System Level Requirements GMT REQUIREMENTS DOCUMENT

Transcription

1 GMT REQUIREMENTS DOCUMENT Prepared By J. Maiten, M. Johns, D. Sawyer and G. Trancho 06/01/2012

2 SIGNATURES Page2 of 85 Author: Date: Jessica Maiten Approval Chief Systems Engineer: Date: Matt Johns Project Manager: Keith Raybould Project Director: Patrick McCarthy Concurrence Date: Date: Project Scientist: Date: Steve Shectman

3 REVISIONS Version Date mm/dd/yyyy Affected Sections Engineering Change # 1 11/30/2011 All None Initial Draft Reason/Initiation/Remarks 2 12/16/2011 All None Intermediate version 3 12/18/2011 All None Intermediate version 4 1/12/2012 All None Intermediate version 5 2/23/2012 All None Intermediate version 6 3/11/2012 All None Intermediate version 7 3/12/2012 All None Intermediate version 8 3/13/2012 All None Intermediate version 9 3/14/2012 All None SAC review version 10 4/18/2012 All None Release for internal project review Page3 of 85

4 Table of Contents Page4 of 85 Table of Contents... 4 Table of Figures... 5 Table of Tables Introduction Purpose Scope System Description GMT Coordinate Systems Definition of Terms References Definitions and Acronyms Definitions Acronyms Project Functional and Performance Requirements Telescope Observing Modes Architecture Optical Thermal Structural Motions Instrument Ports Acquisition, Guiding, and Active Optics Adaptive Optics Operational Readiness Instruments Facilities Summit Site Support Site Infrastructure Base Facility Enclosure General Requirements Enclosure Shutter Enclosure Rotation Enclosure Thermal Observatory Operations Operational Observing modes Observing Tools Engineering Tools Science Data Management Instrument/AO Support Mirror Handling & Maintenance Staff Support General Requirements General Conditions... 65

5 Page5 of Standards Health and Safety Environmental Monitoring Conditions Earthquake Transportation and Storage Services Network and Communications Utilities Electrical Reliability and Maintenance Documentation Appendix A - Verification Matrix Table of Figures Figure 1. Representational deptiction of GMT... 7 Table of Tables Table 1. Defintion of GMTO Terms... 9 Table 2. Definition of GMTO Acronyms Table 3. Applicable Project Documents Table 4. M1/M2 Mirror System Throughput Table 5. M3 Throughput Table 6. DG Corrector-ADC Throughput Table 7. Slew Time Table 8. Offset Time vs. Distance... 44

6 1.0 Introduction Page6 of 85 This document is the System Level Requirements Document (SLRD). It is a systems engineering requirements document and is the project's response to the Science Requirements Document (SRD, GMT- SCI-REQ-00001), Operations Concept Document (OCD, GMT-SCI-DOC-00034), and the Detailed Science Case (DSC, GMT-SCI-DOC-00031). The concepts and requirements in these documents flow down to requirements for the observatory (system). As necessary, requirements from this document flow down to requirements to each of the subsystems. The requirements in this document are numbered in the form of SLR-nnnn where nnnn is the unique number for each of the requirements. The numbering scheme allows for unambiguous reference to individual requirements. 1.1 Purpose This document shall be used as guidance for the subsystem level engineering functional and performance requirements of the GMT Observatory. The requirements documented herein are intended to fully describe the system level engineering requirements to satisfy the criteria of the SRD, DSC, OCD and Safety Plan. By definition, the SLRD will change in response to changes in the SRD and/or OCD but should not require modification due to changes in the subsystem requirements documents. 1.2 Scope This document contains high-level requirements specific to the GMT Observatory. It is site-specific to Cerro Las Campanas. The following areas are defined: General Constraints Environmental Constraints Environmental, Health and Safety Requirements System Attributes High Level Software Requirements Telescope and Instrumentation Requirements Facility and Enclosure Requirements Observation Operational Support Requirements Requirements to meet Standards Utility Requirements Reliability and Maintenance Requirements 1.3 System Description The planned Giant Magellan Telescope (GMT) is a 25 meter alt-azimuth telescope and one of the first of the next generation of Extremely Large Telescopes (ELTs). In operation it will be used to conduct a broad range of astronomical scientific research at visible and infrared wavelengths at its site in Northern Chile. The telescope concept is shown in Figure 1.

7 Page7 of 85 Figure 1. Representational deptiction of GMT GMT is designed around a Primary Mirror that consists of seven 8.4-meter circular segments with an overall diameter of 25.4 meters. The mirrors have the equivalent collecting area of a single 21.5-meter diameter mirror and the diffraction limited resolving power in the infrared of a 24.5-meter mirror. The GMT consists of an altitude-azimuth mount to support the optical assemblies and multiple science instruments while providing the ability to acquire and track astronomical targets over the majority of the visible sky. The mount consists of two rotating structures: the Optical Support Structure (OSS) moves in elevation and houses all of the telescope optics and instruments located at Direct Gregorian and Folded Ports, and the azimuth structure that rotates and supports the OSS and gravity invariant instruments. To allow the OSS design to be optimized, no Nasmyth instrument ports are provided. The Secondary Mirror is also composed of seven segments and is conjugate to the Primary Mirror. There are two Secondary Mirrors to be specified, the Adaptive Secondary Mirror (ASM) and the Fast Steering Secondary Mirror (FSM). The ASM segments employ adaptive face sheet technology. The FSM is built

