5.5.2. Natural Guide Star Adaptive Optics (NGSAO)

A simplified block diagram of the NGSAO WFC is presented in Figure 5.11. There are six main control loops operating simultaneously:

1. The NGSAO Wavefront Controller controls the ASM based on NGWS measurements.

2. The NGSAO Wavefront Controller updates the slope zero-points of the NGWS to correct any aberrations measured by the OIWFS.

3. The M1 and M2 Edge Sensors control the relative positions of the M1 and M2 outer segments with respect to the center segments.

4. The Phasing Controller updates the M1 edge sensor setpoints based on the average ASM segment piston.

5. The Active Optics Wavefront Controller controls field-dependent aberrations based on AGWS measurements.

6. The Pupil Motion Controller adjusts the tilt of M3 to maintain the pupil centered on the instrument.

In addition to these main feedback loops, there are several offloads between controllers. For example the NGSAO Wavefront Controller offloads the time-average segment piston and tilt to the Phasing Controller, and the average surface figure of the ASM to the Active Optics Wavefront Controller.

Fig. 5.11 NGSAO Wavefront Control System Simplified Block Diagram

Figure 5.12 presents a more detailed block diagram of the NGSAO WFC, with all of the real-time components currently envisioned. External components with interfaces to the WFC (e.g., sensor slope processors and mirror controllers) are also represented. Ports are identified with the input or output data type, and data flow with arrows. Telemetry is not explicitly shown, as it is provided as a system service to every component.

Component Descriptions

Fig. 5.12 NGSAO Wavefront Control System Detailed Block Diagram

A list of the NGSAO WFC components is provided in the Table below. A description of the most critical components is given below, followed by an identification of the connections between them.

Note that wavefront aberrations have been identified using the following nomenclature:

• Global aberrations are defined over the full GMT pupil, while segment aberrations are defined over each segment.

• System aberrations represent a measurement of the full telescope optical system, while M1, ASM, etc., aberrations represent only the contribution of one optical surface.

• Temporal averages of an aberration are identified as aberration_av.

At the present time, Zernike polynomials [Noll76] are used as the basis set throughout the WFC (with the exception of ASM and M1 figure control) due to their simple relationship to rigid body motions. Karhunen-Loeve modes which replicate Zernike modes in the low orders may be considered in the future.

Table 5.18 NGSAO Wavefront Control System Components

Component Name	Description	Software Package
wfc_ngsao_ctrl	NGSAO Wavefront Controller	wfc_ngsao_pkg
wfc_aco_ctrl	Active Optics Controller	wfc_common_pkg
wfc_ao_pupil_ctrl	AO Pupil Motion Controller	wfc_common_pkg
wfc_phasing_ctrl	AO Phasing Controller	wfc_common_pkg
wfc_ngsao_recon_srv	NGSAO Reconstructor Server	wfc_ngsao_pkg
wfc_aco_recon_srv	Active Optics Reconstructor Server	wfc_common_pkg
m1_optics_ctrl	M1 Optics Controller	wfc_common_pkg
m2_optics_ctrl	M2 Optics Controller	wfc_common_pkg
m3_optics_ctrl	M3 Optics Controller	wfc_common_pkg

NGSAO Wavefront Controller

The NGSAO Wavefront Controller (wfc_ngsao_ctrl) converts the pyramid WFS signals sent by the NGWS Slope Processor (ngws_sp) to an actuator command vector destined for the ASM Controller. The algorithms of this primary control loop are described in detail in Section 8.8.1 [Bouc13b]. As the interface to the ASM Controller is in terms of absolute actuator position, the wfc_ngsao_ctrl component provides the integrator and must receive the actuator position after the previous step by the ASM. Maintaining the last commanded position in memory is not sufficient, due to the safety checks and clipping performed by the ASM global controller. This architecture has been selected primarily for consistency with the LTAO/NGLAO pseudo-open loop control strategy.

In addition to the primary control loop, the wfc_ngsao_ctrl component performs several offloads to other WFC components at <1 Hz, calculated by projecting the time-averaged difference between the ASM actuator positions and their calibrated “flat” positions onto various modes:

• Segment piston and tilt are sent to the Phasing Controller.

• Global tilt is sent to the pointing kernel and used as the telescope guiding error.

• Global Zernike modes Z4, Z7, and Z8 (focus and coma) are sent to the Active Optics Wavefront Controller, and used to reposition the ASM.

