Technical Overview Section 3

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1 Technical Overview Section 3

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3 Table of Contents Overview Site System Architecture Enclosure & Facilities Telescope Adaptive Optics System Instrument Locations Control System Telescope Observing Modes Optical System Optical Design Primary Mirror Secondary Mirrors Tertiary Mirror Corrector-ADC Optical Configurations Wavefront Control System Telescope Pier Azimuth Track Azimuth Structure Optical Support Structure Lower OSS Structure Gregorian Instrument Rotator Upper OSS Structure Instrument Stations Direct Gregorian Folded Ports Instrument Platform Stations Auxiliary Ports Gravity Invariant Station AO System Adaptive Secondary Mirror Wavefront Sensors Direct Feed Architecture Natural Guide Star Wavefront Sensor Laser Tomography Wavefront Sensor On-Instrument Wavefront Sensor Ground-layer AO Laser Guide Star Facility Phasing System Instrumentation Instruments and the Staged Implementation of GMT Selecting the First Generation Instruments Stage 1 Instruments G-CLEF Description GMACS Description Stage 2 Instruments

4 3.8.5 Stage 3 Instruments GMTNIRS Description MANIFEST Description Enclosure and Facilities Site Layout Enclosure Building Control System Telescope Control System Observatory Operations System Observatory Services Site Characterization Seeing Wind Temperature Humidity Water Vapor Turbulence Profile Site Specific Hazard Analysis References

5 3 3.1 Overview The Giant Magellan Telescope (GMT) is a 25-meter, next generation optical/infrared telescope being developed for the purpose of conducting forefront scientific research in general astrophysics, cosmology, and the study of extrasolar planetary systems. The telescope is built around a segmented primary mirror composed of seven 8.4 m diameter circular mirrors with a collecting area nearly an order of magnitude larger than the largest single apertures now in operation. The GMT optical design is a fast, wide field (20 diameter) aplanatic Gregorian with a plate scale of 1.arcsec/mm at the f/8.2 focus. Adaptive optics (AO) is integral to the GMT, which incorporates a deformable secondary mirror with heritage in the MMT, LBT, VLT, and Magellan AO systems. With an angular resolution of 10 mas at 1 µm, the GMT will image exoplanet systems, probe the sphere of influence around supermassive black holes, and explore distant galaxies on the scale of giant molecular clouds. The GMT will be located at Las Campanas Observatory in Chile, known for its excellent conditions. The GMT project had its seeds in a series of discussions among the original partners in the twin Magellan 6.5 m telescopes, interested parties outside the Magellan community, and international groups, around the years A number of groups were considering the scientific capabilities of a next generation large telescope and a variety of technical approaches to apertures beyond eight to ten meters were being considered. In Astronomy and Astrophysics in the New Millennium 1, the NRC report from the 2000 decadal survey, the scientific case for a large aperture telescope, as it was then understood, was clearly articulated. The case was made even more strongly in the 2010 decadal survey report entitled New Worlds, New Horizons in Astronomy and Astrophysics 2. The GMT addresses these goals and more, through its extreme sensitivity and unmatched angular resolution. 3.2 Site The GMT will be located on Cerro Campanas at Las Campanas Observatory (LCO). LCO is at the southern end of the Atacama Desert, approximately 170 km north of the coastal city of La Serena. Observing conditions at LCO feature a high fraction of clear nights and excellent seeing. These conditions are documented in the final report 3 of the site characterization campaign and borne out through many years of observatory operation. The site environmental conditions are described in Section Figure 3-1 shows LCO with Cerro Campanas on the left, and the Manquis ridge to the right where the Magellan 6.5 m telescopes, DuPont 100-inch, and Swope 40-inch telescopes are located. The distance from the LCO lodge and Magellan telescopes to Cerro Campanas is roughly 7 km. The road leading up to the site (seen mid-left in Figure 3-1) branches off from the access road from the Pan American Highway (Route 5). The summit of Cerro Campanas has recently been leveled in preparation for construction of the GMT. The resulting platform is m in size, with an elevation of 2518 m above sea level. 3 5

6 Figure 3-1. Las Campanas Observatory site 3.3 System Architecture The GMT observatory at Las Campanas consists of the telescope and associated subsystems, the adaptive optics system, science instruments, control systems, the enclosure and support facilities, and infrastructure. The major components of the GMT are identified in this section, and their functions and interfaces explained. The remaining subsections provide an overview of these systems, with more details presented in the following sections Enclosure & Facilities The telescope enclosure and support facilities, illustrated in Figure 3-2, will be sited at the summit of the mountain and accessed by an existing road leading up to the site. Figure 3-2. The Cerro Campanas GMT site The enclosure protects the telescope from the elements in daytime, and must open to allow the telescope an unobstructed view of the sky at night. It must shelter the telescope from wind and moonlight during observation, and provide handling facilities such as cranes, elevators, and access platforms. The enclosure also provides a stiff ~10 m high pier upon which the telescope is mounted, to raise it above the most disturbed ground-layer wind. 3 6

7 The summit support building provides the facilities for operating and maintaining the telescope and associated systems. It provides offices and lab space for the technical staff and visitors, machine shops and assembly areas for telescope equipment and instrumentation, space for the primary mirror wash station and coating plant, and the secondary mirror service and calibration station. It also houses the electrical and mechanical equipment (pumps, compressors, service panels, and chillers) that provide utilities to the telescope. The control system servers and operator consoles are also located in this building. Additional support facilities are located below the summit. These include the connection to the commercial power grid and GMT emergency generators, a warehouse, and a maintenance shop. GMTO staff and visitors will stay in a lodge below the maintenance area. The enclosure and facilities are described briefly in Section 3.9 below and are detailed in Section 7 of this report Telescope The telescope (Figure 3-3) collects the light from astrophysical sources and brings it to one of several science foci at which instruments are located. The alt-az mount points the steerable upper structure of the telescope, the Optical Support Structure (OSS), at objects on sky over the full 360 deg range in azimuth and elevation angles above 30 deg with a 1 deg diameter exclusion zone at zenith. The telescope optics and most instruments are installed in the OSS. Figure 3-3. The Giant Magellan Telescope The GMT aplanatic Gregorian optical design is based on a primary mirror (M1) composed of seven circular 8.4 m diameter segments, and an identically-segmented secondary mirror (M2). Either of two secondary mirror assemblies can be installed: a Fast-steering Secondary Mirror (FSM), or an Adaptive Secondary Mirror (ASM). A Corrector-ADC can be inserted ahead of the direct Gregorian focus to extend the useable field of view from 10 to 20 arcmin diameter and provides atmospheric dispersion compensation over the wavelength range 370 nm to 1µm. 3 7

8 Up to 11 instruments may be installed in the telescope simultaneously. Beam switching is used to select between them. An instrument rotator within the OSS is provided to compensate field rotation caused by alt-az tracking of the mount. The Gregorian Instrument Rotator (GIR) is supported below the primary mirror and rotates about the telescope optical axis. Instruments at the direct Gregorian focus mount inside the GIR. A deployable flat tertiary mirror (M3) directs the beam to additional focal stations on the GIR top surface. Other instrument stations are fed with optical and fiber-optic relays from the direct and folded Gregorian foci but may not take advantage of field derotation using the GIR. Adaptive optics is an integral part of the telescope design. The optical components of the adaptive optics system including the ASM, AO wavefront sensors and the Laser Guide Star Facility (LGSF) are installed the OSS. Natural seeing and adaptive optics observing modes are integrated in the Telescope Control System (TCS). The TCS provides target acquisition, pointing and tracking of the mount and GIR, and real-time control of the active optics system. The active optics system maintains the focus and alignment of the telescope optics and sends surface figure corrections to the primary mirror controller and ASM to control image quality. The cameras and sensors for acquiring targets on the sky, tracking the mount, and providing wavefront information to the active optics system are part of the Acquisition, Guide, and Wavefront Sensing (AGWS) assembly. De-stacking of the images from the seven subapertures and windshake are also sensed and sent to the secondary mirror fast tip-tilt control. The AGWS wavefront sensors also provide feedback to the AO system and for ground-layer AO. It is mounted in the top plate of the GIR. The telescope system provides service platforms and stairs, mounting locations for electrical and mechanical equipment, utility routing, and other telescope mechanisms (bearings, drives, encoders, counterweights, stow pins, mirror covers, and baffles) necessary for operation. Fixtures and procedures for assembling and servicing are also included. A coating plant is provided for applying reflective coatings to the mirrors. An overview of the telescope is provided in Sections 3.5 and 3.6, and it is described in detail in Section 6 of this report Adaptive Optics System The adaptive optics (AO) system sharpens the images delivered to science instruments by correcting wavefront errors introduced by the atmosphere. The primary wavefront corrector is the adaptive secondary mirror (ASM), which can be installed in place of the fast-steering secondary mirror. The AO system is fully integrated with the control system of the telescope. Signals from AO wavefront sensors observing either natural or artificial laser guide stars are combined with signals from instruments, the telescope active optics wavefront sensors, and edge sensors between the mirror segments to compute the commands for the ASM. The operation is coordinated by the TCS. The AO system can operate in three observing modes: Ground-Layer AO (GLAO) corrects only the lowest altitude turbulence, providing a factor of ~2 image size improvement over wide fields of view (up to 10 arcmin) in the near-infrared, with more modest improvement in the visible. The GLAO mode is implemented using natural guide stars sensed by the telescope active optics wavefront sensors, and can therefore be used by any instrument, at any focal station. Natural Guide Star AO (NGSAO) relies on a single bright natural guide star to provide high Strehl, diffractionlimited images over a narrow field of view surrounding the star. Laser Tomography AO (LTAO) uses six artificial laser guide stars and a single faint natural guide star to extend diffraction-limited performance to nearly the full sky, but with lower Strehl. The NGSAO and LTAO modes are 3 8

