For the telescope to meet requirements for science and operational efficiency
the SWCS needs to safely control, automate and coordinate most subsystems to
act in parallel or in sequence.
Operational requirements range from operation modes (classical, queue, etc.),
to observing modes (AO, natural seeing), to instruments, and efficiency. These
and other areas and their requirements are summarized below:
Operational Modes -– Operational modes involve classical, queue, service,
intherrupt, and remote observing, as required by the OCD and the SRD.
Technical demands of the science programs often dictate which observing
modes are the most suited. While most science programs benefit most from
queue observing, “targets of opportunity” will require interrupt or remote
observing modes. In contrast, some experimental, pilot, or high risk/high
reward programs may benefit from classical observing. Operational modes
affect the efficiency of both observatory operations and science, in terms
of scheduling flexibility and instrument readiness requirements. In
addition, the SWCS is required to operate the telescope with a partial
complement of primary and/or secondary mirrors, which will require
customized alignment, calibration, and phasing procedures.
Observing Modes -– The SWCS provides the capability to observe under
natural seeing without any adaptive optics, as well as diffraction limited
modes using natural and laser guide star adaptive optics to correct
wavefront errors in the images delivered to the focal plane. The required
observing modes are: Natural Seeing (NS), Ground Layer Adaptive Optics
(GLAO), Natural Guide Star AO (NGSAO), and Laser Tomographic AO (LTAO). For
details on these AO modes, refer to the Adaptive Optics section in SLPDR.
The SWCS is required to monitor observing conditions and, as conditions
change, to provide the capability to switch between different observing
modes and different optical configurations.
Instrument Operation -– The OCD requires that instruments mounted at any
of the ports can be deployed for use on any given night. The instrument
ports include four Direct Gregorian ports in the GIR, three Folded Ports on
top of the GIR, one Gravity Invariant station on the azimuth disk, one
future Instrument Platform station on the non-rotating IP, and two Auxiliary
Instrument ports on the elevation axis outside of the C-rings. The SWCS
design provides standardized connections to all science instruments,
maintain the readiness of all active instruments, and provide the capability
to operate and switch an active instrument set during the night. Switching
instruments will require the SWCS to automate the process (inserting or
removing M3 and corrector-ADC, adjusting focus, etc.), and to monitor that
different instruments do not interfere or vignette the fields of view.
Availability and Startup –- The operational state of the observatory and
equipment must be precisely defined and, at any given moment in time, be
known due to considerations for operations and safety of the staff and
sensitive equipment. The Figure below shows the observatory states (off,
environmentally controlled, standby, on) and processes (cold, warm, and
standby starts).
An off state is when: all power is removed from the system, mechanical
assemblies are properly parked, enclosure shutters are closed, and the
software is in an inactive state. An environmentally controlled state is
achieved when the system has reached its nominal operating temperature or
pressure. A Standby state is reached when the system has achieved a high
state of readiness to be used, and calibrations have been performed. An On
state is achieved when the system has achieved full performance specs and is
in normal operation.
In most situations, the SWCS will provide the capability to automatically
execute start-up and shutdown processes that are initiated, monitored, and
controlled by an operator. Requirements are specified for the amount of time
to achieve a certain state for the telescope and the AO system.
Blind Pointing –- Blind pointing is used to (re)position the telescope to
within the capture range of the acquisition/guide sensors relying solely on
the accuracy of the mechanical encoders and flexure tables, without
referencing a bright star or science target with known coordinates. The SWCS
is required to provide the capability to initialize and calibrate the
pointing system from cold start, or on-the-fly, by acquiring a position
reference target using an acquisition camera. The initial blind pointing
accuracy for GMT is 10 arcsec RMS, with a goal of 5 arcsec. After initial
pointing for the night, subsequent blind pointings are required to be better
than 5 arcsec RMS over the full range of the telescope motion, with a goal
of 3 arcsec. The SWCS will provide the capability to comply with pointing
budgets that are allocated and related to software and hardware controls as
given by GMT Pointing Budget (GMT-SE-REF-00477).