8 Page8 of 85 with monolithic mirrors and is used in place of the ASM for the initial installation and as a backup. In both cases, fast tip-tilt is provided to deal with low frequency disturbances (below ~20 Hz) such as wind shake. Actuators in the primary and secondary mirrors control mirror segment position counteract slowly varying gravity and thermal deflections of the structure and maintain the alignment of the telescope optics in real time. In addition, the primary mirror support actuators continuously adjust their forces to remove figure errors. Wavefront sensors in the focal plane provide the error signals for these active controls once every 30 seconds to 1 minute. The GMT has an Adaptive Optics (AO) System to correct atmospheric seeing and wind shake at rates up to around 1 KHz. The AO system is comprised of the ASM, a laser system for projecting laser guide stars in the upper atmosphere, wavefront sensors and a phasing camera in the focal plane. Diffraction limited performance is achieved in the near- and mid-ir. A large Instrument Platform (IP) is mounted within the telescope below the Primary Mirror. The 9-meter diameter Gregorian Instrument Rotator (GIR) is mounted below the IP and rotates to compensate for field rotation caused by alt-az tracking. The GIR houses large instruments such as the visible and near-ir multi-object spectrographs and IR instruments that benefit from having a minimum number of warm reflections in the beam. Small to intermediate sized instruments mount on top of the GIR (at the level of the IP) and are fed with a set of fold mirrors and dichroics allowing beam switching between them. A gravity invariant instrument location is provided on the azimuth disk. Instruments in this location are fed by an optical relay or fiber(s). 1.4 GMT Coordinate Systems There are two principal sets of Cartesian coordinates fixed to the physical structure of the telescope: one (x,y,z) to the OSS and the other (u,v,w) to the Azimuth Platform. These are described in the GMTO Coordinate Systems and Vertical Datum (GMT-SE-REF-00189). 1.5 Definition of Terms Throughout this document, the use of the term "Shall" denotes requirements that are mandatory and will be the subject of specific acceptance testing and compliance verification. "Is" or "Will" indicate a statement of fact or provide information and are not subject to any acceptance testing or verification compliance by the supplier. "Can", "May", or "Should" indicate recommendations and are not subject to any acceptance testing or compliance verification by the supplier. The supplier is free to propose alternative solutions. Throughout the document, requirements statements are shown in blue text to allow them to stand out. Statements preceded by "Note:" are support text. Statements preceded by "Rationale:" and in italics are the reasoning behind the requirements. 2.0 References 2.1 Definitions and Acronyms Definitions

9 Page9 of 85 Table 1. Defintion of GMTO Terms Term Acquisition, Guide, and Wavefront Subsystem Acquisition/Guide Sensor Active Optics Definition The mechanical assembly located at the top of the GIR that houses the AGS and WFS probes and mechanisms to move them within the Technical Field of View (TFOV) patrol areas. Sensor for off-axis guiding using position reference stars. Active Optics (AcO) functions to actively maintaining low frequency alignment, focus, and figure of the telescope optics by optical feedback using natural guidestars. The Active Optics typically operates at <1 Hz. Active Optics Wavefront Sensor The wavefront sensor for the Active Optics Subsystem. Adaptive Optics System Adaptive Secondary Mirror Altitude Axis Atmospheric Dispersion Compensator (ADC) Auxiliary Building Azimuth Axis Blind Pointing Control Building Direct Gregorian Port Dithering Dome Seeing A telescope system for correcting rapidly varying wavefront errors by optical means. The GMT Adaptive Optics System uses an Adaptive Secondary Mirror to correct disturbances caused by variations across the pupil of the index of refraction integrated along the line of sight through the atmosphere, and slowly varying telescope and instrument-caused wavefront errors. The distinction between adaptive optics and active optics is one of purpose and correction bandwidth, with adaptive optics operating at >10 Hz and active optics at <1 Hz. The Adaptive Secondary Mirror (ASM) forms the primary corrective element of the GMT Adaptive Optics System. It is formed of 7 independent ASM segments and it is part of the ASM System. See Elevation Axis. An optic or set of optics that compensate for the spectral dispersion introduced by the earth's atmosphere. Detached building houses the mirror washing and coating facility, instrument service areas, and machine shop. This is the vertical axis about which the Azimuth Platform rotates to point the telescope at celestial targets at all bearing angles of the compass. Blind pointing relies on the accuracy of the telescope encoder system and open loop corrections. Blind pointing is used to position the telescope to within the capture range of the acquisition/guide sensors. Building contains the telescope control room, electronics lab, offices and lounge/kitchen. The Direct Gregorian Port (DG), located below the upper GIR platform, provides a high-throughput, low-background focus with the minimum number of reflections. The DG port may be configured in one of two ways, Narrow-Field and Wide-Field. Dithering is the process of repetitively offsetting the telescope between two or more positions on the sky, with a dwell time at each end point where the system may or may not be guiding. Dome Seeing is the image blurring caused by non-thermally equilibrated turbulent air inside of the Telescope Chamber.