9 implemented using wavefront sensors replicated for each client instrument that analyze light reflected from a dichroic instrument window. An overview of the AO system is provided in Section 3.7, and it is described in detail in Section 8 of this report Instrument Locations Science instruments mount on the moving structure of the telescope, utilizing either the direct Gregorian focus (2 reflections), or a folded Gregorian focus provided by a steerable M3 (3 reflections). Some wide-field instruments also require the use of the Corrector-ADC. Instruments can be mounted at five possible stations (Figure 3-4). The Folded Port (FP) on the top surface of the GIR can accommodate up to 3 narrow-field instruments fed by the facility tertiary mirror. The Direct Gregorian (DG) focal station inside the GIR can accommodate up to 4 widefield instruments, which are shuttled into the on-axis position when in use. The GMT does not include a Nasmyth platform, but the Gravity Invariant Station (GIS) on the azimuth structure provides a mounting location for large instruments that require high flexure stability, for example, a precision radial velocity spectrograph. A fixed Instrument Platform (IP) station and two auxiliary ports on the outer surface of the C-rings are also available. Figure 3-4. Instrument mounting locations (auxiliary ports not shown) Utilities are provided to the instruments by the GMT facility. Instruments are installed using the enclosure crane or the lift in the center of the pier. Assembly areas and labs for instrument service are provided in the summit support building. Section 3.8 gives an overview of the GMTO instrumentation program, and the instruments are described in detail in Section 9 of this report. 3 9

10 3.3.5 Control System The Software and Controls System (SWC) encompasses the software and hardware components necessary to control and monitor the GMT optical and electromechanical subsystems and to safely and efficiently operate the observatory. The SWC can be divided into three domains: The telescope control system, which provides all control functions for the enclosure; telescope; and AO system, including wavefront control of the telescope. The observatory operations system provides the capabilities to support efficient operations and workflows of the observatory, such as the sequencing of observations. A set of observatory services underlie both of these, providing common infrastructure components such as user interfaces, logging, telemetry, and configuration management. The hardware and software components of the SWC will be deployed in standard insulated and cooled electronics racks on the telescope, and in the computer room in the summit support building. The system uses an industrial field bus, allowing devices to be controlled either remotely or locally as appropriate, with only Ethernet communication onto the telescope. A fiber-optic network provides connectivity between the two locations, for both standard control and low-latency applications. An independent Interlock and Safety System (ISS) provides facility-wide interlocks, alarms, and emergency stops using a certified safety network. Section 3.10 gives an overview of the control system, and a detailed description is provided in Section 10 of this report. 3.4 Telescope Observing Modes The GMT is designed to operate in a variety of observing modes over a wavelength range starting at the atmospheric cut-off around 320 nm in the UV and extending up to 25 µm in the IR. Four observing modes are supported: 1. Natural Seeing This mode operates over the full wavelength range and delivers images to the science instruments with image sizes limited by wavefront distortions due to the atmosphere. 2. Ground Layer AO (GLAO) This mode delivers image quality improvement at all wavelengths and over wide fields of view, by correcting low-altitude atmospheric turbulence using natural guide stars. It is available to all instruments on the telescope, regardless of their focal station. 3. Natural Guide Star AO (NGSAO) This mode uses a single bright natural guide star to deliver high Strehl ratio (>75% K band), high contrast images at near-ir and IR wavelengths over a field of view limited by natural anisoplanatism. 4. Laser Tomography AO (LTAO) This mode delivers moderate Strehl ratio (>30% H band) at IR wavelengths with broad sky coverage and field of view limited by natural anisoplanatism. It uses multiple laser guide stars and a single faint natural guide star to achieve high sky coverage. To deliver diffraction-limited images in the NGSAO and LTAO observing modes, the GMT mirrors must be phased to a small fraction of the observing wavelength. Since both M1 and M2 are segmented, the average optical path of each M1-M2 segment pair must be equalized. This is done using a combination of wavefront sensors and mechanical displacement sensors along the edges of 3 10

11 the mirror segments. In LTAO mode, the edge sensors are used to detect high-frequency disturbances in the mirror arrays, while the wavefront sensors compensate any long-term drifts in these measurements. In NGSAO mode the wavefront sensors compensate for all phasing errors. The adaptive secondary mirrors, being more agile than the large primary mirror segments, provide the means to keep the telescope phased. 3.5 Optical System Optical Design The GMT optical design is based on an aplanatic Gregorian optical prescription with segmented primary and secondary mirrors. Figure 3-5 shows the layout of the segments. The aperture of the telescope is 25.4 m with an f/0.7 primary focal ratio and f/8.2 final focal ratio. The fast optical system contributes to the compactness of the telescope structure. The m focal length of the two mirror system produces a 1.2 m diameter focal plane with a 20 arcmin diameter field-of-view. The wavelength range is from 320 nm to 25 µm with the baseline aluminum mirror coatings. Figure 3-5. GMT optical layout The basic Gregorian two-mirror optical prescription provides a focus 5.8 m below the primary mirror vertex. Science instruments, guide sensors, and wavefront sensors will be located in this general area. The direct Gregorian (DG) focus has a field of view of 20 arcmin diameter with a 10 arcmin diameter well-corrected field, limited outside of that by field aberrations Primary Mirror The primary mirror is composed of seven 8.4 m circular segments. Six of the seven segments are off-axis. They all share a common parent optical surface. Together the segments provide a total collecting area of 368 m 2 and define the seven subapertures of the telescope. The segments are 3 11

12 being cast, generated, and polished at the University of Arizona, Steward Observatory Mirror Lab (SOML). The first off-axis segment, GMT1, was cast in July 2005, and polishing was completed in October 2012 (Figure 3-6). With 14 mm of aspheric departure peak-valley, these off-axis mirrors are by far the most highly aspheric optics of their size and precision ever made. Fabricating them required the development of a new metrology system including redundant surface figure measurements using independent equipment. Much of the effort in producing GMT1 was associated with the development of the metrology and procedures that are now in place for subsequent GMT segments 4. Three additional segments are currently in the production queue. Figure 3-6. Polishing GMT1 The primary mirror segments will be removed from the telescope in their cells for periodic recoating in order to maintain high reflectivity surfaces and meet the throughput requirement of the system. An additional off-axis mirror will be provided for a total of eight mirror segments. This additional off-axis segment will allow fast turn-around and minimize telescope downtime. The segments will be washed in situ between re-coatings Secondary Mirrors Two secondary mirror assemblies with the same optical prescription are being provided for the GMT. The fast-steering secondary mirror (FSM) consists of seven light-weighted, rigid 1.1 m diameter mirror segments. Fast tip-tilt capability is provided to compensate for telescope wind shake and mechanical vibrations up to around 10 Hz and provide fine alignment control for stacking the images of the seven subapertures. The adaptive secondary mirror (ASM) shown in Figure 3-7 consists of seven 1.1 m segments with deformable front surfaces. The individual segments of the ASM are similar in scale and design to the adaptive secondary mirrors developed for the MMT, LBT, Magellan, and VLT 5. In the case of the GMT, each ASM segment will have 672 electromagnetic actuators supporting a 2 mm thick face sheet, which provides the deformable surface for AO wavefront control. Seven segments make up the full ASM for a total of 4,704 actuators. The ASM will be used for all of the GMT AO observing modes, and can also support the natural seeing mode. 3 12

