by the requirement to see the earliest galaxies (z ~ 10) and the capability to carry out this high z survey in an elapsed time of five years on a dedicated telescope. These considerations lead to a tentative design that uses a 20 - 40 meter diameter telescope with an Integral Field Unit (IFU) high-resolution spectrograph (R=6000 operating in the 1 - 2.5 micron spectral range. We require a 1 - 3 arc minute field of view with a modest adaptive-optics-corrected 0.2 arc-sec half power diameter point spread function (in the near-IR). Simultaneous, complementary observations will be made in the far-infrared/submm (350 - 850) microns to view the "hidden" starbursts known to exist from SCUBA data and the (non-CMB) infrared background. These observations require a low water vapor site. With appropriate instrumentation the same telescope can be used to study proto-planetary disks and star formation regions in the low z Universe. In this paper we present the scientific case for the survey, the basis for our requirements, and the results of our preliminary studies of how best to meet these goals.
Science commissioning of the UnISIS adaptive optics system is underway. In addition to showing test images from this effort, two other topics are discussed. These include a progress report on our continuing effort to improve the UV throughput of the UnISIS laser guide star projection and detection system and our effort to migrate the closed-loop computation engine from OS/2 to real time Linux. The two new improvements to the UV throughput of the laser guide star system involve new high-throughput prisms for the Pockels cell switch and a plan to increase the projected laser energy using anamorphic magnification in the laser beam as it emerges from the Excimer laser.
A laser guided adaptive optics system called UnISIS -- University of Illinois Seeing Improvement System -- has been commissioned at the Mt. Wilson 2.5-m Telescope. The UnISIS laser guide star is created via Rayleigh scattering of 351 nm photons from a 30 W excimer laser. The laser guide star is focused at an altitude of 20 km above msl. The UnISIS adaptive optics system sits at the fixed f/30 Coude focus of the 2.5-m telescope while the 30 W excimer laser sits on the observatory ground floor. The collimated laser beam is projected first into the Coude room where it is converted to f/30 and projected into the sky off the 2.5-m primary mirror. We describe the practical experience gained in installing and commissioning UnISIS, and we present simulations of the expected system performance based on the characteristic Cn2 distribution above Mt. Wilson.
Rayleigh laser guide star technology is discussed here with particular attention paid to the effects of laser pulse length, a parameter that becomes more significant to the design when telescope apertures are greater than 10 meters. After reviewing the relative return signal for Rayleigh versus sodium laser guide stars, a brief review of the pulse length characteristics of sodium lasers is given. Only one of the proposed sodium laser systems is pulsed while the others are CW. To insure star-like sources at the wavefront sensor with FWHM < 1.0 arcsec, lasers that will be most useful for Extremely Large Telescopes must have a short pulse format whereas CW lasers will be of little to no use. A relatively simple Rayleigh laser guide star method is described for Ground Layer Adaptive Optics (GLAO). This method provides a way to average out the effects of high altitude turbulence with a single Rayleigh laser guide star leaves intact the wavefront sign needed to correct ground-layer wavefront perturbations.
Final design details are given for UnISIS, the University of Illinois Seeing Improvement System. The principle components include a 50 Watt Excimer laser working at 351 nm which produces a pulsed Rayleigh laser guide star at 333 Hz and 18 km altitude, a 177-actuator deformable mirror, an atmospheric dispersion correction system, and a science camera configuration with both visual and near-IR cameras that can be used simultaneously. Two high-speed CCD cameras are used to feed dual quad C40 DSP-based reconstructor to close the feedback loop with the deformable mirror. The adaptive optics system is situated on a large optics table that rests above the old Coude spectrograph of the Mt. Wilson 2.5-m telescope and the Excimer laser is located in a basement Coude room.
A 50 watt excimer laser (lambda equals 351 nm) has been installed at the Mt. Wilson 2.5- meter telescope in California as part of the UnISIS adaptive optics system. This laser is used to produce Rayleigh guide stars 18 km above Mt. Wilson. In its initial configuration the projection optics are used to create a single laser guide star. The optical system is designed to allow an easy switch to accommodate three laser guide stars if (1) the laser return signal is sufficiently bright and (2) the laser guide star wavefront sensor has a read noise low enough to detect the split signal. The three guide stars are projected simultaneously in a triangular configuration above the telescope pupil. This three laser guide star system design is the first to confront directly the problem of focal anisoplanatism with an array of laser guide stars. The three guide star array provides a test for theoretical analyses of arrays of laser guide stars which will be an inevitable part of the adaptive optics systems of 8-meter and 10-meter class telescopes.
The relative wander in the positions of the laser beacons in a multiple beacon system limits the accuracy to which the wavefront can be measured. We describe the design of an experiment to test methods for reducing laser beacon wander. The system employs an excimer laser producing Rayleigh guide stars at an altitude of 10 km mounted at the Coude focus of the Mt. Laguna 1 m telescope. The experimental tests are based on simultaneously creating two guide stars and measuring their apparent differential motion in a set of projection and detection schemes that includes: full aperture broadcast, partial aperture broadcast, full aperture reception, and partial aperture reception.
Our group in the Astronomy Department at the University of Illinois is assembling an adaptive optics system that will be installed at the Coude focus of the Mt. Wilson 2.5-meter telescope. The aim of the program is to provide the means to test experimentally, at an astronomically excellent site, many of the newer concepts in laser guided adaptive optics. To accelerate the experimental effort, the system will be built, in part, from existing hardware and from design information provided by U.S. military research groups. Also in the spirit of expediency, the first laser guide star system to be installed at Mt. Wilson will be a single Rayleigh beacon produced by an excimer laser which also exists and has been tested elsewhere. The program as described here is neither final nor fixed because the hardware, the technology, the system configuration, and the ideas that drive it will continue to evolve during the five year duration of the project.
Experiments have been conducted in the atmosphere above Mt. Laguna
Observatory to measure the properties of laser guide stars. The experimental
system consists of a high frame rate video camera which records the
backscattered light from an Excimer laser working in the near-UV at 351 rim.
The Mt. Laguna 1-meter telescope is used to both transmit the outgoing beam and
to image the return beam. The outgoing laser pulse triggers a time-gated image
intensifier within the video camera which, with an appropriately selected time
delay, records a time slice of the backscattered return signal. Preliminary
results from the experiment can be used to calibrate the laser power needed to
operate a large ground-based adaptive optics telescope.