10 Page10 of 85 Term Definition Dwell Time Elevation Angle Elevation Axis Enclosure Enclosure Base Enclosure Base Elevator Enclosure Building Equipment Building Exhaust System Facility Instruments Fast-steering Secondary Mirror (FSM) assembly Folded Port Galactic Pole Dwell time is the amount of time that the telescope remains at a fixed guided position on the sky during offsetting operations. The angle between astronomical horizon and the optical axis of the Telescope. The elevation angle is 90 degrees when the telescope is at zenith and decreases toward horizon. It is the complement of the zenith angle. This is the horizontal axis about which the OSS rotates to point the telescope optics at celestial targets above the minimum elevation angle. Also known as the Altitude Axis. Large structure that forms the basic moving building envelope of the Enclosure Building. It consists of a large diameter ring beam at its base, two large arch girders that form the upper structural portion and supports the shutters and all intermediary structural members, wall systems and overhead building cranes, shutters, wind vents, bogies, enclosure control system, and building services. The Enclosure is capable of full 360 deg rotation about the vertical axis and tracking celestial objects at sidereal and other rates as specified. All of the structures and entities below the enclosure track. It includes the Enclosure Support Structure, Control Building, Enclosure Base Elevator 1 (EB- E1), Instrument Platform Lift, Utility Room, ASM Calibration Room, Foundations, Jib Crane, Building Services, Cart Guides and Rails, Stairs, doors, access, etc. The enclosure base elevator is provided to lift small equipment and personnel between grade level and the observing floor. The Enclosure Building is comprised of the Enclosure, Enclosure Base and the Telescope Pier. Detached building houses the mechanical equipment (hydraulic, pneumatic, HVAC, liquid chillers) for the enclosure. The exhaust system consists of a fan and ducting to remove waste heat from the enclosure and enclosure base and direct it away from the telescope line of sight. Facility Instruments are available to all users and are supported and maintained as part of the GMTO facility. They are generally developed under contract by instrument groups external to GMTO Corporation. Instrument teams may also provide long-term maintenance and continuing development of Facility Instruments under contract to GMTO. Secondary mirror assembly with fast tip/tilt monolithic segments. The Folded Port is located at the upper GIR platform and using the tertiary mirror provides optimized foci for narrow-field instruments which operate at visible, IR and NIR wavelengths (400 nm to 25 microns) where adaptive optics is most effective. The directions perpendicular to the galactic plane point to the galactic poles. The South Galactic Pole is located at RA(2000) = 0:51:26, Dec(2000) = -27:07:42.

11 Page11 of 85 Term Gravity-Invariant Instrument Station (GIS) Gregorian Instrument Rotator (GIR) Ground Layer AO (GLAO) Guide Stars (Position Reference Stars) Instrument Platform (IP) Instrument Platform Lift Interlock Definition The Gravity-Invariant Instrument Station is located on the azimuth disk and provides a gravity invariant mounting location for science instruments. This is a cylindrical structure imbedded in the floor of the Instrument Platform to which mount the science instruments. The GIR rotates to compensate for the rotation of the field of view as the telescope moves in altitude and azimuth. The Ground-Layer Adaptive Optics (GLAO) observing mode uses a guidestar asterism (either LGS/NGS or NGS-only) to detect and correct wavefront errors common to sky objects within a large (up to 10 arcmin in diameter) field of view. These errors are mainly due to low (up to 1 km) altitude components of the atmospheric wavefront aberrations. The wavefront aberration will be detected using multiple wavefront sensors and compensated by the ASM, resulting in improved natural seeing images over a field of view comparable to the GS constellation size. While providing some improvement in the visible, GLAO correction is expected to be particularly useful at wavelengths longer than 1 µm. Position reference stars are fairly bright stars offset from target objects that are used for telescope guiding or wavefront sensing during observations. This is a platform fixed to the C-rings of the OSS below the primary mirror assembly. The platform provides a mounting base for the Gregorian Instrument Rotator (GIR). Instrument Platform Lift (IP Lift) is used to raise instruments and equipment from the enclosure floor to the telescope instrument platform (IP). An interlock is a hardwired connection between two systems or mechanisms that provides time-critical safety information. Laser Tomography AO (LTAO) The Laser Tomography Adaptive Optics (LTAO) observing mode uses a ~1 arcminute diameter constellation of LGS to tomographically reconstruct the highorder components of the atmospheric wavefront aberrations in the direction of a central science target. One or more faint natural guidestars must be used to measure tip-tilt, focus, segment piston, and dynamic calibration terms. The wavefront aberration will be compensated by the ASM, providing diffractionlimited imaging at µm wavelength over a field of view limited by atmospheric isoplanatism. M3 (Tertiary Mirror) Assembly Natural Guide Star AO (NGSAO) This is an assembly at the FP level of the GIR which houses M3 and its mechanisms. The mechanisms move M3 into and out of the telescope beam and also rotate M3 to reflect the incoming light toward the FP instrument in use. The Natural Guide Star Adaptive Optics (NGSAO) observing mode uses a single star and wavefront sensor to provide all of the wavefront correction information for the AO System. The wavefront aberration will be compensated by the ASM, providing diffraction-limited imaging at µm wavelength over a field of view limited by atmospheric isoplanatism.