• M1 segment bending modes [Smit13] 1-45 are sent to the Active Optics Wavefront Controller, used to offload any M1 figure errors that have been compensated by the ASM.

The wfc_ngsao_ctrl component receives residual error measurements from the OIWFS Slope Processor, allowing it to compensate changing non-common path (NCP) wavefront error. While it is typical in current-generation AO systems to perform the high-order NCP calibration in daytime, dynamic calibration will be necessary in the NGSAO and LTAO modes due to the rotation of the pupil on the Instrument Window, which will be distorted by varying atmospheric pressure (see Section 8.5.1.1 [Bouc13b]). Observations of a non-sidereal science target using a nearby fixed NGS will suffer from additional varying NCP aberrations as the NGS footprint tracks across the Instrument Window.

Tilt and focus flexure compensation will typically be performed at <1 Hz, and any faster input data stream will be averaged down. These modes will be sent to the NGWS Supervisor component for correction by the translation stages or modulation mirror. NGWS slope zeropoints will be updated at <0.1 Hz. The wfc_ngsao_ctrl component must rotate the instrument aberrations to the NGWS reference frame, and convert them to an offset in the Sx and Sy signals. This offset is then added to an offset buffer in the ngws_sp component.

The input and output ports of the wfc_ngsao_ctrl component are listed in the following Table:

Table 5.19 NGSAO Wavefront Controller Ports

Port Name	Description	Unit	Size (kB)	Rate (Hz)	Type
ngws fast wfs slopes	NGWS AO camera Sx and Sy signals.	pixel	43.5	2000	Input
ngws slow wfs slopes	NGWS 2nd WL camera Sx and Sy signals.	pixel	43.5	100	Input
oiws system global tilt	System global tip-tilt error measured by an OIWFS, in instrument reference frame.	mas	0.008	1000	Input
oiws system global focus	System global focus error measured by an OIWFS, in instrument reference frame.	nm	0.004	100	Input
oiws system global lo	System global residual Zernike modes Z4-Z200 (approx.), in instrument reference frame.	nm	0.800	10	Input
asm pos	ASM actuator actual positions.	nm	18.8	2000	Input
ngsao recon	NGSAO wavefront reconstructor matrix.	n/a	102227	0.02	Input
asm segment piston av	Time-averaged ASM segment piston, to offload to M1 Positioner.	nm	0.028	1	Output
asm segment tilt av	Time-averaged ASM segment tilt, to offload to M1 Positioner.	μrad	0.056	1	Output
asm global tilt av	Time-averaged ASM global tilt, to offload to the Mount.	μrad	0.008	1	Output
asm global lo av	Time-averaged ASM global Zernike modes Z4-6.	μm RMS	0.012	1	Output
asm segment lo av	Time-average of ASM actuators, projected onto M1 segment bending modes 1-45.	μm RMS	1.26	1	Output
ngws pos err	Position update for NGWS patrol stages or modulation mirror.	mas	0.012	1	Output
ngws fast slope ref err	Update of NGWS fast channel slope zero-points	pixel	43.5	1	Output
ngws slow slope ref err	Update of NGWS slow channel slope zero- points.	pixel	43.5	1	Output
asm cmds	ASM actuator commands.	nm	18.8	2000	Output

Active Optics Wavefront Controller (NGSAO Mode)

The Active Optics Wavefront Controller (wfc_aco_ctrl) is responsible for maintaining the collimation and focus of the telescope, and controlling the figure of the M1 segments. While the NGWS and ASM minimize the wavefront error on-axis, the AGWS will detect any off-axis aberrations resulting from M1 figure errors being corrected at M2, and vice versa.

The inputs to the wfc_aco_ctrl component are the AGWS centroids, offloads from the NGSAO Wavefront Controller, and a reconstructor matrix. The four AGWS probes can each be configured in any of 3 modes, measuring system global tilt, system segment tilt, or 25×25 Shack- Hartmann centroids. In the NGSAO mode, one probe would typically be configured to measure global tilt (used for acquisition, but ignored thereafter), and three probes configured as Shack-Hartmann sensors to control field-dependent aberrations and GIR rotation. The AGWS WFS centroid vector is multiplied by a reconstructor matrix to derive the coefficients of various output modes, which correspond to degrees of freedom of the available optical compensators. The controlled modes are listed in Table 10-25 [TBA], and in the general case include M2 segment and global rigid body motions, M1 global tilt, and up to 45 bending modes of each M1 segment. Updates to the AGWS probe position and GIR rotation angle are also computed.