13 Figure 3-7. Adaptive secondary mirror (ASM) and top frame The segments of the ASM and FSM are each supported by a set of six position actuators providing individual alignment and focus control. Each secondary mirror will be mounted in a frame that attaches to the main truss and comprises the top frame assembly of the telescope. The two secondary mirror assemblies, FSM and ASM, will be exchanged by removing the top frame from the telescope using the enclosure overhead crane. The ASM will normally be used for both natural seeing and AO operation. The FSM is the telescope s commissioning secondary mirror, and it will be used when the ASM is removed for service Tertiary Mirror A deployable and steerable tertiary mirror 3.9 m below the M1 vertex will direct a 3 arcmin diameter beam to folded port instruments (Figure 3-8). It will be coated with a Gemini-like silver based coating to optimize throughput and minimize emissivity at visible and infrared wavelengths. The tertiary mirror will be mounted on the Gregorian instrument rotator (GIR) described in Section Figure 3-8. Tertiary mirror and GIR 3 13

14 3.5.5 Corrector-ADC The Corrector-ADC provides a well-corrected 20 arcmin diameter field of view over the wavelength range 370 nm to 1 µm (Figure 3-9). It also compensates for atmospheric dispersion at those wavelengths. The Corrector-ADC mounts on rails in the center primary mirror cell allowing it to be deployed in and out of the beam. The field lens for the corrector (L2) is located in the science instrument just above the Direct Gregorian Wide Field (DGWF) focus. The wide-field focus is 177 mm below the bare DG focus. Figure 3-9. Corrector-ADC Optical Configurations The three GMT optical configurations, shown in Figure 3-10, are: Direct Gregorian Narrow Field (DGNF). This two mirror (M1/M2) combination delivers the beam to the nominal Gregorian focus with two reflections. Vignetting limits the field of view to 20 arcmin diameter but field aberrations become significant for field angles beyond ~5 arcmin radius. Folded Port (FP). The tertiary mirror directs the beam to instruments mounted on the top surface of the Gregorian instrument rotator. The unvignetted field of view is 3 arcmin. Direct Gregorian Wide Field (DGWF). A 20 arcmin well-corrected field is delivered to the direct Gregorian focus with the Corrector-ADC inserted in the beam. The optical pick-off and relay for gravity invariant instruments mounted on the azimuth disk, auxiliary port instruments, and IP instruments will be instrument-dependent variants of the three principle configurations. 3 14

15 Figure The GMT optical configurations (DGNF, FP, DGWF) Wavefront Control System The surface figure of M1 and relative alignment of M1 and M2 will be actively maintained by a wavefront control system. Optical misalignment, distortion, and low frequency (<20 Hz) mechanical vibrations will be detected by the acquisition, guiding, and wavefront sensing system (AGWS) using four natural guidestars in the direct Gregorian focal plane. Telescope focus and segment misalignment will be corrected using position actuators supporting the primary, secondary, and tertiary mirrors, while higher order aberrations are corrected by the primary mirror support system. The wavefront control system also provides the feedback for telescope guiding. The four AGWS sensors patrol an annulus between 6 and 10 arcmin off-axis. A separate on-axis probe can be deployed to the direct Gregorian field center for initial and periodic alignment and calibration of the wavefront control system. In the GLAO observing mode, the AGWS additionally provide feedback for controlling the ASM surface figure. In the diffraction-limited AO observing modes (NGSAO and LTAO), feedback to the ASM is provided by the natural and laser guidestar AO wavefront sensors and segment edge sensors described in Section Telescope The GMT is an altitude-azimuth telescope designed around the 25 m segmented primary mirror. The alt-az mount provides complete sky coverage for elevation angles from 30 deg to 89.5 deg over 360 deg of azimuth. The telescope drives and controls acquire and track targets over the full range of motion. All of the optical subsystems, adaptive optics system, and science instruments are supported by the structure of the telescope. The major telescope assemblies are shown in Figure They consist of: (1) Pier provides a rigid base for the telescope and supports utility systems. (2) Azimuth track the interface between the pier and the telescope structure. It provides bearing surfaces for the azimuth motion. (3) Azimuth structure turntable assembly that contains the bearing systems that allow the telescope to point in azimuth and elevation. (4) Optical support structure (OSS) carries the optical and science instruments of the telescope and has the bearing surfaces that allow the telescope to rotate in elevation. 3 15

16 These assemblies are introduced in this section and described in detail in Section 6 of this report. Figure Major telescope assemblies Pier The pier is part of the enclosure foundation and provides the base for the GMT (Figure 3-12). In addition to supporting the telescope, an intermediate landing ( diaphragm ) within the pier and an external walkway near the top provide access to the telescope components below the azimuth disk. These components include the azimuth bearings and drives, the utility platform, and azimuth cable wrap. A portal through the walls of the pier provides a passageway for moving direct Gregorian instruments and equipment onto the central Pier Lift Platform (PLP) and from there into the telescope. Utilities (power, compressed air, oil for the hydrostatic bearings, and signal cables) also enter the telescope through the base of the pier. 3 16

17 Figure Pier and azimuth track (cut-away view) Azimuth Track The azimuth track assembly is the stationary structure that forms the interface between the rotating azimuth structure and the pier, as shown in Figure Two planar runner bearing tracks on top and a single cylindrical radial runner bearing on the m inside diameter define the azimuth axis. The azimuth track is constructed from seven segments. Adjustment screws between the track and pier allow for leveling the track. Figure Azimuth track Azimuth Structure The azimuth structure is the lower of the GMT s two rotating assemblies (Figure 3-14 and Figure 3-15). This structure defines the elevation axis with 24 hydrostatic bearings on its top side that interface with the optical support structure (OSS) runner bearings. It also interfaces to the azimuth track runner bearings with a second set of 24 hydrostatic pads on its lower side. The lower azimuth pads and azimuth tracks define the azimuth axis of the telescope. The weight of the OSS is transferred to the azimuth track through the pedestal structures on the top surface of the azimuth structure. The forcer heads for the main axis drives are located between the hydrostatic bearing pads. 3 17

18 Figure Azimuth Structure (top view) Figure Azimuth structure (bottom view) Optical Support Structure The optical systems and science instruments are located, with one exception, in the optical support structure (OSS). The exception is the gravity invariant instrument station on the azimuth structure. The adaptive optics system components are also integrated within the OSS. The major structural assemblies of the OSS consist of: C-Ring assembly Instrument platform (IP) Cell connector frame (CCF) 3 18

19 Primary mirror cells and braces Upper truss and top frame Lower OSS Structure Figure 3-16 shows the assemblies on the lower OSS structure. The OSS structure is described in detail in Section 6 of this report. Figure OSS lower structure The C-ring assembly supports the OSS structure and assemblies above. It consists of a pair of C-rings, so called because of their shape, and bracing that connects them. The runners for the elevation bearings are machined into the cylindrical rims of the C-rings. The elevation drive magnets, encoders, and limit switches also mount in this area. The instrument platform (IP) spans between the C-rings and runs their length (Figure 3-17). The IP provides access to folded port instruments mounted at that level. The Gregorian instrument rotator (GIR), described below, mounts in the center of the C-ring assembly flush with the top of the IP. The cell connector frame (CCF) sits on top of the C-ring assembly. The CCF is a welded and bolted structure that ties together the seven primary mirror (M1) cells. The M1 cells support the primary mirrors and are vital structural members of the OSS assembly. While the telescope can perform science operations with as few as four primary mirror segments, it cannot operate without the full complement of cells. Counterweights will be used to balance the telescope with less than seven segments. Braces that reach back to the C-rings stabilize the outboard ends of the M1 cells. 3 19