12 Page12 of 85 Term Natural Seeing Nodding Offsetting Definition This is the seeing limited image quality that relies on the imaging and tracking properties of the telescope without the use of adaptive optics. Slowly varying effects that affect image quality such as gravitational and thermal distortion of the structure and optics, tracking errors and telescope shake are corrected but rapidly varying atmospheric effects (seeing) are not. Our definition of natural seeing does not include "dome" seeing effects. Nodding is the process of repetitively offsetting the telescope between two or three positions on the sky, with a dwell time at each end point where the system may or may not be guiding Offsetting is the process of accurately moving from one guided pointing to another guided position relative to the first. Optical Support Structure (OSS) This consists of all of the telescope structure that moves with the elevation axis. The major subassemblies of the OSS are (a) the secondary mirror, (b) the top-end truss, (c) the primary mirror assembly (7 mirrors and cells) and connector frame, (d) the C-ring assembly, and (e) the instrument platform assembly that includes the instrument rotator. Wide-field correctors and the wide-field atmospheric dispersion compensator mount in the central primary mirror cell just above the Folded Port focus. Science instruments, except those at the GIS, are attached to the OSS structure. PI Instruments Pier Lift Platform Pointing A PI Instrument is one developed by an external organization for private use at GMT by the instrument team and its collaborators. PI Instruments on GMT will require approval and authorization by GMTO. GMTO will provide limited support for logistics, mounting PI Instruments on the telescope, and normal telescope scheduling and operation. The instrument team will be responsible for instrument operation, support, and maintenance. Details will be spelled out in an agreement between GMTO and the instrument team institution before the instrument can be authorized for installation on the GMT. Pier Lift Platform is located in the center of the Telescope Pier and is used to raise instruments from ground level into the GIR from inside the Telescope Pier. Pointing is the process of repositioning the telescope to new sky coordinates with precise placement of science objects in the field of view. Pointing relies on the use of guide sensors in the TFOV to accurately center objects in the SFOV. Primary Mirror Assembly (PMA) The primary mirror structural assembly consists of the seven primary mirror cells and a Cell Connector Frame (CCF). Reference Optical Axis (ROA) Scanning This is the axis of revolution of the primary mirror parent optical surface. Scanning is the process of moving the image in the focal plane at a set rate relative to a reference. Sky coverage at the Galactic Pole How often a random pointing around the Galactic Pole will allow to find a suitable guidestar. Step and Integrate Step and Integrate is the process of moving the image in the focal plane discrete angular steps with pauses to integrate between steps.

13 Page13 of 85 Term Definition Technical Field of View (TFOV) The Technical Field of View is the area accessible to guide and wavefront sensors in the AGWS. Telescope Azimuth Disk Assembly Telescope Azimuth Track Assembly Telescope Chamber Telescope Pier Tracking Zenith Angle Acronyms AcO AcWFS ADC AGS AGWS AO AOS ASM CG COTS CS DG Acronym The structural elements that transfer load from the Elevation Structure to the Azimuth Track and provide for motion of the telescope about the azimuth axis. The Telescope Azimuth Track is the large steel ring upon which the Mount sits and allows it to rotate about the vertical Azimuth axis. The Azimuth Track provides a smooth and flat surface for the hydrostatic axial bearings. They also provide appropriate mounting surfaces for a variety of azimuth Mechanical Systems. The Telescope Chamber is the volume inside of the Enclosure where GMT resides. This is the approximately cylindrical structure at the center of the enclosure that supports the telescope. The top of the pier interfaces to the Telescope Azimuth Track and Telescope Lower Utility Transfer system Tracking is the process of maintaining alignment of the telescope pointing with the science target during an observation using position feedback from reference guidestars to track the telescope mount in azimuth, elevation, and GIR angle. The angle between vertical and the optical axis of the Telescope. The zenith angle is zero when the telescope is at zenith and increases toward horizon. It is the complement of the elevation angle. Table 2. Definition of GMTO Acronyms Active Optics Active Optics Wavefront Sensor Definition Atmospheric Dispersion Compensator Acquisition/Guide Sensors Acquisition, Guide, and Wavefront Subsystem Adaptive Optics Adaptive Optics System Adaptive Secondary Mirror Center of Gravity Commercial Off The Shelf Continuous Scan Direct Gregorian

14 Page14 of 85 Acronym Definition DGNF DGWF DIQ EDMS FOV FP FSM GIR GIS GLAO GMT GMTO HBS ICD IP LCO LTAO M1 M2 M2 Lab M3 NF NGS NGSAO OSS P-V PI PMA RD RMS ROA Direct Gregorian - Narrow Field Direct Gregorian-Wide Field Delivered Image Quality Electronic Document Management System Field of View Folded Port Fast-steering Secondary Mirror Gregorian Instrument Rotator Gravity-Invariant Instrument Station Grounds Layer Adaptive Optics Giant Magellan Telescope Giant Magellan Telescope Organization Hydrostatic Bearing System Interface Control Document Instrument Platform Las Campanas Observatory Laser Tomography Adaptive Optics Primary Mirror Secondary Mirror Secondary Mirror Lab Tertiary Mirror Narrow Field Natural GuideStar Natural Guidestar Adaptive Optics Optical Support Structure Peak to Valley Principal Investigator Primary Mirror Assembly Reference Document Root Mean Square Reference Optical Axis

15 Page15 of 85 Acronym Definition RSS SFOV SLE SOML SRD TBC TBD TBR TEL TFOV UPS VAO WF WFC (Corrector) WFS Root Sum of Squares Science Field of View Survival Level Earthquake Steward Observatory Mirror Lab (at Univ. of Ariz.) Science Requirements Document To Be Confirmed To Be Determined To Be Reviewed Telescope Technical Field of View Uninterruptible Power Supply Virtual Astronomical Observatory Wide Field Wide Field Corrector Wavefront Sensor 2.2 Project The following documents form a part of this specification to the extent specified. In the event of conflict between the documents referenced and the contents of this specification, the requirements in this specification shall take precedence. Table 3. Applicable Project Documents Reference # Document Number Version Title RD-1 GMT-SE-REF GMT Glossary of Terms and Abbreviations RD-2 GMT-SCI-REQ GMT Science Requirements Document (SRD) RD-3 GMT-SCI-DOC GMT Operations Concept Document (OCD) RD-4 GMT-SCI-DOC GMT Detailed Science Case (DSC) RD-5 GMT-PM-RVW GMT CoDR Report RD-6 GMT-SE-DOC GMT Optical Design RD-7 GMT-SE-REF GMT Electrical Power Systems RD-8 GMT-SE-DOC Site Specific Seismic Hazard RD-9 GMT-SE-REF GMT Environmental Conditions RD-10 GMT-SE-DOC GMT Image Quality Budgets

