instrument ports (as the current Gemini AO system Altair does)9, f, and maintains a minimal number of optical elements. The design includes an Adaptive ...
A Proposed Implementation of a Ground Layer Adaptive Optics System on the Gemini Telescope Kei Szetoa, David Andersena, David Cramptona, Simon Morrisb, Michael Lloyd-Hartc, Richard Myersb, Joseph B. Jensend, Murray Fletchera, W. Rusty Gardhousea, N. Mark Miltonc, John Pazdera, Jeff Stoesza, Doug Simonse, Jean-Pierre Vérana a NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, Canada b Department of Physics, University of Durham, Rochester Building, Science Laboratories, South Road, Durham, UK c Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ, USA d Gemini Observatory, 950 N. Cherry Avenue. Tucson, AZ, USA e Gemini Observatory, 670 North A`ohoku Place, Hilo, HI USA ABSTRACT We describe a simple and cost-effective concept for implementing a Ground Layer Adaptive Optics (GLAO) system on Gemini that will feed all instruments mounted at the Cassegrain focus. The design concept can provide a GLAO correction to any of the current or future seeing-limited optical or near-infrared Gemini instruments. The GLAO design uses an adaptive secondary mirror and provides a significant upgrade to the current telescope acquisition-and-guide system while reusing and building upon the existing telescope facilities and infrastructure. This paper discusses the overall design of the GLAO system including optics, opto-mechanics, laser guide star facilities, natural and laser guide stars wavefront sensors. Such a GLAO system will improve the efficiency of essentially all observations with Gemini and also will help with scheduling since it virtually eliminates poor seeing. Keywords: Adaptive Optics, Ground Layer, Adaptive Secondary, Laser Guide Star, Acquisition and Guide
1. INTRODUCTION Ground Layer Adaptive Optics (GLAO) offers the potential of improving image quality over very large fields of view (FOVs) under almost all atmospheric conditions, even at optical wavelengths.1-7 Although GLAO does not produce diffraction limited images, it increases the concentration of the point spread function (PSF) by sensing and correcting only the lowest turbulent layers of the atmospheres. Because the corrected layers are so close to the ground, the correction is expected to be the same over a large FOV while the remaining turbulent layers at higher altitudes degrade the spatial resolution isoplanatically.3 GLAO will benefit any astronomer using optical or near infrared seeing-limited instruments; a decrease in the mean Full-Width-Half-Max (FWHM) of the PSF will reduce required exposure times to reach a given signal to noise and allow more top tier science projects, which require good image quality to be completed, especially in a queue-scheduled observatory environment. The advantages a GLAO system can provide to an observatory led the Association of Universities for Research in Astronomy (AURA) in July 2004 to commission a team of scientists and engineers at the Herzberg Institute of Astrophysics (HIA), the University of Arizona (UA) and the University of Durham (UD) to conduct a feasibility study of a GLAO System for the Gemini Observatory. HIA was responsible for coordinating the overall design study, along with project management and system engineering responsibilities, and led the engineering development of the GLAO concept. UA coordinated the modeling trade study and organized the modeling effort among the three groups. The trade study provided effective means for exploring the performance parameter space and was a powerful optimization tool for determining the baseline GLAO design. The modeling effort by the three groups is described in detail in a complementary paper in this conference.8 UD led the important science case development which was used to identify the final science requirements and the initial operational concepts of the GLAO system. The science case development also consolidated the GLAO system performance characteristics required in terms of observational efficiency and scientific competitiveness.
The study consisted of the three distinct components described above: science case, modeling trade study and design trade study. While no truly unique science enabled by GLAO was identified, the science case process was very useful in realizing the “across-the-board” gains in image quality and efficiency gains for Gemini GLAO. The modeling tools, developed independently by the three groups, allowed a thorough GLAO modeling study to demonstrate these gains within a short period of time while working in parallel. Moreover, the modeling team also took time to institute consistency checks of each group’s modeling tools before going on to perform a large number of system performance trade studies. At the outset of this study, the feasibility of GLAO was not well-understood and our team was able to effectively “de-mystify” GLAO. The success of the GLAO system design for the Gemini Observatory was facilitated by the team’s extensive modeling capabilities that allowed the refinement of the scientific requirements based on realistic performance expectations. This was followed by a comprehensive design and system configuration trade study to optimize performance gain resulting in a compact and cost effective design that has minimal impact on the existing observatory infrastructure. The Gemini GLAO system design is well integrated into the Cassegrain-focus Instrument Support Structure (ISS) environment and is capable of servicing every Gemini instrument while not occupying one of the limited number of instrument ports (as the current Gemini AO system Altair does)9, f, and maintains a minimal number of optical elements. The design includes an Adaptive Secondary Mirror (ASM) as the Deformable Mirror (DM). The baseline design for the Gemini GLAO system also incorporates four low power sodium beacons (Laser Guide Stars; LGSs) launched using a modified version of the existing Gemini beam transfer and launch optics designed for the Multi-Conjugate Adaptive Optics (MCAO) systemg. The LGS Wavefront Sensors (WFSs) will be incorporated into a new acquisition and guide unit design which preserves the functionality of the current unit while adding the enhanced GLAO capabilities to Gemini.
