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Development underway on next-generation ground-based telescope

The Giant Magellan Telescope, an extremely large, segmented-mirror instrument, should help answer leading astronomy and astrophysics questions in the coming decade.
25 July 2006, SPIE Newsroom. DOI: 10.1117/2.1200607.0248

Based on tantalizing glimpses of distant solar systems, the early universe, and dark energy's influence on visible matter, the coming decade holds great promise for our understanding of the universe. In its astronomy and astrophysics decadal survey published in 2000, the National Academy of Sciences identified the scientific goals and instrumentation needed to achieve this promise. The survey ranked a giant segmented-mirror telescope as the highest-priority ground-based project. The proposed Giant Magellan Telescope (GMT) is an effort by a consortium of universities and research institutions to fill this need. The telescope will combine the latest innovations in adaptive optics with proven technology developed for the current generation 6–10m ground-based telescope.

The GMT, shown in Figure 1, is designed with a primary mirror that consists of seven 8.4m circular segments in an altitude-azimuth mount.1 The segments provide the equivalent light collecting area of a single 21.9m circular mirror with 16 milliarcsecond diffraction-limited IR resolution (H band, centered on 1.65μm) using adaptive optics. The fast f/0.7 focal ratio of the primary mirror permits a compact and stiff structure that reduces the cost of the telescope and its enclosure (shown in Figure 2), and resists wind buffeting that can blur images. A concave, segmented, 3.2m diameter secondary mirror reflects the light from the primary mirror to the ‘Gregorian’ focus below it. The seven secondary segments are aligned with the segments in the primary.

Figure 1. The proposed Giant Magellan Telescope (GMT) features seven mirror segments.

Figure 2. The GMT and its enclosure are shown in this rendering.

The techniques for making the fast, highly aspheric, on-axis 8.4m mirror, although demanding, are well developed.2 The six off-axis segments will be more challenging because of their non-circularly-symmetric surfaces. The surface height variation around the edges of the off-axis segments is 13mm peak-to-valley. No mirror of comparable size and difficulty has yet been made with the added complication that the radii of curvature of the seven mirrors must match to 1 part in 120,000. The key to fabricating these optics will be the ability to accurately measure the surface figure while polishing with a stressed lap.3 To accomplish this, four independent and redundant tests will be used during the polishing phase.

The principal test, shown in Figure 3, will use an interferometer at a segment's center of curvature. A 3.8m-diameter fold sphere at the top of the test tower will remove most of the off-axis waveform's astigmatism. A second fold sphere, together with a computer-generated hologram, will correct the residuals. Maintaining the optics' alignment during the test will be critical. A laser tracker and distance-measuring interferometers will be used to set the positions of the optics.

Figure 3. The principal off-axis test to be used during mirror polishing is shown. CGH: computer-generated hologram. (click to enlarge)

Two prototype off-axis segments are currently in production at the Steward Observatory Mirror Laboratory in Arizona. The first—a nearly-completed 1/5 scale model of a GMT segment—is being finished with the same polishing techniques and metrology that will be used for the real segments, but without the tight radius of curvature tolerance. This prototype will used as the primary mirror of the New Solar Telescope at the Big Bear Solar Observatory.

The second prototype will be the first full-size GMT segment. The 8.4m mirror blank, shown in Figure 4, has been cast, and preparations are underway for polishing it. The purpose of this effort is to completely develop the technology and production pipeline for these key project components.

Figure 4. Shown is an 8.4m-diameter GMT off-axis cast-mirror blank.

Adaptive optics (AO) will provide diffraction-limited performance at wavelengths greater than 1μm. The AO system will consist of an adaptive secondary mirror with ∼4,700 actuators; a six-laser projection system for creating artificial guide stars in the sodium layer of the atmosphere above the telescope; and wavefront and guide sensors in the focal plane for closing the loop.4 The secondary mirror will be scaled up from the adaptive versions developed for the Multiple Mirror and Large Binocular telescopes. It will replace the non-adaptive secondary mirror used during telescope commissioning.

Conceptual designs for five instruments that address GMT's science goals are complete.5 These include: wide-field multi-object spectrographs for the visible and near-IR bands; mid-IR imaging and near-IR high-resolution spectrometers; and a near-IR AO imager. The instruments mount below the primary mirror on a platform with a turntable to remove image rotation from the altitude-azimuth tracking. Beam-directing mirrors will allow rapid instrument swapping.

GMT will be located in northern Chile's Atacama desert, where three candidate sites are being tested at Las Campanas Observatory. Full science operations are expected at the start of 2016.6

Matt Johns
Carnegie Observatories
Pasadena, CA