In the NGSAO and LTAO modes the wfc_aco_ctrl component maintains the ASM actuators near mid-range by adjusting telescope collimation and focus (M2 global rigid body motion), but does not control M2 segment rigid body motions. Doing so could compromise the phasing of the segments, which are instead controlled directly by the M2 Edge Sensors.

The wfc_aco_ctrl component also corrects field-dependent aberrations not detected by the AO wavefront sensors. If the ASM internal metrology were perfect, then the offload of the average actuator positions projected onto the M1 segment bending modes should leave the ASM with the ideal shape over long timescales. However, any reference body flexure or drift in the internal capacitive sensors will lead to quasi-static aberrations on the ASM face sheets with compensating aberrations on the M1 segments, resulting in field-dependent aberrations. Simulations of the WFC demonstrate that low-order aberrations up to Z10 can be distinguished between M1 and M2 segments with 3 AGWS WFS (see Section 6.12.2.5 [John13]). The wfc_aco_ctrl component will correct these by applying a fixed offset to the M1 segment figure and telescope pointing. This is mathematically equivalent to updating the ASM reference shape and allowing the ASM to offload to this new shape.

The outputs of the wfc_aco_ctrl component are offset commands to the M1 and M2 optics controllers (m1_optics_ctrl and m2_optics_ctrl), and updates to the pointing kernel, which control the AGWS probe locations and GIR angle.

AO Phasing Controller (NGSAO Mode)

The Phasing Controller (wfc_phasing_ctrl) in the NGSAO mode provides updates to the M1 and M2 edge sensor control points. The relative position and tilt of the M1 and M2 segments is controlled at low bandwidth (<1 Hz) by the M1 and M2 edge sensors in closed loop with the M1 and M2 segment positioners. Meanwhile, the NGWS detects any residual segment piston and tilt at high bandwidth (>>10 Hz), and compensates this error with the ASM actuators. The role of the wfc_phasing_ctrl component in the NGSAO mode is therefore only to offload the time- average segment piston and segment tilt from the ASM actuators to the edge sensor setpoints.

Since the NGWS sees only the system segment piston and tilt error, it is not possible to determine whether it is M1 or M2 that has caused it. Offloading small amounts (microns) of segment piston to the wrong mirror has no negative impact, but segment tilt on the wrong mirror will cause field- dependent segment piston (see Section 8.1.5 [Bouc13b]). We expect the M2ES to be an order of magnitude more stable than the M1ES over long timescales, due to their construction of Zerodur and the lower sensitivity of capacitive sensors to environmental effects. The time-averaged ASM segment piston and tilt will therefore be offloaded to the M1ES setpoints, with the M2ES left unchanged.

The input and output ports of the wfc_phasing_ctrl component are listed in Table 10-26 [TBA]. The ports used in the NGSAO mode are identified in the last column.

Deployment

A representation of the deployment locations of the NGSAO WFC is provided in Figure 5.13. Most WFCS software components run on standardized rack-mounted servers in the Electronics Room of the Summit Support Building. The two exceptions which require specialized hardware are the ASM Master Control Unit crate on which the ASM Controller runs, and the AO Real-time Computer System (AORTS) which includes the NGSAO Wavefront Controller.

The current baseline design calls for a single AORTS (identified as wfc_rts in Figure 5.13) to be re-used for all the observing modes. The RTS will likely be composed of multiple server nodes in a small high-performance computing cluster (see Section 8.8.4 [Bouc13b]), and a smaller number of computing nodes might be used in the less demanding modes (e.g., NGLAO).

Fig. 5.13 NGSAO wavefront control system deployment diagram

Simulations

No complete simulations of the NGSAO wavefront control system with all of the control loops presented in this section have yet been performed. However, the fast NGSAO control loop (Loop A, see Section 8.8.1 [Bouc13b]), the edge sensor and Phasing Controller behavior (Loops C and D, see Section 10.3.2.5.3.3), and the interaction between the active optics controller and NGSAO/LTAO on-axis wavefront compensation (Loop E) described in this section have been independently simulated. Active Optics Controller simulations can be found in Section 6.12.2.5 [John13].