2. SCIENCE CASE DEVELOPMENT A key component of the GLAO feasibility study was to further develop the science case given in the document “Scientific Horizons at the Gemini Observatory: Exploring a Universe of Matter, Energy and Life” which came out of the 2004 Gemini Aspen meeting.10 A science team was assembled to collaborate on this aspect of the study under the leadership of UD. All instrumentation science cases had to be developed in an iterative manner, with astronomers’ wish lists being compared with the predicted instrument performance from the modeling effort, and the hard reality of metal and glass. The novelty of GLAO made such iterations particularly necessary, while the relatively short timescale for the study also dictated that the number of iterations must be done as efficiently as possible. A workshop to develop the science case was held in Tucson. The timing was carefully chosen so that, at least, preliminary modeled performance results for a straw man GLAO system were available, but also so that the results of the workshop could be used to guide the overall design development. The workshop was meant to generate a science case document showing the gains from GLAO. Where possible, quantitative measures such as changes in required exposure times or improvements in astrometric measurement accuracies were computed and presented. An additional goal was to discuss the GLAO implementation and investigate whether a partial GLAO implementation was scientifically useful. Finally, the workshop was meant to generate a set of science requirements (based on the science cases) which would be used by the design team. The workshop successfully delivered on the above goals and the full science case was delivered to AURA in the final feasibility report.11 The science case for a GLAO System for Gemini has to demonstrate that substantial gains can be made over the current Gemini AO facilities. The science team did this by quantifying the shortening of exposure times, PSF uniformity over a large FOV (square 50 arcminutes), and improvements of measured science parameters for a range of science cases. The team focused on the ‘First Light’ science (see Figure 1) identified by the Aspen process, along with the gains in the study of stellar properties in our galaxy, but also included a range of other science cases to illustrate the wide range of applications which would benefit from GLAO. Even while these specific ‘Aspen’ science cases are addressed, a crucial feature emerged that every non-diffraction limited Gemini science proposal will benefit from a GLAO facility. f g j
http://www.gemini.edu/sciops/instruments/altair/altairIndex.html http://www.gemini.edu/sciops/instruments/adaptiveOptics/AOIndex.html The implementation of an ASM will enable high Strehl ratio mid-infrared observations.
Figure 1 The predicted numbers of “first-light” Lyman alpha emitting sources per square arcmin as a function of redshift and brightness (See Le Delliou et al.12). Predictions are made for three redshift ranges: 7.9-10.1, 11.4-13.8, 15.2-18.6 (top to bottom) for two escape fractions: 2% (solid lines) and 100% (dashed lines). A large survey for “first light” objects conducted over many square arcminutes will benefit from GLAO due to improved cumulative seeing distribution (Figure 2) and the improved signal to noise in shorter periods of time.
Figure 2: Cumulative histogram of FWHM measured in arcseconds based on the nine Cerro Pachòn model atmospheres13-14 for both seeing-limited (dashed lines) and GLAO (heavy solid lines) cases at a wavelength of 1.25µm, J-band. Simulations used 4 LGS, 17 actuators across the DM and a 10 arcminute FOV. A GLAO correction can alter the image quality statistics at a site; the proportionally greater improvement when seeing is worst (and presumably the ground layer turbulence is greatest) means GLAO can virtually eliminate bad-seeing nights; the poorest image quality occurring 30% of the time without GLAO will only occur ~10% of the time with GLAO.15
Image quality statistics at all wavelengths will be drastically improved by GLAO. Image quality conditions which occur only 20% of the time currently will occur 60-80% of the time with GLAO correction. These GLAO-improved seeing statistics will ensure that top ranked proposals which require good image quality will be successfully observed and, in general, will ease scheduling constraints and substantially improve the operational efficiency of the observatory. Similarly, the improved image quality also translates into shorter exposure times and an increase in the number of programs which can be executed. Gemini should realize a net 30 to 40 percent improvement in overall efficiency.15 Since the large GLAO-corrected FOV and the potential of having a deployable Integral Field Unit (d-IFU) spectrograph will make Gemini a more efficient survey telescope, two new Gemini science instruments were proposed: A 7x7 square arcminute imager to utilize the large GLAO-corrected FOV and a d-IFU spectrograph capable of observing several objects simultaneously within the GLAO FOV. The ‘flagship’ science surveys of large areas for first light objects can be done with much higher efficiency with GLAO. At lower redshifts, the d-IFUs combined with the improved GLAO image quality will produce a significant multiplexing advantage for surveys of velocity fields thus leading to better understanding of dark matter on galactic scales. The improved image quality and uniform PSF produced by GLAO will also facilitate proper motion studies within the Local Group. The science team also recommended a phased approach of GLAO implementation starting with an ASM serving Gemini’s strong contingent of Mid-Infrared instrumentsj, and eventually culminating with a proposed Multi-Object AO (MOAO) system feeding deployable IFUs. With this phased development of GLAO, they also felt that GLAO can be delivered in a staged manner, greatly reducing the schedule risk and telescope downtime, while delivering exciting science at each stage.
