- Initial evaluation of the performance requirements for a next generation telescope based on analysis of selected research areas which appear to exploit the potential discovery spaces for a 30-m class telescope;
- Identification, via analysis of a point design concept, of the key technical challenges that must be met in order to develop design concepts for a GSMT and its instrument complement; and
- Analysis of technical issues common to the design of all extremely large telescope concepts.
In this section, we outline the next steps in a decade-long process aimed at the design and construction of a next generation telescope via a partnership involving the US national observatory and one or more national and/or international collaborators. Our proposed plan is directly responsive to the recommendation of the Astronomy and Astrophysics Survey Committee (AASC) decadal survey that a GSMT be built via such a partnership, and that the National Optical Astronomy Observatory (NOAO) represent the US community in all phases of the program: design, construction, and operations.
With timely funding from federal sources and partnering institutions, this plan can meet the essential goal of having a GSMT in place and functioning in time to serve its central role in complementing the James Webb Space Telescope (JWST) and Atacama Large Millimeter Array (ALMA).
In the following sections, we outline the steps needed to:
- Understand the scientific context in which GSMT will operate - and as a consequence, the key science drivers for GSMT - to a depth sufficient to enable broad community agreement on a document describing the flowdown from science to GSMT design goals and requirements;
- Understand the role of GSMT in the "system" of US ground- and space-based observatories and to describe processes to begin integration of science and operations planning for the era of GSMT, JWST, and ALMA;
- Advance and coordinate the technical studies common to all ELT efforts. In addition, the Draft Plan for the GSMT Design develops a plan, schedule, and budget for the GSMT design and development project phase that is consistent with completion of GSMT within 10 years after project initiation.
6.1.1 FLOWDOWN FROM SCIENCE TO REQUIREMENTS
Chapter 2 outlines a selection of science programs that exploit the discovery spaces opened by a GSMT, while Chapter 3 summarizes the initial GSMT performance requirement targets that follow from our analysis of individual program needs. Although these programs reflect significant input from a broad cross section of the community, they must be viewed as the first iteration in a process leading to a community- endorsed set of science-based GSMT performance requirements and goals.
We propose here a process that builds on the efforts summarized in Chapter 2, and that leads to an initial science-to-requirements flowdown document. The New Initiatives Office (NIO) believes it essential that significant investment of federal funds - either early on in the process or later, during the operations phase - requires as a quid pro quo strong US community input in setting initial performance requirements and in guiding cost-performance tradeoffs during the design phase.
The first step in this process was the creation of the GSMT Science Working Group (SWG) at the request of the National Science Foundation (NSF). The SWG is chaired by Rolf Kudritzki of the University of Hawai`i Institute for Astronomy. Its membership is broadly representative of the US community and it includes individuals with research interests that link with those central to the JWST and ALMA missions.
Contributions of the SWG members and supporting NOAO scientists will outline the exciting science programs enabled by GSMT. Based on these scientific opportunities, and an understanding of the role of GSMT in the era of JWST, ALMA, and the Keck and VLT interferometers, the SWG will propose a priotitized list of performance goals and requirements. With NIO support, the SWG will develop a science requirements document that defines the top-level focus of GSMT science, and thus the range of basic GSMT design concepts that should be considered during the conceptual design phase.
The SWG will guide development of a "scientific merit function," as described in Section 5.7 for use in optimization studies during the conceptual design phase.
The SWG is also charged with advising the NSF regarding appropriate short- and long-term investments in technology and fabrication studies. The SWG may recommend staged implementation plans that would enable GSMT to meet the desired requirements over time.
. 6.1.2 GSMT AND THE SYSTEM OF US GROUND AND SPACE-BASED OBSERVATORIES
Upon its completion, GSMT will represent the apex of the pyramid of facilities comprising the US system of ground-based O/IR observatories, and will be an essential part of the suite of next generation observatories (including JWST, ALMA, SKA, and Constellation-X) spanning the full range of the electromagnetic spectrum. Time on GSMT will be highly coveted, and in order to make optimal use of its power, considerable community planning will be necessary. Key questions that must be addressed are:
- What are the surveys, pre-planning observations, and calibration observations required for efficient GSMT operations?