20 Figure Instrument platform and Gregorian instrument rotator Gregorian Instrument Rotator Direct Gregorian (DG) and folded port (FP) instruments are mounted on the Gregorian instrument rotator shown in Figure The GIR is installed in the center of the C-ring assembly on bearings that allow it to rotate about the OSS Z-axis and compensate for field rotation in the instruments caused by the alt-az motion. The rotation axis of the GIR defines the Reference Optical Axis (ROA) of the telescope. The top plate of the GIR is flush with the top plate of the IP. The GIR includes instrument bays for four direct Gregorian and three quadrants for mounting folded port instruments. The tertiary mirror (M3) mounts on top of the GIR in the fourth quadrant. The acquisition, guide, and wavefront sensor assembly is recessed into the top plate Upper OSS Structure The upper OSS structure, the truss and top frame, mount on top of the CCF as shown in Figure Figure Upper OSS structure and mechanisms 3 20

21 The secondary mirror positioner, seven secondary mirror segments, and the M2 baffle attach to the top frame. The prime focus calibration arm shown in Figure 3-19, used to deploy the alignment and instrument calibration source, is attached to the upper truss near the top. Figure Prime focus calibration arm and source Instrument Stations Section lists the science instrument mounting locations (Figure 3-4). This section provides further detail Direct Gregorian Large science instruments and those requiring an unvignetted field of view greater than 3 arcmin will operate at the direct Gregorian (DG) focus. Four bays within the GIR structure each accommodate instruments up to 5.5 m in height and 2.78 m square and a maximum weight of 11,250 kg (see Figure 3-4). DG instruments will be mounted on slides to move them from their bays into the center of the GIR and place them at the focus. Counterweights in the GIR will maintain its balance about the z-axis. Utilities (power, data lines, coolant) will be routed to the DG instruments through cable chains at the base of the GIR. DG instruments may be designed for the bare (primary/secondary mirror) configuration or widefield operation with the Corrector-ADC as described above. Due to the height difference in the focus, DG instruments will, in general, be designed for one configuration or the other. Service access to the DG instruments is provided by platforms within the GIR. DG instruments will be installed in the GIR from below. A lift platform in the center of the pier will raise the instrument into position from ground level (Figure 3-20). A cart is provided for moving DG instruments from the auxiliary building to the telescope. Section 6.14 describes the handling equipment and procedures. 3 21

22 Figure Pier lift for installing DG instruments Folded Ports Folded port (FP) instruments mount on the top surface of the GIR. The telescope beam is directed to the instrument by the tertiary mirror as described in Section The focus is located 1.9 meters from the optical support structure Z-axis. FP instruments will be smaller than the DG instruments. The maximum instrument height is 1.9 m and the instruments are constrained to fit within the 9.2 m diameter of the GIR top surface. Medium and high-strehl AO instruments are placed in this area. Figure 3-21 shows GMTNIRS and the tertiary mirror in the folded port configuration. Figure Folded port instrument (GMTNIRS) and M3 on the instrument platform 3 22

23 FP instruments are lifted onto the telescope using the enclosure overhead crane and moved across the instrument platform to the GIR. The installation procedure is described in Section Instrument Platform Stations The instrument platform (IP) provides space to mount one or more instruments to the front or back of the GIR. Mounting large instruments on both sides would block access to the folded port instruments on the GIR. Instrument platform instruments are removed the same way as FP instruments, via the IP extension using the overhead crane Auxiliary Ports The Auxiliary Ports (AP), as shown in Figure 3-22, are provided for future installation of instruments or an optical feed to the gravity invariant station. They consist of mounting bosses on the outside of the C-rings centered on the elevation axis. The 750 mm diameter hole through the C-rings provides clearance for an optical relay from a folded port focus. The platform on the outside is removable to provide additional clearance for the AP instrument. Installing AP instruments requires a rail hoist to be attached below the primary mirror assemblies. Figure Auxiliary port and GIS instrument locations Gravity Invariant Station The gravity invariant station (GIS) is provided for instruments that require the utmost mechanical stability. The GIS is located on top of the azimuth structure (see Figure 3-4 and Figure 3-22). Either a fiber or optical relay is required to feed light to the instrument from the direct Gregorian or folded port focus. GIS instruments will be moved to the observing floor using the enclosure overhead crane and skids to move the instrument into its final position. 3.7 AO System The adaptive optics system corrects wavefront aberrations caused by the atmosphere, providing improved resolution and sensitivity to appropriately designed science instruments. The adaptive 3 23

24 secondary mirror is the primary wavefront corrector for the AO system. This choice provides distinct advantages over post-focal correction of the atmospheric wavefront errors, particularly when combined with a direct feed of the high-order wavefront sensors from the cryostat window of each AO instrument. The high throughput, low emissivity, and wide field of view (in GLAO mode) of the GMT AO system will be unique among the next generation of extremely large telescopes. This section provides a brief introduction to the AO system components and control loops. They are described in greater detail in Section Adaptive Secondary Mirror The ASM consists of seven segments and their embedded control system (see Figure 3-7). The segmented GMT optical design leads to specifications for each ASM segment that are similar to those of present-generation adaptive mirror telescopes (e.g., the Large Binocular Telescope 6, Magellan Telescope 7, and ESO Very Large Telescope 8 ). The ASM benefits greatly from the design heritage of those mirrors, and represents the 4 th generation of such devices designed by a consortium of Microgate Corp. and ADS International. Each 1.05 m diameter ASM segment has 672 voice coil actuators, supporting a 2 mm thick Zerodur face sheet. The optical prescription of the face sheets is identical to that of the fast-steering secondary mirror (FSM). Capacitive sensors maintain the shape of the face sheet with respect to a light weighted ULE reference body with ~5 nm precision, at up to a 2 khz update rate. This internal closed-loop control of the actuators operates at a rate of 30 khz, and includes feed-forward control 9. The reference body and actuators are both supported by an aluminum cooling plate, which also provides an interface to the M2 positioner. The 4704 voice coil actuators act on permanent magnets bonded to the rear of the Zerodur face sheets, leading to the desirable property that failed actuators exert no force, and can be removed from the control with little reduction in performance. Local control electronics for each segment are housed in cooled crates, which are attached to the top end. All global control tasks are performed by the electronics located remotely in the electronics room, which provide the control and data interfaces to the telescope control system. Once commissioned, the ASM will remain installed for long periods, operate in all operational weather conditions, and support every observing mode. Yearly maintenance campaigns are likely, during which time the FSM would be installed for several weeks, precluding use of the AO observing modes Wavefront Sensors Direct Feed Architecture The direct-feed wavefront sensing architecture 10 is so named because light from the telescope is passed directly from the tertiary mirror (M3) to the science instrument operating in the NGSAO and LTAO modes. High-order wavefront sensing is performed using visible light reflected from a dichroic instrument window tilted 20 deg to the incoming beam (Figure 3-23). The most obvious advantage of the direct feed architecture is the higher throughput and lower emissivity that it naturally provides to the science instruments compared to post-focal AO systems. The number of optical surfaces between the instruments and the sky is vastly reduced with respect to a post-focal AO relay. 3 24

25 A Visible Wavefront Sensor system (VWS), which includes both natural and laser guide star sensors, must be replicated for each client instrument, and supported from the front of the instrument. Flexure between the VWS and the instrument focal plane is sensed by an On- Instrument Wavefront Sensor (OIWFS) inside the instrument cryostat, and corrected by the AO system control loops. The OIWFS must also sense fast tip-tilt and segment piston error in the LTAO mode. Figure 3-23: Schematic of the direct feed architecture, as implemented in GMT AO system design Natural Guide Star Wavefront Sensor The NGSAO observing mode will use the light from a single natural guide star, located within 90 of the science target, for wavefront sensing. The Natural Guide Star Wavefront Sensor (NGWS) must therefore sense all wavefront aberrations due to the atmosphere and telescope, including tiptilt, segment piston, and high-order modes. The need to sense segment piston led to the selection of a pyramid wavefront sensor 11,12, which also provides high sensitivity and low aliasing properties. The NGWS components are mounted on a vertically oriented board that patrols the 180 diameter focal plane on two linear stages (Figure 3-24). Two wavelength channels allow segment piston errors to be measured unambiguously (one nm, and the other approximately nm). Both cameras sample the GMT pupil with subapertures, providing wavefront measurement on approximately the same spatial scales as the ASM actuator spacing. 3 25