16 Reference # Document Number Version Title Page16 of 85 RD-11 GMT-SE-DOC GMT Site ing at Las Campanas Observatory - Final Report RD-12 GMT-SE-REF GMT Design Codes and Standards RD-13 GMT-SE-REF GMT Coordinate Systems and Vertical Datum RD-14 GMT-SE-REF GMT Common Utilities RD-15 GMT-SE-REF GMT Cabling, Connectors and Cabinets RD-16 GMT-SWC-REF GMT Software Standards RD-17 GMT-SWC-REF GMT Hardware Standards RD-18 GMT-SWC-REF GMT Communications Standards and Protocols RD-19 GMT-PM-DOC GMTO Safety Plan RD-20 GMT-SE-DOC GMT Critical Spares Document RD-21 GMT-SE-REF GMT CAD Standards and Guidelines 3.0 Functional and Performance Requirements 3.1 Telescope GMT is a next-generation 25m-class telescope intended for astronomical scientific research at UV, visible, near and mid infrared wavelengths. The telescope will be located in Chile at Las Campanas Observatory. A suite of natural-seeing and adaptive optics instruments will allow a flexible program of observations that address the science goals set forth in Section 3 of the Science Requirements Document (GMT-SCI-REQ-00001) Observing Modes The GMT will provide different observing modes to enable the science goals of the observatory and to allow observing conditions to be maximally exploited. The modes differ in the manner and degree to which adaptive optics is used to correct wavefront errors in the images delivered to the focal plane. All of the Observing Modes rely on Active Optics (AcO) for maintaining optical alignment, focus, and figure of the telescope optics using optical feedback from wavefront sensors using natural guidestars in the focal plane. The Active Optics typically operates at <1 Hz. The AcO control is supplemented by the AO system when operative. The AO System uses an Adaptive Secondary Mirror to correct disturbances caused by variations across the pupil of the index of refraction integrated along the line of sight through the atmosphere, and slowly varying telescope and instrumentcaused wavefront errors. This includes effects of Dome Seeing. The AO system will also sense and correct to some level for telescope vibrations caused, for example, by wind disturbance and mechanical equipment mounted on the structure. Seeing Limited Modes Images delivered to the focal plane are not corrected for atmospheric distortion in the Natural Seeing mode. This mode is available over the full wavelength range of GMT. The Ground-Layer Adaptive Optics (GLAO) observing mode uses a guidestar asterism (either LGS/NGS or NGS-only) to detect and correct wavefront errors common to sky objects within a large (up to 10

17 Page17 of 85 arcmin in diameter) field of view. These errors are mainly due to low (up to 1 km) altitude components of the atmospheric wavefront aberrations. The wavefront aberration will be detected using multiple wavefront sensors and compensated by the ASM, resulting in improved natural seeing images over a field of view comparable to the GS constellation size. While providing some improvement in the visible, GLAO correction is expected to be particularly useful at wavelengths longer than 1 µm. Diffraction Limited Modes The diffraction limited observing modes include Natural Guide Star Adaptive Optics (NGSAO) and Laser Tomography Adaptive Optics (LTAO). These modes provide a much higher level of wavefront correction over a smaller field of view than the Natural Seeing or GLAO modes limited primarily by anisoplanatism is the incoming wavefront. The Natural Guide Star Adaptive Optics (NGSAO) observing mode uses a single star and wavefront sensor to provide all of the wavefront correction information for the AO System. The wavefront aberration will be compensated by the ASM, providing diffraction-limited imaging at µm wavelength over a field of view limited by atmospheric isoplanatism. Sky coverage depends on the availability of suitably bright reference stars nearby the target objects. The Laser Tomography Adaptive Optics (LTAO) observing mode uses a ~1 arcminute diameter constellation of LGS to tomographically reconstruct the high-order components of the atmospheric wavefront aberrations as a function of altitude in the direction of a central science target. One or more faint natural guidestars must be used to measure tip-tilt, focus, segment piston, and dynamic calibration terms. The wavefront aberration will be compensated by the ASM, providing diffraction-limited imaging at µm wavelength over a field of view limited by atmospheric isoplanatism. LTAO provides a larger fraction of sky coverage than NGSAO. SLR-3101: Natural Seeing Observing Mode The GMT shall provide a natural seeing observing mode that will be operative with FSM or ASM. Note: A calibration procedure prior to the start of observing may be required to meet this requirement. Rationale: This requirement is a flowdown from the SRD. SLR-0930: GLAO Observing Mode The GMT shall provide a GLAO observing mode in which the light of astrophysical sources is corrected using a combination of multiple laser guide stars and/or multiple natural guide stars. Note: is required to determine the expected level of correction using natural guide stars versus LGS. Rationale: This requirement is a flowdown from the SRD. SLR-0928: NGSAO Observing Mode The GMT shall provide an NGSAO observing mode in which the light of astrophysical sources is corrected using a single natural guidestar. Note: Additional tip/tilt and segment phasing input from other sensors (e.g. accelerometers, segment edge sensors, off-axis tip/tilt sensors) may be used in all AO modes Rationale: This requirement is a flowdown from the SRD. SLR-0929: LTAO Observing Mode The GMT shall provide an LTAO observing mode in which the light of astrophysical sources is corrected using multiple laser guide stars and one or more natural guide stars. Note: Additional tip/tilt and segment phasing input from other sensors (e.g. accelerometers, segment edge sensors, off-axis tip/tilt sensors) may be used in all AO modes