AGWS Pointing –- After stars are acquired by the AGWS, systematic pointing
errors typically shrink to a size smaller than the natural seeing FWHM. The
amount of residual offset errors depends on guide star brightness, location
in the focal plane, the length of time averaging on the guide sensors, and
differential flexure between the AGWS and the instrument. The pointing
accuracy requirements are specified at the intersection of the telescope
reference optical axis (ROA) and the instrument focal plane. For natural
seeing, the GMT should achieve pointing accuracy to better than 0.2 arcsec
at the DG ports, averaged over one second. At the center of a Folded Port
science instrument, the requirement relaxes to 1 arcsec, to allow for
relative flexure with M3. Differential flexure between the Telescope ROA and
the Instrument ROA will add to the AGWS pointing uncertainty. The SWCS will
provide the capability to calibrate and correct differential flexure between
the AGWS and/or AO wavefront sensors, using guide sensors in the science
instruments, and to maintain compliance with pointing budgets given by GMT
Pointing Budget (GMT-SE-REF-00477).
Tracking, Guiding, and Scanning -– During science observations, the
telescope structure, GIR, and guiders will move along specified trajectories
to track the sky or to guide on objects at sidereal or non-sidereal rates.
Tracking involves using pre-determined telescope mount and GIR encoder
motion rates to track the rotation of the sky without a guide star. Guiding,
in which pointing feedback comes from one or more off-axis guide stars, is
necessary to compensate for pointing model errors and wind perturbations of
the telescope structure and optics. When guiding sidereal astronomical
targets, the AGWS probes are either fixed or may move around the telescope
optical axis if GIR rotation is disabled. Guiding for non-sidereal targets
requires the telescope to track along a non-sidereal trajectory, while the
AGWS probes move along either linear or more complex trajectories. The same
is true in the NGSAO and LTAO observing modes, in which the AGWS does not
provide the fundamental pointing reference but is used to control
field-dependent aberrations. In these modes, either the non-sidereal target
itself, or an off-axis guide star on the On-Instrument Wavefront Sensor
(OIWS, which must be steered along the predicted non-sidereal trajectory)
provides the guiding feedback. Continuous / Drift Scanning is a
non-sidereal tracking mode where the telescope moves at a set rate relative
to sky rotation as the science instrument integrates on the data and reads
out at a predetermined rate. The SWCS will provide capabilities to track,
guide, and scan, at sidereal and non-sidereal rates. The GMT is required to
guide/track on an object with apparent motion of less than 6 arcsec/min
(with a goal of 20 arcsec/min) relative to the sky rotation. The SWCS will
comply with image quality requirements given by Natural Seeing Image Quality
Error Budgets (GMT-SE-REF-00145) and AOS IQ Error Budget (GMT-AO-REF-00518)
under AO observing modes.
In addition to standard sidereal tracking and guiding modes, the SWCS will
operate in several other more specialized modes. For certain science
observations it may be necessary to guide on a science target by holding the
GIR fixed (e.g., extrasolar planet imaging), up to 60 degrees without
rotation, or by rotating it at particular rates (e.g., to maintain
parallactic angle). For continuous, or drift, scan mode, the SWCS will
provide a guided mode for linear scanning at any specified angle, with a
drift rate that is selectable up to a maximum, and up to a travel distance
limited by the guiders.
Offsetting -– An offset is where the telescope repositions by a small amount
typically without having to reacquire a guide star. Several variations on
the theme that are commonly used are “Nodding,” “Dithering,” and “Step and
Integrate.” The SWCS will provide capabilities to perform offsets of those
and other types, with varying complexity on the patterns, and to coordinate
observations after performing offsets. The offset distance has an absolute
maximum of 3 arcmin. The SWCS will provide the capability to achieve
relative offsets with overall accuracy requirements that comply with the
allocations in GMT Pointing Budget (GMT-SE-REF-00477).
Acquisition -– Target acquisition is used for: telescope guiding, stacking
the 7 telescope apertures, active optics corrections, adaptive optics
corrections (natural and laser guide stars), and positioning science
targets. The SWCS will provide the capability to select and acquire
observing targets, with acquisition times that comply with GMT Efficiency
Budget (GMT-SE-REF-00593).
Guiding and Active Optics -– After guide star acquisitions, the telescope
guides and delivers natural seeing corrected images to the instruments
and/or AO system. During observations, the Active Optics (AcO) use guide
stars to align the optics, collimate the telescope, and control the mirror
figure for the primary mirror. The SWCS will enable AcO corrections after
the end of the telescope slew motion. The SWCS should also allow telescope
guiding even with AcO disabled. The SWCS will also monitor the active optics
probe positions to prevent shadowing of the science DG narrow field, as well
as < 20% of the DG wide field, during observations. Lastly the SWCS will
monitor, and prevent, potential interference and collisions of the AGWS
guide probes.