26 Figure 3-24: Visible wavefront sensor system shown in context on a folded port AO instrument (GMTIFS) Laser Tomography Wavefront Sensor The Laser Tomography Wavefront Sensor (LTWS) measures the wavefront of the six laser guide stars, allowing the telescope control system in the LTAO observing mode to tomographically reconstruct the high-order components of the atmospheric wavefront aberrations in the direction of a central science target. The LTWS is fed by light reflected off the instrument window, and again off a dichroic located ahead of the NGWS (see Figure 3-23). The LTWS consists of six Shack-Hartmann wavefront sensors that each divides the GMT pupil into subapertures. The spot patterns are recorded by CMOS cameras, which can operate at up to 800 Hz frame rate. The sensors must translate axially to track the changing range to the atmospheric sodium layer, and rotate around the optical axis to track the laser guide star asterism, which is fixed with respect to the telescope pupil. The LTWS also includes a powerful slope processor, which processes the massive flow of pixel data into wavefront slope vectors, which are then sent to the telescope control system via the low-latency network On-Instrument Wavefront Sensor Each instrument using the NGSAO or LTAO observing modes must provide an on-instrument wavefront sensor (OIWFS) that observes a single on- or off-axis star, performing different functions depending on the observing mode. In the NGSAO mode, it compensates primarily for flexure, and must therefore measure tip-tilt, focus, and pupil position at slow rates (~0.1 Hz). In the LTAO mode, the OIWFS must sense all atmospheric and telescope aberrations that cannot be measured with the lasers. These are: Tip-tilt of the natural guide star at rates up to 1 khz in order to compensate for the image motion introduced by the atmosphere, and the wind induced vibration of the telescope Focus at rates of approximately 10 Hz, to compensate for sodium layer altitude variations Telescope segment piston at ~0.1 Hz, to correct slow drifts in the M1 edge sensors Low-order aberrations (<200 modes) at ~0.1 Hz, to detect tomographic reconstruction errors and changing non-common path aberrations 3 26

27 The design of the OIWFS for GMTIFS is illustrated in Figure It has four separate wavefront sensors fed by dichroics, mounted on an optical bench that is in turn mounted to the GMTIFS main optical bench. Tip-tilt sensing is performed using an imaging quad cell in the K-band ( µm). The H-band is split between a 5 5 Shack-Hartmann focus sensor ( µm) and an integrated optics piston sensor ( µm). These three sensors are fed by a small deformable mirror (DM) with actuators, to correct the anisoplanatism of the off-axis natural guide star. A subaperture Shack-Hartmann Truth wavefront sensor using the J-band ( µm) is located ahead of the DM. The OIWFS includes a slope processor which processes all sensor data into modal coefficients, which are then sent to the Adaptive Optics Real Time System (AORTS) via the low-latency network. It also receives DM updates from the AORTS on the same network. Figure 3-25: The On-Instrument Wavefront Sensor for GMTIFS Ground-layer AO Wavefront sensing in the GLAO mode will be provided using natural guide stars, and the fast wavefront sensing capabilities of the AGWS. An extensive trade study comparing this architecture to one using laser guide stars was performed during the preliminary design phase. The NGS GLAO system was selected based on its higher throughput and observing efficiency and far lower cost. In the GLAO mode, the wavefront of four natural guide stars will be sensed by the AGWS at up to 200 Hz, the ground layer turbulence will be tomographically reconstructed from these signals, and then corrected by the ASM. GLAO correction will thus be available to any instrument on the telescope, regardless of focal station, whenever the adaptive secondary mirror is installed Laser Guide Star Facility The laser guide star facility (LGSF) generates the LTAO asterism, which consists of six guide stars equally spaced on a 60 arcsec diameter circle. Approximately 20 W of optical power per laser are required to generate sufficient photon return in seasonal minimum sodium conditions to assure meeting the LTAO wavefront error budget (<260 nm rms high-order wavefront error). The LGSF uses six self-contained laser guide star units, each of which includes a laser system, a Beam Conditioning and Diagnostic System (BCDS), and a Laser Launch Telescope (LLT) (see 3 27

28 Figure 3-26). The baseline laser system is the Toptica/MPB SodiumStar Raman fiber laser, which generates a single laser beam with the required power, spectro-temporal, and spatial characteristics. This is formatted by the BCDS prior to projection on the sky by the LLT. Together, the BCDS and LLT ensure proper pointing and focusing of the laser beam on the sky. The laser guide star units are located at the periphery of the M1 mirror structure, mounted in pairs. The location at the end of the M1 cell connector frame provides a very stiff mount and ease of access for maintenance. In order to aid in the acquisition of the laser guide star by the LTWS, the laser guide star facility includes a laser guide star acquisition system whose purpose is to image the laser guide star asterism, and to determine the coordinates of each LGS with respect to the GMT optical axis. This system also provides important diagnostic capabilities for the LGSF to measure performance in terms of LGS brightness and spot size. Figure Laser guide star facility components and layout, including the Laser Projection Assembly (LPA), the beam diagnostic and conditioning system (BCDS), fiber laser, and laser launch telescope Phasing System Achieving the diffraction limit of the 25.4 m aperture of the GMT will require the primary and secondary mirrors to be phased to <50 nm rms. Due to the large separation between primary segments (30-40 cm) and their construction of borosilicate glass, which has a non-zero CTE, capacitive or inductive edge sensors alone are not expected to be sufficiently stable over timescales longer than a few minutes. In the NGSAO observing mode, the natural guide star wavefront sensor can sense and correct segment piston at up to 2 khz. However, only faint natural guide stars are generally available in the LTAO mode. The GMT therefore uses a 3-stage phasing system consisting of a coarse optical phasing sensor with a large capture range to initially phase the telescope, primary and secondary mirror edge sensors to maintain alignment over short timescales, and a high-sensitivity optical sensor to correct long-term drifts in the edge sensors. Since the primary and secondary segments are matched one-to-one, errors can be rapidly compensated using the agile adaptive secondary mirror, then off-loaded to the primary or secondary segment positioning actuators as appropriate. A more detailed description of the phasing system is provided in Section 8.5 and in the Adaptive Optics PDR Report

29 3.8 Instrumentation To achieve the science goals of the GMTO partners, all subsystems must perform well in order to deliver the photons collected by M1 to the instrument focal plane with minimal aberrations and maximal efficiency. From there, it is the responsibility of the instruments to convert as many photons as possible into meaningful information for scientific analysis. Instruments can be designed to perform specific experiments that address a few specific questions. Or, they can be designed to collect data in ways that are more generally useful. The first generation of GMT instruments falls into the category of general purpose instruments, as these have proven to have the greatest scientific impact in the early years of a new telescope Instruments and the Staged Implementation of GMT The GMT s staged implementation plan influences the order of instrument development. Stage 1, in which four primary mirror segments and four fast-steering secondary mirror (FSM) segments are operational, is a purely natural seeing telescope. Consequently, the adaptive optics (AO) instruments are deferred until Stages 2 and 3. Stage 1 also will be a period when several telescope systems are maturing, but may not be fully optimized. The GMT will be most effective with instruments that are tolerant of on-going technical developmental and varied atmospheric conditions. Two instruments are targeted for this period: a high resolution (R=25, ,000), single object, optical echelle spectrograph (G-CLEF), and a medium resolution (R=1,000-5,000), multi-object, optical spectrograph (GMACS). By the end of Stage 2, the GMT will be able to support AO-fed instruments. One instrument is planned for this period: a combined diffraction-limited, AO-fed, high spatial resolution imager (5 mas pixels) and a medium resolution (R=5,000 and 10,000) integral field spectrograph (IFS) with a selection of four (6, 12, 25, 50 mas) spaxel scales (GMTIFS). During Stage 3, the telescope will be fully populated with its primary and adaptive secondary mirrors and with the wide-field corrector and atmospheric dispersion compensator. Two instruments are planned for this period: a diffraction-limited AO-fed high-resolution (R=50,000 and 100,000), single object, IR echelle, having nearly complete coverage of the JHKLM bands (GMTNIRS), and a full-field (20 arcmin) fiber feed that works with multiple instruments (e.g., G- CLEF and GMACS) to enhance their wide-field, multi-object capabilities (MANIFEST). Subsequent to Stage 3, GMT will enter its long-term full operational period. As old science questions are answered and new ones emerge, the instrumentation must evolve for the GMT to remain effective and competitive. That is, second generation (and beyond) instruments, as well as upgrades to existing instruments, are a natural component of the operations plan. New capabilities will be delivered to the GMT on a semi-regular interval of ~3 years Selecting the First Generation Instruments Initial concepts for GMT instruments were prepared for the 2006 GMT Conceptual Design Review 14. Seven of the concepts were developed in greater detail in , culminating in their conceptual design reviews (CoDRs) in Sep.-Oct The technical and scientific reports from those studies, the review panel comments from the CoDRs, and the GMT Science Case 15 provided the basis for selecting a subset of the seven instruments to advance to a preliminary design phase, with the intent to fabricate those few. The instrument cost, relative to the total budget allocation for instruments, was an important consideration and a solid constraining factor in the selection. 3 29