18 Rationale: This requirement is a flowdown from the SRD. Page18 of Architecture This section includes the requirements for the architecture of the GMT. SLR-2557: Telescope Configuration GMT shall have an altitude over azimuth structure. Rationale: This is a design choice adopted and approved by the GMTO Board. SLR-2661: Gregorian Optical Design The GMT optical system shall be based on an aplanatic Gregorian prescription with segmented primary and secondary mirrors as specified in the Optical Design document GMT-SE-DOC Rationale: This is a design requirement adopted and approved by the GMTO Board. SLR-2701: Optical Prescriptions The GMT optical system shall be designed with multiple configurations according to the prescriptions specified in the Optical Design document GMT-SE-DOC Note: There are three optical configurations specified in this document: Direct Gregorian - Narrow Field (DGNF) Direct Gregorian - Wide Field (DGWF) Folded Port (FP) Rationale: This provides multiple observing modes for a variety of science programs Telescope SLR-1012: Primary Mirror (M1) Configuration The GMT shall be designed around a segmented M1 consisting of seven circular, 8.4-meter segments arranged in a hexagonal configuration. Note: The layout is shown on UA/SOML drawing rev. F. Rationale: This is a design requirement adopted and approved by the GMTO Board. SLR-1013: Fast-Steering Secondary Mirror (FSM) The GMT shall provide a secondary mirror (FSM) composed of seven monolithic segments with fast tip-tilt capability conjugated 1:1 with the primary mirror segments. Rationale: This mirror assembly will be used for initial commissioning of GMT and as a backup for the Adaptive Secondary Mirror (ASM). The diameters and clear apertures of the segments are prescribed in the Optical Design document GMT-SE-DOC SLR-3070: Incomplete Telescope Segmentation To the extent practical, GMT systems shall be operable with fewer than the seven M1 or M2 segments during the commissioning or laboratory test period. Note: The performance of some systems will be degraded when operating in this mode. This mode is intended to help in the commissioning. Alignment and Phasing the Telescope will be challenging without the center segments. Rationale: This is to allow Telescope commissioning to start prior to the delivery of all segments.

19 SLR-2546: Tertiary Mirror (M3) GMT shall provide a deployable monolithic flat tertiary mirror (M3) to direct the beam to instruments located at Folded Ports. Note: M3 must be deployable so that it can be removed from the beam for DG operation. Page19 of 85 Rationale: The diameters and clear apertures of M3 are prescribed in the Optical Design document GMT-SE-DOC SLR-1020: Focal Stations GMT shall provide eight (8) stations for mounting Science Instruments on the telescope. Note: The required focal stations are: a) the Direct Gregorian Port (DG) with stations for 4 deployable instruments, b) three Folded Ports (FPs), and c) the Gravity Invariant Station (GIS). Rationale: The focal stations are: a) the Direct Gregorian Port (DG) with stations for 4 deployable instruments, b) three Folded Ports (FPs), and c) the Gravity Invariant Station (GIS). SLR-4197: Future Auxiliary Ports The GMT shall not preclude the addition of auxiliary locations for instrument mounting on the IP and outside of the c-ring. Note: A clear optical path will be provided for these mounting locations. Rationale: This will allow for future expansion of instrument stations. SLR-4120: IP Instrument Mounting GMT shall provide pads on the fixed IP for mounting science instruments. Rationale: This allows for future installation of instruments not requiring field de-rotation. SLR-2692: Gregorian Instrument Rotator (GIR) GMT shall provide a GIR to compensate for field rotation due to the alt-azimuth tracking motion of the telescope at sidereal and non-sidereal rates and deliver a non-rotating field of view to DG and FP Science Instruments mounted on the rotator. Rationale: The GIR provides field de-rotation for Science Instruments mounted at the DG and FP stations. SLR-3513: Wide Field Correction The GMT shall provide field dependent aberration correction over a FOV greater than or equal to 20 arcmin over the full DGWF wavelength range. Note: The wavelength range is specified in requirement SLR Rationale: This is required to meet image quality requirements for fields of view greater than ~10' diameter and up to 20' diameter. The optical glass limits transmission in the short end of the wavelength range. SLR-2702: Atmospheric Dispersion Compensation The GMT shall provide compensation for atmospheric dispersion over the full DGWF wavelength range. Note: The wavelength range is specified in requirement SLR Rationale: This is required for fields of view greater than ~10' and correction of atmospheric dispersion for broad band observations in the visible. The optical glass limits transmission in the short end of the wavelength range.

20 Page20 of 85 SLR-2848: Acquisition, Guide, and Wavefront Subsystem GMT shall include a subsystem within the GIR with pick-off probes, Acquisition/Guide Sensors (AGS) and Active optics Wavefront Sensors (ACWFS). Rationale: This instrument performs the acquisition and guide function for the telescope and is a derived requirement to meet the imaging and pointing performance specifications. SLR-3671: Mirror Covers GMT shall provide mirror covers for the primary and tertiary mirror that can be efficiently deployed for daily use. Rationale: Mirror covers will help preserve the quality of the mirror coatings, minimize scattered light and will extend the period between cleanings and re-coatings and protect the mirrors from damage. SLR-3360: Telescope Balance GMT shall provide a means to balance the Optical Support Structure (OSS) and Gregorian Instrument Rotator (GIR) about their rotational axes for different instrument configurations. Note: Moveable counterweights will be required for instruments and mechanisms that can be deployed during normal operations. The method for re-balancing during nighttime instrument changes will be designed to minimize lost observing time. Rationale: This is required to accommodate varying instrument masses and moments AO System SLR-2608: Adaptive Secondary Mirror Subsystem (ASM) GMT shall provide a secondary mirror composed of seven (7) adaptive deformable mirror segments conjugated 1:1 with the primary mirror segments for seeing-limited and diffraction limited observing mode. Rationale: The mirror assembly will be used for both natural seeing and adaptive optics observing modes when it is installed. The diameters and clear apertures of the segments are prescribed in document TBD. SLR-2612: Laser Guide Star Facility The GMT shall utilize Laser Guide Stars in the LTAO and/or GLAO observing mode. Rationale: Laser Guide Stars are required to meet the LTAO and GLAO [TBC] sky coverage requirements. This requirement is at Level 2 due to Interlock & Safety System flowdown. SLR-3831: AO Direct Feed The GMT AO system in NGSAO and LTAO observing modes shall utilize an AO direct feed architecture. Note: Direct feed means AO corrected images are delivered directly to the Science Instrument without re-imaging fore-optics. Rationale: Direct feed means AO corrected images are delivered directly to the Science Instrument without re-imaging fore-optics and it a design decision to optimize throughput. SLR-3833: Telescope Subaperture Phasing The GMT subapertures shall be phased to one another for LTAO and NGSAO operation using a combination of sensors on the primary and secondary mirror segments and wavefront sensors in the focal plane.