- How can the "smaller" telescopes in the US system be used to carry out these observations? Does providing such access require targeted federal investment in the 4-10-m class telescopes comprising the US system?
- What planning mechanisms are needed now to ensure that the current system evolves to complement and support GSMT?
- How will GSMT be integrated into programs requiring synoptic, target of opportunity, and other interrupt-driven observations?
- How will large, coherent science programs requiring joint access to multiple facilities on the ground and in space be planned, reviewed, coordinated, and funded?
- In view of the central role of GSMT in addressing frontier astrophysical problems, how should the planning of its instrument suite be tied to the needs of space- and radio observatories?
- What planning and funding mechanisms can ensure the timely availability of new capabilities that are essential to achieving key NASA mission objectives?
Although first light on GSMT lies more than a decade in the future, NIO believes that these issues require careful discussion now so that the community has adequate time to reach consensus on the changes (in relationships among the public and private observatories and between NASA and NSF) needed to accommodate GSMT in the era of next generation ground- and space-based observatories. Consequently, NIO plans during FY 2003 to organize a number of targeted community workshops aimed at defining the key issues embedded among the questions above.
6.1.3 NEAR-TERM TECHNICAL STUDIES
Many of the preceding sections of this "book" contain descriptions of "next steps." The following list provides links to these sections.
4.3. The Enclosure
4.4. Telescope Structure
4.5.B Image Motion and Image Quality of the GSMT Optical System
4.5.C. Conceptual Design of Primary Mirror Segment Support System of the GSMT Point Design
4.6.1. Adaptive Optics
4.7.1. Wide-Field Multi-Object Multi-Fiber Optical Spectrograph (MOMFOS)
4.7.2. Near-Infrared Deployable Integral Field Spectrograph (NIRDIF)
4.7.2.A OH Suppression - Current Status and Future Prospects
184.108.40.206. High Dynamic Range Imaging and the GSMT
220.127.116.11. Diffraction-limited Coronagraph
4.10. Cost Estimate
5.2. Site Testing and Selection
5.4. Cost Effective Mirror Segment Fabrication
5.5. Characterization of Wind Loading
5.6. Summary of AO Technology Requirements for GSMT
In selecting which tasks should be given priority at the present time, we will rely on the advice of the NIO Advisory Committee regarding relative emphasis and how best to structure the activities. In general, we plan to emphasize activities that: (1) are necessary to support the discussions on GSMT scientific context; (2) address key issues of technical feasibility; (3) show promise of enabling significant cost reductions; (4) have long lead times and will be needed relatively early in the GSMT project; and (5) are applicable to a number of design concepts and to several potential ELT programs.
The tasks that meet the above criteria can be roughly divided into three categories, which are listed below and then discussed in the following paragraphs.
- Further development of the point design
- Technology development studies of general value to ELT programs
- Development of tools and capabilities to prepare for the GSMT project
18.104.22.168. Further Development of the Point Design
The point design was created as a tool to serve several purposes, including:
- To identify fundamental technical challenges and their solution paths
- To understand the dependence of scientific performance on the design of the instruments and key telescope subsystems
- To evaluate the dynamic performance of a complete telescope when subjected to disturbances, including wind-buffeting and atmospheric seeing
- To identify cost-drivers and key cost-performance trade areas
- To develop the analytical models and tools critical to next-step studies
Sections 4.1 through 4.10 describe the studies that have defined and investigated the point design. Although the design is fairly complete, some further development is needed; this work will concentrate on fulfilling the purposes listed above, rather than trying to improve or refine the design. Efforts are planned in the following areas:
- Further elaboration of the structural models. The goal of this work is to better represent the rigid body degrees of freedom of the segments and secondary mirror, to allow development of integrated modeling capabilities that incorporate feedback and control of those degrees of freedom.
- Additional work on instrument concepts. Predicting the potential performance of instruments is part of the process to set scientific priorities, which requires evaluating a number of instrument concepts. This is an area where collaboration with other organizations is particularly beneficial. NIO plans to work with, and in some cases fund, universities and other scientific organizations that are developing conceptual designs of instruments for ELTs.
- Additional work to predict the potential performance of the AO systems. This is another area that is important to help inform the discussions of scientific priorities.
