5.3. Requirements on TCS, Pointing, Tracking & Guiding

The requirements governing the TCS and telescope motion control flow several levels down from the System Level to the SWCS level(s). Broadly, the requirements of the TCS address: initial blind pointing accuracy, pointing accuracy, tracking modes, continuous scan, pointing offsets, acquisition and reacquisition and, finally, special requirements on the active optics, with details to follow below.

Table 5.5 Initial Blind Pointing Requirements (Level 3)

Title	Statement
Initialization for Target Acquisition	Provide the capability to initialize and calibrate the telescope pointing system from a cold start by acquiring a position reference star with a target acquisition camera.
Telescope Initial Blind Pointing Accuracy	Provide the capability to perform blind pointing from cold start with pointing accuracy required by the Pointing Budget (GMT-SE-REF-00477).
Blind Pointing Accuracy	Provide the capability to perform blind pointing after start-of-the-night encoder initialization with pointing accuracy required by the Pointing Budget (GMT-SE-REF-00477).

Blind pointing relies entirely on the mechanical positional accuracy of the telescope encoder systems on the telescope axes and lookup tables to slew the telescope to the desired sky [α, δ] or mount [Az, El] coordinates. Blind pointing occurs in open loop (active optics and adaptive optics), and is used to position the telescope to within the capture range of the AGWS in the GIR, with which to achieve higher accuracy pointing and guiding on targets. Typically, an initial blind pointing is performed once in the beginning of the night to remove systematic errors due to temperature and flexure, so as to establish a nominal pointing origin, and fine-tune the initial pointing model for the night. Subsequent blind pointing checks may be required throughout the night to achieve the higher precision.

After the telescope positions a target to within the capture range of the AGWS via blind pointing, the four probes of the GIR will reposition to acquire guide stars in the telescope focal plane, which improves the pointing to sub-arcsecond accuracy. The fine pointing accuracy requirements are referenced to, and measured at, the center of the field of view delivered to the science instruments located at either the direct Gregorian or folded ports, under natural seeing. To meet the requirements for fine pointing, it is necessary to correct for the physical offsets and differential flexure between the science instrument and the telescope via the pointing model.

Telescope tracking keeps a celestial coordinate position fixed in the field of view, by rotating the telescope elevation, azimuth, and GIR axes. Tracking uses look-up tables that account for mount errors and relative flexure between the telescope structure and the instrument.

Several tracking modes will be implemented: Sidereal, non-sidereal, GIR fixed rotator, GIR field, GIR parallactic, and GIR fixed pupil. Non-sidereal tracking is used for scanning or for solar system targets where the AGWS and the OIWS track at the sidereal rate while the telescope mount guides on the science target to maintain its position fixed in the science detector reference frame.

Table 5.6 Tracking Mode requirements (Level 3)

Title	Statement
Sidereal Tracking	Support tracking and guiding at sidereal rate in all observing modes.
Non-Sidereal Tracking	Support tracking and guiding on an object itself.
GIR Fixed Rotator Tracking	Provide capability to track objects in fixed GIR mode.
GIR Field Tracking	Provide capability to track while the GIR rotates to maintain a fixed offset relative to the cardinal directions.
GIR Parallactic Angle Tracking	Provide tracking mode where the GIR rotates to maintain a fixed parallactic angle.

Guiding operates in closed loop, using the positions of guide stars on the AGWS or On-Instrument Wavefront Sensor (OIWFS) to make slight adjustments to maintain much more accurate telescope positions, removing the effects of wind buffeting, mechanical errors, time-dependent flexures, or other perturbations. Guiding adjustments are small corrections on top of the telescope sky-tracking rate. GMT will use four guide-probes in the AGWS to sense pointing and guiding errors in three axes (elevation, azimuth, and rotation in GIR).

Table 5.7 Pointing Accuracy Requirements (Level 3)

Title	Statement
Telescope Pointing Accuracy at the DG Ports	Provide the capability to point the field of a DG science instrument, under natural seeing, with an overall accuracy that complies with the Pointing Budget (GMT-SE-REF-00477).
SWC Telescope Pointing Accuracy at the Folded Ports	Provide the capability to point the field of an FP science instrument, under natural seeing, with an overall accuracy that complies with Pointing Budget (GMT-SE-REF-00477).
Differential Flexure Correction	Provide the capability to correct the differential flexure between the AGWS and/or AO Wavefront Sensors and the Science Instruments, using internal guide sensors provided by Science Instruments, and to comply with the GMT Pointing Budget (GMT-SE-REF-00477).