21 Rationale: This is required to achieve high Strehl performance in LTAO and NGSAO modes. Page21 of 85 SLR-4656: GLAO Facility The GMT shall provide a GLAO Facility to implement the GLAO observing modes for multiple instruments. Note: Unlike NGSAO and LTAO, GLAO will not be a replication system. Rationale: Direct Feed Architecture Calibration Systems A facility calibration system will be provided, per the requirements below, that is available to all natural seeing and AO instruments. Instruments with additional calibration requirements will provide the capability as part of the instrument system. SLR-4665: Flat-Field Calibration GMTO shall provide a deployable general-purpose flat-field calibration system for natural seeing and AO instruments operating from 0.34 um um. Note: This will be a general purpose calibration facility and may not meet specialized requirements of some instruments. Those instruments may have to include built-in calibration capabilities. Rationale: This requirement is a flowdown from the SRD. SLR-4666: Spectral Calibration GMTO shall provide a deployable general-purpose spectral calibration system with beam characteristics that mimic the light coming from astronomical sources for natural seeing and AO instruments operating from 0.34 um um. Note: This will be a general purpose calibration facility and may not meet specialized requirements of some instruments. These instruments may have to include built-in calibration capabilities. Rationale: This requirement is a flowdown from the SRD. SLR-2819: AO Calibration GMT shall provide, at the telescope, a system for calibrating all deformable mirrors and wavefront sensors required by all the AO observing modes. Rationale: This requirement is a flowdown from the SRD. SLR-4709: Calibration System Deployment Position The GMT calibration systems shall be deployable at any elevation angle within the observing range of the telescope (including zenith). Note: This includes the deployable systems for flat-field, spectral, and AO calibrations. Rationale: This is required to allow calibrations to be obtained under the same structural (e.g. flexure) conditions as observations and to minimize the observing overheads. SLR-4710: Calibration System Deployment Time The GMT calibration systems shall be deployable and ready for calibrations within 2 minutes [TBC]. Note: This includes the deployable systems for flat-field, spectral, and AO calibrations. Rationale: This is required to allow calibrations to be obtained during the night with minimal observing overhead.

22 Page22 of 85 SLR-2823: Daytime Calibration Efficiency The GMT daytime calibration of available instruments and/or AO system shall not exceed 4 hours [TBC]. Note: The length of time needed to perform the daytime calibrations will vary dependent on the instruments but this is the limit. Rationale: This requirement is a flowdown from the SRD Optical The GMT optical system consists of a segmented primary mirror (M1) and a segmented secondary mirror (M2) that deliver light to a variety of instrument ports with various fields of view and at various wavelength ranges to enable a wide range of science capabilities. Two secondary mirror assemblies that share the same optical prescription are provided. The Fast-steering Secondary Mirror (FSM) uses monolithic segments. The Adaptive Secondary Mirror (ASM) has deformable front surfaces to provide wavefront correction by the Adaptive Optics (AO) System. A flat fold mirror (M3) is used in some optical configurations of the telescope to send the beam to instrument stations located around and at right angles to the Reference Optical Axis (ROA). Wide-field operation over a 20 arcmin diameter field of view and atmospheric dispersion compensation at visible wavelengths is enabled by the use of a deployable atmospheric dispersion compensator/widefield corrector (Corrector-ADC). The optical prescription for GMT is specified in the GMT Optical Design document (GMT-SE-DOC-00010). Many of the following requirements are derived from the natural seeing image sizes specified in the SRD that have been budgeted in the Natural Seeing Image Quality Error Budgets document (GMT- SE-DOC-00145). The sources of image blur allocated in the budgets include optical design, optical fabrication, thermal and gravitational distortion of optical elements, alignment, tracking errors, vibrations in the telescope structure, and mirror seeing. The total delivered image quality to science instruments is the RSS of telescope sources of blur and the natural seeing. The conditions under which the image budgets were derived are summarized in the following table: Criterion Zenith Angle Wind Speed Temperature Range Temperature Rate of Change Primary Mirror Configuration Condition 0 degrees 0.4 m/s to 9.8 m/s +7 C to +18 C C/hr to +0.2 C/hr segmented, not phased Secondary Mirror Configuration segmented, fast steering mode Configurations The GMT provides three optical configurations for science instruments that include Direct Gregorian Narrow-Field (DGNF), Direct Gregorian Wide-Field (DGWF) that implements a Corrector-ADC, and Folded Port (FP). In addition, a technical field, within the DGWF will be used for active optics, acquisition and guide functions.