3. GLAO DESIGN REQUIREMENTS In general, the GLAO design requirements are derived from two categories: those that are dictated by the GLAO science case and those that are strategically planned based on the predicted performance. The science requirements are listed in Table 1 and they include considerations for establishing scientific competitiveness. The requirements reflect the overall scientific and strategic objectives and are intentionally developed independent of instrumentation design. In the absence of an actual instrument concept, contingency is incorporated into the science requirements and provides the future instrument builder with realistic design margin (e.g. the proposed 7x7 arcminute FOV imager). The primary scientific goal of the GLAO system is to provide moderate improvement in the image quality over a large FOV; the system is designed for multiplexing gains (compared to a classical AO system) and is optimized for maximizing S/N ratios, shortening exposure times for observations with existing instruments, and providing a sufficiently uniform PSF across a large FOV so as to enable greater astrometric accuracy. In addition to the science case, additional design drivers were formulated specifically to achieve scientific competitiveness even with the moderate improvement in image quality GLAO can provide. To be scientifically competitive among the other “Aspen Process” Gemini instruments identified, GLAO must attain observation efficiency by having a large FOV and be able to accommodate as many as possible of the current suite of relevant Gemini instruments; therefore, GLAO will be required to operate with a different AO light path configuration from the current Gemini AO facilities which are “fed” light from a 2.3 arcminute diameter AO fold mirror inside the existing telescope acquisition and guide unit. Based on the modeling results, the required image quality can only be met using a combined Laser Guide Star (LGS) and Natural Guide Star (NGS) wavefront sensing system even though an all-NGS system will likely meet the sky coverage requirement. Although the GLAO performance is slightly better with Rayleigh LGS, sodium LGS is adapted due the impending implementation of sodium LGS at Gemini. Therefore the baseline requirements for the GLAO system are to: •
Operate with a FOV as large as practical given the current telescope structural configuration; nevertheless, it must be greater than the current AO systems where FOV are limited by the size of the AO fold mirror (not to mention anisoplanatism).
•
Feed GLAO corrected light to all relevant Gemini instruments, in particular Flamingos-2 k and GMOSl.
k
http://www.gemini.edu/sciops/instruments/flamingos2/Flam2Index.html
•
Integrate “internally” within the telescope system so that, unlike Altair and MCAO, GLAO does not occupy an ISS mounting port.
•
Utilize LGS to achieve the required image quality specifications.
•
Operate in both GLAO and non-GLAO mode.
To achieve the above requirements, GLAO will incorporate a deformable ASM system, which also functions as the conventional secondary mirror for non-GLAO operation, with a hexapod system to provide low-bandwidth and highstroke X-Y translation, tip, tilt and focus adjustment. GLAO will also include a new Acquisition and Guide Unit (AGU) that incorporates all existing functionalities (acquisition, guiding, active optics control and reconfigurable instrument light paths) and new GLAO functionalities related to wavefront sensing, calibration and telescope operations. As much as possible, the GLAO system will utilize the existing Gemini LGSF facilities with its optical design modified to accommodate the larger 10 arcminute FOV. Table 1 GLAO System Science Requirements
Field of View Wavelength Range Delivered Image Quality: - Ensquared Energyn - PSF FWHMo PSF Uniformity and Stability - Uniformity across 10’ FOV - Stability during exposure Field Distortionp - Calibratable - Uncalibratable Total System Emissivity Total System Throughput Sky Coverage Observing Efficiencyq Compatibility with Science Instrumentsr Dithering Chopping
Science Requirements 50 arcmin2, with a goal of 70 arcmin2 0.8-2.5 µm, with a goal of 0.6-26mµm 50% improvement within a 0.2" square IFU element for median H-band conditions 0.35" in J band for median conditions