22.214.171.124 Technology Development Studies of General Value to ELT Programs
The required studies address three areas of concern: (1) feasibility of key technologies necessary to the success of GSMT; (2) technology developments needed for cost reduction; (3) efforts that require a long lead time. These are discussed below.
Several technologies must be extended in order for a 30-m telescope to fulfill its scientific promise. These mainly fall in the areas of active optics, adaptive optics, and instrument technology.
- Develop segment position control technology. A key challenge for a GSMT is to control the relative positions of the segments, in the presence of disturbances such as wind-buffeting, to tolerances of less than 100 nanometers. There are fundamental questions to be answered regarding sensors, actuators, and the design of the associated control systems. This is an area in which a great deal of work has been done by other groups, and NIO will make sure its efforts build on and are complementary to the work of others.
- Evaluate feasibility of fast tip-tilt-focus secondary mirrors. Another important challenge is to control the rigid body motion of the secondary mirror. Fast tip, tilt, and focus of the secondary mirror will be necessary to stabilize the image, and when working in the diffraction-limited regime at near infrared wavelengths, the angular tolerance for control of secondary mirror tilt will be of the order of a few milli-arcseconds. Needed studies include evaluation of system design issues, position sensors and actuators, reaction-compensation methods, and development of advanced control systems.
Needed technology developments in the field of AO are described in Section 5.6. These include the following:
- Develop new adaptive mirror technologies. This includes development of: adaptive secondary mirrors; large, high-order "conventional" (i.e., flat) deformable mirrors; high-order Micro Electrical Mechanical Systems (MEMS) deformable mirrors; and large tip-tilt mirrors. NIO plans to work with and support organizations and commercial firms that are developing these technologies.
- Develop high-speed, low-noise, large format CCDs. Detectors suitable for use in the high-order wavefront sensors needed for ELTs are not currently available. NIO plans to work with and support commercial firms that are developing appropriate detectors.
- Establish feasibility of sodium laser guide star. Multi-conjugate adaptive optics (MCAO) on an ELT will require artificial guide stars, and sodium laser guide stars are a strong candidate for this application. Further work is necessary to develop practical and affordable sodium lasers and to deal with the apparent elongation of the laser guide stars as viewed from different parts of the telescope aperture. NIO will work with other groups to develop technical solutions, and if possible, test these concepts on existing telescopes.
- Develop more efficient real-time control algorithms and electronics. Because of the high order of correction required in an ELT AO system, current real-time control methods are too computationally intensive for existing computers. NIO is working on enhanced wavefront reconstruction and control algorithms, and plans to work with commercial firms on the design of the associated electronics.
In all of these AO studies, NIO will collaborate and coordinate with other organizations in the field, particularly the Center for Adaptive Optics. NIO will continue to advocate studies in the areas most important for ELTs, and will lobby for additional funds through the implementation of the Adaptive Optics Roadmap. In all development activities, NIO will ensure that our efforts are complementary with those of other organizations.
As the instrument conceptual designs are developed, they highlight technology development challenges. Some of the challenges identified to date are listed below.
- Extend grating fabrication technology. Several enhancements are needed to (1) produce half- meter-sized volume-phase holographic (VPH) gratings, (2) develop two-dimensional mosaicing techniques for the fabrication of large surface-relief gratings, and (3) produce R10 echelle gratings for use in the near-IR, and possibly optical wavelength regimes. NIO plans to work with and support commercial firms to develop these fabrication technologies.
- Investigate the photometric stability of long fiber optics cables. NIO will conduct tests and measurements to quantify the photometric stability achievable when using 60-m long fiber optic cables.
- Investigate the feasibility of large cryostats. Some of the instrument designs, such as MIHDAS (Mid-IR high-dispersion AO spectrograph), require room-sized cryostats operating at 10-20 K. Other scientific applications (e.g., particle physics experiments) have used similarly large cryostats, but it is important to understand the technical difficulties, limitations, and costs. NIO plans to study and evaluate similar cryostats that have been used successfully.