Efficiency –- Efficiency requirements influence the design of the SWCS by
affecting the sequence of activities (performed in parallel or in series),
and the partitions of time or error budgets. All efficiency requirements are
passed onto, or identified, by the SWCS at Level 5 after other GMT subsystem
requirements are better refined at Level 4. At Level 3, the SWCS has a
requirement to later identify and define execution sequences for
instruments, telescope, and science operations to optimize on-sky observing
efficiency, so as to comply with GMT Efficiency Budget (GMT-SE-REF-00593).
Calibration -– Calibration requirements ensure that the system performs
reliably, accurately, and consistently. The SWCS will provide the capability
to support the calibration of all subsystems and instruments in active or
ready state, and the calibration of all wavefront sensors during daytime
prior to the start of observing. All GMT instrument calibrations must comply
with GMT Efficiency Budget (GMT-SE-REF-00593). For AO, WFS calibrations
occur during daytime; the amount of time allocated for calibrations for both
routine and non-routine operations will be dictated by AO Calibration
Efficiency Performance Budget document (GMT-AO-REF-00515).
Thermal Control Requirements
Thermal control requirements manage the heat dissipation in the telescope
chamber. Thermally conditioned electronic cabinets are used to control heat
dissipation from electronic sources. The SWCS is required to remotely control
these cabinets to keep their heat dissipation in the telescope chamber to less
than 10 watts per square meter.
Architectural Requirements
Software architecture pertains to the organizational structure; the components
and their relationships; the properties of both; and the semantics necessary
for understanding or reasoning about the system. Software Architectural
Requirements establish the high level structure of the observatory software
system. The SWCS Architectural Requirements are divided into and elaborated in
the sections below.
General -– General requirements outline the high level command and control
structure of the SWCS. The software architecture is based on a component
model organized in a hierarchy of control and supervision. The SWCS will
provide central control for every subsystem, and the capability to switch
between central and local control. To facilitate efficiency in maintenance
and operations, the SWCS must be able to discover, navigate, and access any
feature provided by a software component, from a central location.
User Profiles -– To enable users to perform their functional roles, the
SWCS defines several levels of access, including the telescope operator,
instrument specialists, AO specialist, and astronomers, with appropriate and
consistent graphical user interfaces appropriate for each use case.
Telescope Control -– At Level 3, the telescope control requirements for
the SWCS remain at high level, requiring that all subsystems have a standard
behavior, and that the SWCS provides control for all optical and mechanical
degrees of freedom. The notion of “Control” includes functions like:
physical control, logic, sensor reading (optical, mechanical, pressure,
temperature), commands, diagnosis, calibration, safety, configuration,
monitoring, or fault management. The SWCS is required to contribute to the
performance of the system to within the error budget allocations. In
addition, electronic cabinets in the enclosure should contain only equipment
that requires short electrical connections to field elements. Remote
input/output modules should be used whenever possible. The SWCS is required
to use fiber optic cables that provide electrical isolation when needed.
Observatory Operations -– Observatory operations involve observations
management and execution, and data management. Briefly:
Observation Management and Execution -– From the SLR, the SWCS is required
to provide tools to assist astronomers in the proposal process to define,
plan, and execute observing programs. The SWCS will facilitate observing
by implementing sequences that allow astronomers and operators to automate
complex operations. The SWCS must also provide the capability to schedule
and manage observatory workflows and tasks, and monitor the observing
status of programs. Lastly, the SWCS will implement tools to assess and
validate data quality.
Data Management –- Data management requirements call for providing: a data
archive system for collecting, storing, and retrieving all data acquired
during observations, including metadata, for the lifetime of the
observatory; methods to generate data bundles (engineering, science,
calibration, etc.); a method to group, access, and query science,
engineering, and telemetry data from the engineering archive; and a
capability to distribute data offsite.
Observatory Services -– Observatory services include common
infrastructure, networking, time synchronization, and data storage systems.
The requirements for those services are briefly:
Common Services -– Telemetry, system health assessment, log system,
configuration management and alarm systems are commonly present in all
subsystems and components. The SWCS will provide centralized capabilities
to implement and monitor those services. The SWCS will also provide users
a universal way to gather information, via visualization displays, or an
ability to search on existing observatory subsystems for the presence of
software and hardware components, and their available commands.