23 Direct Gregorian - Narrow Field Page23 of 85 The Direct Gregorian- Narrow Field (DGNF) optical configuration consists of the segmented primary (M1) and secondary (M2) mirrors, as described in the Optical Design document GMT-SE- DOC The DGNF configuration delivers a nominal f/8.2 beam with a m focal length to Science Instruments mounted at the DG port. SLR-2556: DGNF Wavelength Coverage The GMT in the Direct Gregorian-Narrow Field (DGNF) in natural seeing configuration shall operate over a wavelength range of 320 nm to 25 microns. Rationale: This requirement is a flowdown from the SRD. SLR-2710: DGNF Science Field of View The GMT in the DGNF natural seeing configuration shall provide a Science Field of View (SFOV) greater than or equal to 10 arcminutes in diameter. Rationale: This requirement is a flowdown from the SRD. SLR-3572: DGNF Optical Image Quality The GMT optical components shall contribute no greater than [Goal: 0.100] arcsec 80% encircled energy diameter to the DGNF Natural Seeing Image Quality budget due to residual fabrication and support errors after correction by the AcO. Note: The optical components include the primary mirror and secondary mirror segments. The residual errors include surface figure errors and mounting and support errors after correction by the AcO. The DGNF Natural Seeing Image Quality Budget is specified at the reference wavelength of 500 nm. This contribution will be combined in quadrature with other contributions to the Natural Seeing Image Quality budget for the DGNF configuration to arrive at the total allowance. The total DGNF Natural Seeing Image Quality budget and the conditions under which the budget applies are specified in the SRD, SCI-1876 and SCI Rationale: This requirement was derived from the Natural Seeing DIQ Error Budget analysis. SLR-2687: DGNF Pupil Stability The GMT shall maintain the position of the exit pupil at the DGNF port to within ±0.5 % [TBC] peak-to-valley of the pupil diameter. Rationale: This requirement is needed for GLAO. The estimate is derived from NIRMOS discussion on pupil blurring, and reduced by a factor of 8 relative to 0.5" slit diffraction effects Direct Gregorian-Wide Field The Direct Gregorian-Wide Field (DGWF) optical configuration consists of the segmented primary (M1) and secondary (M2) mirrors and the Atmospheric Dispersion Compensator/Wide Field Corrector (Corrector-ADC), as described in the Optical Design document GMT-SE-DOC The DGWF configuration delivers a nominal f/8.3 beam with a m focal length to Science Instruments mounted at the DG port. The position and curvature of the DGWF focal surface is not required to be coincident with the DGNF focus. Instruments will, in general, be designed for one or the other configuration or will be able to compensate for the difference. SLR-2711: DGWF Wavelength Range The GMT in the Direct Gregorian Wide-Field (DGWF) configuration shall operate over a wavelength range of 370 nm to 1.0 microns [Goal: 350 nm microns].

24 Page24 of 85 Note: The band pass is limited by absorption in available optical glasses for refractive correctors (cf. GMT Optical Design Report GMT-SE-DOC-00010, rev C or higher). Rationale: This requirement is a flowdown from the SRD. SLR-2712: DGWF Science Field of View The GMT in the DGWF configuration shall provide a Science Field of View (SFOV) not less than 20 arcminutes in diameter. Note: Some instruments will be required to share the Science Field of View with the facility active wavefront sensors and guiders operating within the Technical Field of View (TFOV). Rationale: This requirement is a flowdown from the SRD. SLR-3641: DGWF Optical Image Quality The GMT optical components shall contribute no greater than [Goal: 0.107] arcsec 80% encircled energy diameter to the DGWF Natural Seeing Image Quality budget due to residual fabrication and support errors after correction by AcO. Note: The optical components include the primary and secondary mirror segments and the Corrector- ADC. The residual errors include surface figure errors, variations in refractive element properties, and mounting and support errors after correction by the AcO. This contribution will be combined in quadrature with other contributions to the Natural Seeing Image Quality budget for the DGWF configuration to arrive at the total allowance. The total DGWF Natural Seeing Image Quality budget and the conditions under which the budget applies are defined in the SRD, SCI Rationale: This requirement was derived from the Natural Seeing DIQ Error Budget analysis Corrector-ADC Per the Optical Design, the ADC and WFC have been combined into a single optical assembly. The dual purpose of the Corrector-ADC is to increase the usable science field of the telescope from the approximately 10 diameter provided by the base Gregorian primary-secondary mirror optical system to 20 and also correct for atmospheric dispersion. It is used in the Direct Gregorian Wide-Field (DGWF) configuration The Corrector-ADC includes a strong field lens which is optimized so that the chief ray is approximately perpendicular to the focal surface across the full field of view. There is a complicated trade-off with a refractive ADC between the accuracy of correction that can be achieved, wavelength range, and the maximum amount of correction [GMT CoDR, Chapter 6, GMT-PM-RVW-00146]. The availability of broadband coatings is also a consideration for throughput and emissivity. The specifications in this section represent a compromise to optimize image quality and throughput over the range of elevation angle and at wavelengths where dispersion is greatest. SLR-4318: Corrector-ADC Bandpass The GMT Corrector-ADC dispersion correction shall be optimized over the wavelength range of 380 nm to 1.0 microns [Goal: 340 nm to 1.8 microns]. Rationale: This requirement is a flowdown from the SRD. SLR-1023: ADC Minimum Elevation Angle The GMT shall provide an ADC optimized to compensate for atmospheric dispersion from zenith pointing down to an elevation angle of 40 degrees. Note: Below 40 degrees, images will be partially corrected by the correction at 40 degrees.