30 A high-level panel was assembled, consisting of 12 representatives across the partnership and including two members from the general community, to advise the GMTO Board on the question of instrument selection. Their recommendations are presented in detail later in this document, but can be summarized here, as presented to the GMTO Board in March 2012: 1. Develop G-CLEF (high resolution optical echelle) 2. Develop GMTIFS (diffraction-limited IR imager and IFS) 3. Develop GMACS (optical multi-object medium resolution spectrograph) 4. Develop gratings for GMTNIRS (diffraction-limited high resolution JHKLM spectrograph) 5. MANIFEST, the facility fiber feed system, is needed to realize the high A-Ω potential of GMT 6. Create a process for developing second generation instruments The five instruments identified by the panel are described in detail later in this document, and highlights of their technical and functional capabilities are provided here Stage 1 Instruments The two Stage 1 instruments are G-CLEF 16 and GMACS 17. G-CLEF has multiple fiber input configurations. These include a high-throughput mode with single 1.2 fibers, a precision abundance mode with a single 0.7 diameter fiber and a precision radial velocity mode with an array of 0.7 fibers that match the primary mirror array. Table 3-1 and Table 3-2 summarize the functional parameters of these instruments. Table 3-1. Main functional parameters for G-CLEF Parameter Value Notes Wavelength range, nm Goal: 350-1,000 Spectral resolution 25K, 40K, and >100K Depends on fiber mode (HT, PA, or PRV) Fiber observing modes HT, PA, PRV, MOS HT = high throughput (1.2 arcsec fiber) PA = precision abundance (0.7 arcsec fiber) PRV = precision radial velocity (0.7 arcsec fiber array of 7 x 0.23 arcsec fibers for pupil slicing) MOS = multi-object spectroscopy with MANIFEST feed (0.7 arcsec fibers) Number of targets 1, ~5, ~40 Single object without MANIFEST Up to ~5 with MANIFEST and full spectrum coverage Up to ~40 with MANIFEST and 2 echelle orders of coverage Peak throughput 10% Goal: 15% (including telescope and all losses) Velocity precision Better than 50 cm/s Goal: better than 10 cm/s 3 30

31 Table 3-2. Main functional parameters for GMACS Parameter Value Notes Wavelength range, nm Goal: 350-1,000 Full coverage with low resolution gratings Spectral resolution Blue: 1,000-3,400 Red: 2,000-6,000 Depends on grating selection, wavelength, and slit width/seeing (0.7 arcsec slit) Gratings, l/mm (blue / red) Low: 728 / 783 Preliminary selection High: 1300 / 1300 Field of view, arcmin At least 4 x 7 Goal: more than 4 x 9 Number of targets (slitlets 60 / 300 Goal: 85 / 500 / MANIFEST fibers) Peak throughput (including telescope and all losses) 30% Goal: 35% G-CLEF Description The critical driver for the G-CLEF design is that it must provide superbly precise radial velocities. This functional requirement places very strong demands on the entire instrument, with special focus on the thermal and mechanical stability of all components. Thus, the instrument cannot flex and is therefore located at a gravity invariant location on the GMT azimuth disk. It is also mounted with thermal and vibration isolators. Aside from being large and requiring large optics, the instrument concept is otherwise a relatively simple double-pass white spectrograph design. In order to provide simultaneous full-spectrum coverage and to maintain efficiency, it employs two channels for the blue and red octaves of the spectrum. The large echelle grating poses the largest development challenge for the optics, and the thermal stability control poses the largest mechanical challenge. Because of its position on the azimuth disk, light from the telescope must be relayed to the spectrograph via a fiber. The optical pickoff is located at one of the telescope s folded port pickoffs. A deployable tertiary sends the light to a fixed position on the instrument platform and onto one of the three fibers selected for the observing program. A separate fiber feed can be accepted from the MANIFEST multi-fiber feed for multi-object programs that do not require the highest spectral resolution GMACS Description Although large, GMACS is a straightforward dual channel, multi-object spectrograph. It uses VPH gratings as the dispersion elements, and therefore the gratings and cameras must articulate in its high-resolution configuration in order to optimize performance for a given wavelength setup. GMACS is mounted in one of the four bays of the Gregorian instrument rotator, below the instrument platform disk. Like G-CLEF, GMACS must accept a separate fiber feed from MANIFEST when used for very high target multiplexing programs. The MANIFEST fiber configurations have not been defined yet, but options are available to provide additional or enhanced capabilities for GMACS. For example, MANIFEST can feed GMACS with multiple small deployable fiber IFUs, or it can deploy very small fiber bundles that image slice the image plane to increase the spectral resolution of GMACS by a factor of

32 3.8.4 Stage 2 Instruments GMTIFS 18 is the sole Stage 2 instrument, although it serves two high performance diffractionlimited roles as an imager and as a spectrograph. GMTIFS is mounted at one of the folded ports. Its main features are given in Table 3-3. Table 3-3. Main functional parameters for GMTIFS IFU Parameter Value Notes Wavelength range, μm Spectral resolution 5,000 and 10,000 5,000: ZJ, JH, or HK bands 10,000: Z, J, H, or K bands Spaxel scales, mas 6, 12, 25, 50 User selectable; square on the sky Field of view, arcsec 0.53x0.27, 1.06x0.54, Corresponding to the spaxel selection Peak throughput (including telescope and all losses) 2.20x1.13, 4.40x %, 28%, 32%, 32% For Z, J, H, K Imager Parameter Value Notes Wavelength range, μm Pixel size, mas 5 Field of view, arcsec 20.4 x 20.4 Corresponds to the pixel scale and detector elements (4K x 4K pixels) Filters 13 slots available Z, J, H, Kn, K, and half-band filters (short and long versions of ZJHK), plus room for narrow band Peak throughput (including telescope and all losses) 23%, 30%, 32%, 32% For Z, J, H, K GMTIFS is designed explicitly for use with the AO observing modes, NGSAO and LTAO. The majority of the science goals require high sky coverage, and therefore LTAO will greatly improve the science value of GMTIFS when it becomes available later in the telescope implementation plan. The instrument design is driven by the expected superb AO performance of the GMT. The design anticipates spatial resolutions comparable to the J-band diffraction limit (~10 mas). At the focal plane scale of the GMT, this resolution represents a physical scale of 10 μm. Thus, mechanical tolerances must be held to a small fraction of this scale (~1 μm), and optical performance must match (~1 mas). As a consequence, numerous components throughout the instrument are designed close to known manufacturing limits. Otherwise, the imager is fairly straightforward. Still, there are complex components; for example, the IFS and imager both share a high performance 8-element atmospheric dispersion compensator that is needed to maintain image quality. Both the imager and IFS plan to use H4RG detectors that are not yet available. Like any IFS, the spectrograph side is more complicated than the imager. The integral field reimaging is accomplished using an array of image slicer mirrors. These provide a set of 45 slitlets, each being 88 spaxels in length. The spectral dimension is limited by the detector array to 4096 pixels. In order to achieve image quality at the diffraction limit, GMTIFS must include an on-instrument wavefront sensor (OIWFS) to provide tip-tilt, focus, phasing, and truth sensing corrections. The OIWFS is further complicated by the fact that the science field is unlikely to have any bright stars, 3 32