Scanning is an observing technique to obtain imaging or spectroscopy data over an area of the sky by allowing the telescope pointing to drift at a constant rate relative to the sky motion. During scanning, depending on the science goal, the science detectors may be synchronized and read out continuously to minimize image smearing, or they may stay in continuous integration. In continuous scanning mode, the AGWS continuously tracks guide stars to maintain mirror figure and pointing accuracy. However, as the guide stars may move outside the guider field during integration, the pointing kernel is responsible for finding and reacquiring guide stars somewhere in the field seamlessly and automatically.

Table 5.8 Continuous Scan Requirements (Level 3)

Title	Statement
Continuous Scan Guide Mode	Provide guiding for linear scanning of target at a fixed position angle while continuously reading out data.
Continuous Scan Rate	The maximum continuous scan rate is limited by the non-sidereal tracking rate.
Continuous Scan Distance	The maximum continuous scan distance is limited by the travel distance of the autoguiders.
Coordination of Continuous Scans	The SWCS will coordinate the operation of continuous scan and science instrument operations.

Offsets are used during observing to reposition the science detector relative to the pointing origin of the telescope. Offsetting is used for many purposes, such as to obtain sky observations, to position objects onto a slit or aperture, to create mosaic observations, etc.

Table 5.9 Offset Requirements (Level 3)

Title	Statement
Offset Distance	Provide the capability to perform telescope offsets, limited by guide star changes, and an absolute maximum radius of 3 arcmin.
Seeing-Limited Offset Accuracy	Provide the capability to perform telescope offsets with an accuracy given by SLR-4100 under seeing-limited operation, and comply with Pointing Budget (GMT-SE-REF-00477).
Diffraction Limited Offset Accuracy	Provide the capability to perform telescope offsets with an accuracy given by SLR-4110 under diffraction-limited operation, and comply with Pointing Budget (GMT-SE-REF-00477).
Seeing-Limited Offset Dwell Time	After a telescope offset, provide the capability to deliver seeing- limited images that meet the image quality budget after at least a 45 second dwell time.
Coordination of Offsets and Data Collection	Provide the capability to coordinate offsetting of telescope and operation of science data collection.
Offset Efficiency	Provide capability to execute offsets in less than the amount of time specified in the GMT Efficiency Budget document (GMT- SE-REF-00593). Note: The offset tie is for unlocking guide system, making the move, and re-engaging the guiders

The telescope pointing kernel, in cooperation with the WFCS system, performs telescope pointing, tracking, acquisition, and guiding. There are requirements for the different observing modes (natural seeing, LTAO, NGSAO). In the event that the active optics system is disabled, the pointing kernel is required to continue telescope guiding and tracking.

Table 5.10 Active Optics Requirements (Level 3)

Title	Statement
AGWS Setup Time	Provide capability to set up the AGWS in less than the amount of time specified by the GMT Efficiency Budget (GMT-SE- REF-00593) after the telescope is in position and the sensor probes are deployed.
AGWS Probe Position Time	Provide the capability to position the AGWS probes anywhere in their patrol area in less than the amount of time specified by the GMT Efficiency Budget (GMT-SE-REF-00593).
AO Acquisition Efficiency	Provide the capability to complete target acquisition, at the end of Active Optics acquisition/setup, in less than the amount of time specified by the GMT Efficiency Budget (GMT-SE-REF- 00593).
AO Re-acquisition After Opening NGS Loop	Provide the capability to re-acquire a natural guide star, after opening the NGS loop, in less than the amount of time specified by the GMT Efficiency Budget (GMT-SE-REF-00593).
LTAO Re- acquisition of LGS after open loop	Provide the capability to re-acquire LGS guide stars in the LTAO mode after opening the LGS loop in less than the amount of time specified by the GMT Efficiency Budget (GMT-SE- REF-00593).
NGLAO NGS Re- acquisition after open loop	Provide the capability to re-acquire NGS guide star in NGLAO mode, after opening the NGS loop in less than the amount of time specified by the GMT Efficiency Budget (GMT-SE-REF- 00593).
SWC Active Correction	Actively control the optical system to obtain the best image quality performance and comply with Natural Seeing Image Quality Error Budgets (GMT-SE-REF-00145)
Tracking in AcO Disabled Mode	Provide capability to track when the guiders and active optics disabled. [Goal to track with guiders enabled, but AcO disabled]
AcO Setup Time	Provide the capability to complete the set up and configuration of active optics after telescope slew in less than the amount of time specified by the GMT Efficiency Budget (GMT-SE-REF- 00593).