126.96.36.199.2 Cost Reduction
- Develop cost-effective segment fabrication methods. This is a key cost-reduction area, with potentially tens of millions of dollars of savings possible. Two related aspects must be considered. First is an investigation of possible configurations for lightweight segments, which could include Keck-type meniscus face sheets on whiffletree supports, or structured silicon carbide segments on 3-point supports, for example. Second is the development of low-cost optical polishing and testing methods. There are several ELT groups interested in fabricating aspheric segments in the 1-2-m size range, so this is an area where collaboration is very beneficial.
188.8.131.52.3 Long Lead-time
- Site selection. Section 5.2 describes the continuing NIO program of site characterization and testing. Because of the nature of this work, which requires first the identification of the most promising candidate sites and then collection of data over a period of more than a year at each of the prime sites, the selection process will take several years. Tasks include remote sensing studies using satellite data, developing suitable site test equipment, modeling of airflow over candidate sites by computational fluid dynamics, and on-site measurements. This is an area where collaboration and cost sharing with other programs is very beneficial, and NIO is actively collaborating with other organizations in all aspects of this work.
Other long lead-time development efforts (e.g., sodium lasers, large deformable mirrors) have already been discussed in previous sections.
184.108.40.206 Development of Tools and Capabilities
- Integrated modeling. Because the performance of a system as complex as GSMT will be highly dependent on the ability of the active and adaptive systems to compensate for disturbances, that performance can only be adequately simulated by integrated modeling techniques that incorporate structural models, optical system models, and a simulation of the effects of the layered control systems. In recent years, NASA, aerospace corporations, and some astronomy organizations (e.g., (European Southern Observatory's (ESO) Very Large Telescope Interferometer) have developed software tools that allow integrated modeling of complex optical systems. This capability can provide essential insight into key technical issues that must be addressed on a system-wide basis, and it will be crucial during the conceptual design phase of GSMT to evaluate the performance of alternative concepts. Building on the work of other organizations and using existing software tools as much as possible, NIO plans to develop an in-house capability to model the performance of GSMT designs.
- AO modeling. NIO has been a leader in the development of advanced AO modeling capabilities. As described in Section 5.6, additional work is needed to be able to simulate the performance of the proposed AO systems. It will also be necessary to develop techniques to allow AO system models to work within an integrated modeling framework.
- Characterization of wind loading. Wind-buffeting is likely to be the most difficult disturbance of the telescope to deal with, and it could be a factor that would fundamentally limit the performance of GSMT. Therefore, it is important to be able to quantify the dynamic wind loading of a proposed telescope structure in support of integrated modeling of the telescope's performance. NIO has already done groundbreaking work in this area, as described in Section 5.5, with the help of collaborators from the University of Arizona, the University of Massachusetts at Lowell, Tennessee State University, and the Marshall Space Flight Center. However, more information is needed to predict the wind loading on a 30-m telescope in a particular enclosure, and NIO plans additional studies in the following areas: in-situ wind measurements; computational fluid dynamics modeling; and additional reduction of existing data. Wind-buffeting is an area of concern for all ELT programs, and NIO will encourage collaborations with other astronomy organizations through a workshop to be held early next year.
- Parametric cost estimating. As described in Section 5.7, parametric cost estimating is essential during the conceptual design phase of a design-to-cost project, because it can efficiently produce cost estimates for a number of different design concepts. However, the method requires research to collect cost information from similar programs in order to identify cost estimating relationships. NIO plans to work with other observatories and ELT development groups to collect and evaluate cost data from a number of large telescope projects and other similar endeavors.
The integrated modeling and parametric cost estimating tools that NIO is developing represent capabilities that can benefit design work on any ongoing efforts within the US community to explore different concepts for GSMT, thus enabling selection of an optimized concept that can meet the scientific aspirations of the community.
220.127.116.11 Role of the NIO Advisory Committee
This section has identified 18 technology development areas for GSMT. The key to moving forward on so many initiatives will be to combine resources with other groups having parallel interests. NIO plans to seek out mutually beneficial collaborations in all of the above-mentioned areas. The NIO Advisory Committee can play a crucial role by serving as a catalyst to help define and promote these collaborations.
In choosing which studies to emphasize, NIO will be guided by advice from the NIO Advisory Committee and by the consensus reached with collaborators.