33 and so, the corrections must be based on off-axis sources. To achieve good sky coverage, stars may be quite far off axis, up to 90 arcsec. Consequently, the AO correction for these stars will be different than that for the science field on-axis, and so, the sky coverage requirements flow down to having a deformable mirror in the OIWFS to correct the guide star for its off-axis aberrations Stage 3 Instruments The two Stage 3 instruments are GMTNIRS 19 and MANIFEST 20. Table 3-4 and Table 3-5 summarize the functional features of these instruments. Table 3-4. Main functional parameters for GMTNIRS Parameter Value Notes Wavelength range, μm Spectral resolution JHK: 50,000 LM: 100,000 Depends on delivered image quality (85 mas slit) Gratings Silicon immersion Field of view 1.2 arcsec slit Principally, a single object instrument Peak throughput (including telescope and all losses) TBD Table 3-5. Main functional parameters for MANIFEST Parameter Value Notes Wavelength range, nm 350 1,000 Limited in the blue by fiber lengths, which can be short or long (e.g., GMACS vs. G-CLEF) Configuration time, sec Less than 195 Goal: 120 Fiber arrangements Single, bundles of 7, 19, The mix of fiber options will be determined and large IFUs during MANIFEST preliminary design Fiber positioning 0.1 accuracy, arcsec Peak throughput to GMACS / G-CLEF (includes entry losses) ~75% Efficiency has been corrected for similar entry losses that affect the other instruments GMTNIRS Description GMTNIRS is relatively simple, despite being a near-diffraction-limited AO-fed instrument. It has only one moving part the rotating pupil mask. To provide full and simultaneous JHKLM spectral coverage, GMTNIRS implements five small echelle spectrographs, with the appropriate passband selected via an array of dichroics. The JHK spectrographs (R=50,000) are nearly identical copies of each other. The LM spectrographs (R=100,000) are also very similar. The optical elements are relatively small as a consequence of using silicon immersion gratings. Thus, costs are controlled through replication and size reduction, with much of the instrument risk placed on the grating development. GMTNIRS does require five IR detectors. The JHK detectors are readily available H2RG sensors while the LM detectors will be the not-yet-available H4RG sensors in order to provide the higher resolution needed at those wavelengths. As a high-resolution spectrograph, many of the science targets will be bright objects, and so GMTNIRS will work well using NGSAO. However, it s very broad-band sensitivity makes the 3 33

34 instrument attractive for observing fainter, extragalactic targets, especially for very red and obscured objects. Thus it must also work with LTAO using an off-axis guide star. The plans for its OIWFS must account for that science driver. Like GMTIFS, GMTNIRS is mounted at an FP location, on top of the rotating GIR MANIFEST Description MANIFEST is not a true instrument in the sense that it has no detector. Nevertheless, it is considered within the GMT instrumentation program because it adds significant capability to other instruments. MANIFEST is currently the only means planned for accessing the full 20 arcmin field of view provided by the telescope. When MANIFEST is implemented, the GMT will realize its potential as the ELT with the largest A-Ω the product of collecting area and areal field of view. In doing so, the GMT will be a uniquely powerful facility for follow-up spectroscopy of objects identified via survey imaging programs already under way (e.g., DES) as well as from those under development (e.g., LSST). Fibers will be positioned using a new technology called Starbugs 21,22. These are independent robots based on piezo tubes to provide their motion. Prototype Starbugs are highly developed and will be integrated into a demonstrator on-sky facility over the next two years. The Starbugs are attached by suction to a glass plate at the curved focal plane of the Gregorian focus. They can move at speeds up to 3 mm/s, are small enough to be placed within 8-10 arcsec of each other, and can carry payloads of either single or many fibers. The versatility of the Starbug system enables a spectrograph like GMACS to simultaneously obtain spectra of several hundred objects at once, or to be used as a multi-ifu spectrograph, depending on the fiber cables being deployed for a given observation. In addition, MANIFEST can feed both GMACS and G-CLEF simultaneously, thereby allowing two observational programs to execute at once. MANIFEST must have access to the full field, so it is located in one of the GIR instrument bays. It requires the Corrector-ADC to provide the optical correction over the 20 arcmin field. Although it is a fairly straightforward instrument, having no significant large optics, the Starbugs must follow the focal surface that is defined by a transparent plate. The 1.3 m diameter plate is large, but it is not an optical element. It must match the focal surface well enough to avoid defocusing the image. Prototyping of the plate is in progress. 3.9 Enclosure and Facilities Site Layout GMTO will provide facilities at Las Campanas as described in Section 7.3 and shown on the Site Master Plan shown in Figure The site has been arranged such that structures at the summit include only those that are essential to support efficient science operations and daytime operations including maintenance. The design goal is to minimize any facility-related degradation of site conditions necessary for science observations. Other facilities are located at the support site, a few kilometers away from and a few hundred meters below the summit. The summit site (elevation 2,518 m) includes the GMT and its enclosure and the summit support building. These structures are oriented along a line approximately normal to the predominant wind direction and separated by a distance of approximately 40 meters, minimizing the effects of 3 34

35 disruption of the air flow across the summit support building on the air circulation around the enclosure. The summit also includes space for a second giant telescope in the future. The summit support building includes the facility building where the control room, electronics room, staff offices and labs are located. It also includes an auxiliary building containing large highbay areas for primary mirror segment maintenance, secondary mirror maintenance and calibration and instrument maintenance. An equipment building houses equipment used in the operation of the telescope, including pumps, chillers, compressors, electrical switch gear, etc. The primary mirror coating plant and mirror washing equipment are located in the auxiliary building. Pumps and power supplies used in the coating process are also located in the equipment building. A rendering of the summit site with the GMT, enclosure and summit support building is shown in Figure 3-2. Figure Site master plan drawing The support site includes a utilities building, warehouse and lodge. A water storage facility is located mid-way between the summit and support sites to provide potable and fire suppression water for the support site. The utilities building includes heavy equipment and vehicle maintenance shop spaces, maintenance management offices, an electrical room and a communications room. The entry point for the site power is in the electrical yard, just outside of the utilities building. Communications fiber trunk lines enter the facility in the communications room where they are then distributed to the site through underground conduit. Diesel generators used for backup power are located in the electrical yard. A fuel depot for generator and vehicle fuel storage and dispensing is located adjacent to the electrical yard. The warehouse will be located across from the utilities building. The warehouse will initially be used to store the primary segments in their transport containers prior to integration into their mirror cells. During operations, the warehouse will store handling equipment and spares. The utilities building and warehouse are shown in Figure

36 Figure (Left) Views of the utilities building and (Right) warehouse located at the support site A lodge consisting of dormitories, a cafeteria and a recreation facility will also be located at the support site, away from the utilities building and warehouse in order to minimize daytime noise near the dormitories. The capacity of the lodge will be sized to meet the observatory staffing level. The design and development of the lodge is a Stage 2 activity and has not yet begun. LCO has a reliable source of water that is pumped up to a water tank on the site from a well at the base of the mountain. Water treatment will be provided locally at the facility. Electrical power will be supplied by commercial sources at the base of the mountain on high-voltage overhead lines. An existing 23 kv line provides power to LCO. The service will have to be upgraded in order to supply the GMT s estimated 2.5 MW average load. Clearing and leveling of the site was completed in August, 2012 (see Figure 3-29). Approximately 55,000 m 3 of rock and soil was removed from the summit, 20,000 m 3 of which was used as fill on the site. The cleared platform measures approximately 280 m long by 100 m wide. Geotechnical testing of the rock at the summit site and granular soil at the support site has been completed and the results have been used to refine the foundation designs. Figure The GMT site as of July