Networking –- The SWCS provides the following networking infrastructure to
support different levels of quality-of-service and scalability: low
latency control network, ultra- low latency network to enable GMT
wavefront control modes, high bandwidth bulk and streaming data transfer,
industrial Ethernet fieldbus; fiber optics connection between electronic
room and equipment in the enclosure and telescope; and redundant network
cabling connection between mission critical components.
Time Synchronization -– Time synchronization is fundamental to observatory
operations. The SWCS will coordinate the operations of subsystems to
within time budgets allocated by the SLR. The SWCS will also obtain and
serve observatory-wide accurate time references.
Storage –- Storage requirements define the need for different types of
storage: non- permanent local storage, transient, permanent, and backup.
Critical engineering data and permanent data backup will exist for the
lifetime of the observatory. The SWCS will provide a high bandwidth
storage network for data access.
Interface Requirements
SWCS shall comply with Internal, Telescope, AOS, Instrument, Facilities,
Enclosure, Interlock and Safety System and System Services interface control
documents (ICD). The SWCS main interfaces have been identified and the
corresponding interface control documents are summarized in the Table below.
The overall strategy for interface specification is described in Sections
on Software Development and Subsystem Specification and Modeling.
Table 2.2 Software and Controls ICDs (sorted by SWCS PBS)#
ICD Number
ICD Name
GMT-3.0_6.1.1-ICD-00629
Software and Controls to GMTIFS ICD
GMT-3.0_6.2.1-ICD-00630
Software and Controls to GMACS ICD
GMT-3.0_6.2.2-ICD-00631
Software and Controls to MANIFEST ICD
GMT-3.0_6.3.1-ICD-00632
Software and Controls to G-CLEF ICD
GMT-3.0_4.1.2-ICD-00594
Software and Controls to M1 Subsystem ICD
GMT-3.0_4.1.7.2.6-ICD-00595
Software and Controls to M2 Positioner Assembly ICD
GMT-3.0_4.1.1-ICD-00596
Software and Controls to FSM Subsystem ICD
GMT-3.0_4.1.7.2.7-ICD-00597
Software and Controls to M2 Baffles ICD
GMT-3.0_4.1.5-ICD-00600
Software and Controls to M3 Subsystem ICD
GMT-3.0_4.1.7-ICD-00539
Software and Controls to Mount Subsystem ICD
GMT-3.0_4.1.4-ICD-00599
Software and Controls to GIR Subsystem ICD
GMT-3.0_4.1.3-ICD-00598
Software and Controls to Corrector-ADC Subsystem ICD
GMT-3.0_4.1.6-ICD-00601
Software and Controls to Acquisition, Guide and Wavefront
GMT-3.0_6.6-ICD-00633
Software and Controls to Facility Calibration System ICD
GMT-3.0_7.3.4-ICD-00496
Enclosure Control System to Enclosure Building Control System ICD
GMT-3.0_7.3.4-ICD-00492
Software and Controls to Enclosure Building Control System ICD
GMT-3.0_2.2-ICD-00634
Software and Controls to GMT Environmental Monitoring Facility
GMT-3.0_2.1-ICD-00635
Software and Controls to Interlock and Safety System ICD
GMT-3.0_5.1-ICD-00433
Software and Controls to Adaptive Secondary Mirror System ICD
GMT-3.0_5.2.3-ICD-00183
Software and Controls to VWS Support Subsystem ICD
GMT-3.0_5.2.1-ICD-00436
Software and Controls to Natural Guide Star WFS Subsystem ICD
GMT-3.0_5.2.2-ICD-00437
Software and Controls to Laser Tomography WFS Subsystem ICD
GMT-3.0_5.3-ICD-00435
Software and Controls to On-Instrument WFS System ICD
GMT-3.0_5.5.1-ICD-00438
Software and Controls to Phasing Camera Subsystem ICD
GMT-3.0_5.5.2-ICD-00434
Software and Controls to M1 Edge Sensor Subsystem ICD
GMT-3.0_5.5.3-ICD-00440
Software and Controls to M2 Edge Sensor Subsystem ICD
GMT-3.0_5.6-ICD-00439
Software and Controls to AO Real Time System ICD
GMT-3.0_5.7-ICD-00252
Software and Controls to Laser Guide Star Facility ICD
GMT-3.0_5.8-ICD-00441
Software and Controls to AO Calibration System ICD
GMT-3.0_5.9-ICD-00442
Software and Controls to AO Commissioning Camera System ICD
GMT-3.0_2.3.2.2-ICD-00636
Software and Controls to Electronics Cabinets and Enclosures ICD