37 3.9.2 Enclosure Building The GMT enclosure is a quasi-cylindrical structure that houses the telescope. Large doors ( shutters ) in front and on top of the building retract to allow unobstructed observation through the full range of azimuth and elevation angles. During observing, the shutters can be closed down around the incoming beam to shield the telescope from wind and moonlight. The enclosure is comprised of a rotating structure on top of a fixed base. The enclosure rotation is completely unconstrained, allowing arbitrary positioning relative to the telescope. The telescope and enclosure are mechanically decoupled with separate load paths to ground and independent drive systems. A circular track at the top of the fixed base and bogies beneath the major structural columns of the upper structure comprise the rotation system. The overall height of the enclosure is 62 m above grade level. The observing floor is 11.8 m above grade, level with the top of the GMT azimuth disk. This puts the elevation axis of the GMT at 22.5 m above grade. A 65 metric ton bridge crane (with a 10 metric ton auxiliary hoist) at the top of the enclosure will be used in the initial assembly of the GMT and for servicing the telescope and equipment during operations. In particular, the crane will be used to exchange primary mirror segments and cells in the telescope for re-coating in the auxiliary building. It will also be used for instrument handling. The thermal environment in the enclosure is critical to astronomical observations. Measures being taken to minimize the effects of dome seeing include: Maximize wind-driven flushing through large vent openings and the shutters. The current design has an approximately 30% open area with the vents and shutters fully open. Promote rapid equilibration of the enclosure structure by the use of thin cross-section structural members. The goal is to achieve a less than one-hour time constant for a major portion of the structure. Insulate the building to reduce heat buildup during the day. The walls and roof are made of insulated metal panels and the floor will be insulated from below. Seal the openings to prevent air infiltration during the day. Select exterior coating to reduce radiative over-cooling at night and solar gain during the day. Cool or trap active sources of heat, and exhaust the waste heat well away from the telescope line of sight. Actively ventilate the massive main structural elements, enclosed mechanical area housing the enclosure bogies and telescope pier to keep waste heat from entering the telescope chamber. Cross-sectional images of the enclosure are shown in Figure The left figure shows the fully closed enclosure, indicating the swept volume of the telescope. The figure on the right shows the fully open enclosure and unobstructed viewing aperture through the full range of telescope elevation motion. 3 37

38 Figure The GMT enclosure closed (Left) showing the telescope swept volume and open (Right) showing the unobstructed viewing aperture Extensive wind tunnel testing and computational fluid dynamic modeling of the structure has been conducted. These models included the local terrain of Cerro Campanas, enclosure with the telescope, and the summit support building. The objective was to investigate the wind-flow around and through the enclosure as well as interior flushing times, and to compute wind forces on the building and telescope. Various configurations of the shutters and vents (ranging from fully open to fully closed) have been investigated over a range of wind speed and direction. Under unfavorable (20 th percentile, 3 m/s) low-wind conditions with the enclosure and telescope pointed downwind, complete flushing occurs within approximately three minutes in the current design. Methods and results of the wind-flow studies are described by Farahani, et al Control System The GMT Software and Controls System (SWCS) encompasses the software and hardware components necessary to control and monitor the GMT optical and electromechanical subsystems and to safely and efficiently operate the GMT observatory. The design of the SWCS is driven by a set of general guidelines: The use of industry components and standards that improve the cost-effectiveness of the system; an architecture based on wellestablished practices and design patterns; the validation of the technical platform and architecture through prototyping and incremental delivery; a model-based development approach integrated with an Agile management process; and the efficient support and collaboration with the parties involved in the development of the different subsystems. The SWCS is divided into a set of subsystems to facilitate their specification, development and integration. These subsystems, when integrated, are organized in a modular architecture that improves the internal consistency of subsystems and minimizes the interfaces between them. The SWCS subsystems are classified into three main categories or domains (Figure 3-31): The Telescope Control System (TCS) The Observatory Operations System, which groups subsystems that provide the capabilities to support the efficient operation of the observatory The Observatory Services, which groups subsystems that provide common infrastructure services 3 38

39 Figure Software and control top level view Telescope Control System The telescope control system encompasses the software and hardware necessary to efficiently control and monitor the optical and electromechanical devices of the telescope. This includes all components of the AO system. The TCS is organized as a hierarchy of supervisory and control subsystems. At the lowest layer of the hierarchy are the subsystems that directly interface with optical and electromechanical devices (i.e., actuators and sensors). At a higher level, the wavefront control and the pointing kernel subsystems coordinate these low-level device control subsystems to point the telescope and move probes to the corresponding targets, achieve and maintain the nominal optical configuration, and close the wavefront correction loops during the execution of an observation. Observations and other type of operations are defined and automated using the tools provided by the observatory operations system (e.g., observing tools, sequencer) Observatory Operations System The Observatory Operations System (OOS) provides telescope staff and astronomers the high-level software tools necessary to operate the GMT observatory. Before observing, astronomers will use observing tools to define observing parameters and instrument configurations, such as selecting targets and AO guide stars (brightness, proximity to science targets) positioning instrument apertures (slits, fibers, image mosaic, etc.) and setting up instrument parameters (exposure time, filter settings, wavelength, grating, etc.). The pre-defined parameters and setups are pre-loaded into the observatory operations database to be recalled on-the-fly during observations. Astronomers and operators may also define sequences to automate complex observing procedures that may be performed repeatedly during an observing program, such as executing mosaic observations, switching between AO guide stars and science targets, dithering to blank sky, etc. During observations, the OOS provides tools to visualize the location of the science targets in the sky, so as to facilitate planning of the observations or to work around unexpected contingencies due to weather or other constraints (e.g., clouds, wind). After the observations are taken, quick-look reduction and calibration of the incoming data is performed, to help observers gauge the quality of the data and to inform decisions on follow-up observations. In addition to the above tasks, staff will use tools to schedule long term observing programs, or perform observations on-the-fly, working with an observer or a queue scheduling system. During the night, the OOS provides the telescope operator with the means to efficiently monitor the health of the system, and to adapt to changing runtime conditions. 3 39

40 Observatory Services The observatory services domain groups subsystems that provide common infrastructure. Each service (engineering user interface, logging, telemetry, configuration, etc.) addresses a specific structural function. Structural functions are systemic in nature and they provide interfaces for any component in the system (e.g., fault management, telemetry). Application components can access the services via a small interface that hides the service implementation details. Services are partitioned in a way that facilitates a modular design where different services can evolve independently through the life of the project to address new scalability needs (e.g., new hardware subsystems require higher performance telemetry) or to address different optimization strategies Site Characterization The environmental conditions at LCO are well known thanks to over 40 years of telescope operation and recent surveys conducted on the site. In , surveys were conducted at several sites including Cerro Manqui (where the Magellan 6.5 m telescopes are now located) and at Cerro Campanas, the site of the GMT. From 2005 through 2011, GMTO conducted an extensive campaign to characterize conditions at the possible sites for the GMT. The survey included both atmospheric conditions that affect imaging performance (seeing, atmospheric turbulence profiles, and precipitable water vapor) and basic weather parameters (wind direction and speed, temperature, pressure, and humidity). Weather stations and Differential Image Motion Monitor (DIMM) telescopes (shown in Figure 3-32) were located at three GMT candidate sites. Data were also taken at the Magellan telescope site to provide a baseline. Over the course of the campaign, an extensive database of information was collected and analyzed. The results are documented in the GMT Site Testing at Las Campanas Observatory Final Report 24, which also summarizes results from the previous campaign, and provides a general overview of the process. After a review of the data collected in the survey, Cerro Campanas was selected as the GMT site. Figure DIMM tower and weather station mast The operational and survival conditions that govern the design of the GMT facility (based on the site characteristics described below) are specified in the GMT Environmental Conditions document

41 Seeing A key factor in comparing sites was the image quality as measured by the on-site DIMMs, located 8 m above the summit elevation. Table 3-6 illustrates the exceptional seeing at the Cerro Campanas site, which has a median value of 0.63 arcsec FWHM at 500 nm wavelength. Table 3-6. DIMM seeing percentiles Percentile 10% 25% 50% 75% 90% Image FWHM (arcsec) Wind The weather stations at the candidate sites recorded both wind speed and direction. Table 3-7 shows the day and night wind-speed statistics. Table 3-8 shows the nighttime-only statistics when the weather was clear or partially clear. Wind speeds at Cerro Campanas are approximately 10% higher than the lower-altitude Magellan site on Cerro Manqui. Higher wind speed promotes better ventilation of the enclosure but also increases telescope shake and dust contamination. This must be accounted for in the design. Table 3-7. Wind speed percentiles (day and nighttime) Percentile 10% 25% 50% 75% 90% Wind Speed (m/s) Table 3-8. Wind speed percentiles (nighttime clear and partly clear) Percentile 10% 25% 50% 75% 90% Wind Speed (m/s) Figure Wind speed seasonal variation (nighttime clear and partly clear) During the survey period, the maximum wind speed (33 m/s) was measured during the day at Cerro Campanas. Figure 3-33 shows the variation of the median wind speed on clear and partly clear nights throughout the year for three years of nighttime data. The error bars are the standard 3 41