5.5.4. Natural-Guidestar Ground Layer Adaptive Optics (NGLAO)

The NGLAO WFC is illustrated in Figure 5.21. It has strong similarities to the Natural Seeing WFC, with four main control loops operating simultaneously:

1. The Slow Guiding control loop updates the pointing kernel guiding offsets based on low-pass filtered AGWS average tilt measurements.

2. The NGLAO Wavefront Controller tomographically reconstructs the ground-layer wavefront from fast AGWS WFS measurements, and corrects this with the ASM.

3. The Active Optics Wavefront Controller controls field-dependent aberrations based on time- averaged AGWS WFS measurements.

4. The Flexure control loop updates the position of the AGWS probes based on On-Instrument Wavefront Sensor measurements (typically tip-tilt only, but could also include focus).

The NGLAO Wavefront Controller is a simplified version of the LTAO Wavefront Controller. The Active Optics Wavefront Controller behavior is a cross between that in the natural seeing mode and the diffraction-limited AO modes.

Fig. 5.21 NGLAO Wavefront Control System Simplified Block Diagram

Component Descriptions

The list of the NGLAO WFC components is provided in Table below. Only the three components unique to the NGLAO mode are described below.

Table 5.21 NGLAO Wavefront Control System Components

Component Name	Description Software	Package
wfc_slow_guide_pipe	Slow Guiding Pipeline	wfc_common_pkg
wfc_nglao_ctrl	NGLAO Wavefront Controller	wfc_nglao_pkg
wfc_aco_ctrl	Active Optics Controller	wfc_common_pkg
wfc_nglao_recon_srv	NGLAO Reconstructor Server	wfc_nglao_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

NGLAO Wavefront Controller

The NGLAO Wavefront Controller receives input from the AGWS slope processor and controls the surface figure of the ASM, including fast tip-tilt. In the NGLAO, all four AGWS sensors would generally be configured as 25×25 Shack-Hartmann wavefront sensors, operating at up to 206 Hz frame rate. The ASM figure command derived from each set of AGWS WFS measurements is that which minimizes the wavefront error over a given region of the telescope focal plane. Mathematically, the reconstruction is implemented as a single matrix multiplication and the controller operates in pseudo-open-loop using position feedback from the ASM at each step to compute the next command (see Figure 5.22). The details of this computation are presented in the AO Preliminary Design Report [Bouc13b].

Fig. 5.22 NGLAO Wavefront Controller Block Diagram

The NGLAO Wavefront Controller provides the following offloads to the Active Optics Wavefront Controller:

• Average global M2 tip and tilt, to be offloaded to the M2 Positioner

• The average ASM actuator positions projected onto global Zernike modes Z4, Z7, and Z8 (focus and coma). These are used to reposition the ASM.

• The average ASM actuator positions projected onto M1 segment bending modes 1-45. These are sent to the Active Optics Wavefront Controller, used to offload any M1 figure errors that have been compensated by the ASM.

The data interfaces of the NGLAO Wavefront Controller are listed in Table 10-32 [TBA].

Active Optics Wavefront Controller (NGLAO Mode)

The Active Optics Wavefront Controller (wfc_aco_ctrl) in the NGLAO mode is very similar to the diffraction-limited AO observing modes. All four AGWS sensors will generally be configured as 25×25 Shack-Hartmann wavefront sensors. The wfc_aco_ctrl component will receive time- averaged slope vectors from the AGWS Slope Processor (nominally every 30 s), from which it derives the corrections to M1 global tilt, global coma-neutral tilt, and segment Zernike modes Z5, Z6, Z9, and Z10. It also receives offloads of the ASM global focus and coma, and up to 45 M1 bending modes for each segment. These are summed and sent to the M1 and M2 optics controllers.

Finally, unlike in the other AO modes, the wfc_aco_ctrl component may receive residual tilt and focus updates from an OIWFS Slope Processor (or other instrument-based wavefront measurement), which must be rotated into the reference frame of the telescope and used to reposition the AGWS.

NGLAO Reconstructor Server

The NGLAO Reconstructor Server is another key component in this observing mode. It computes the minimum variance tomographic reconstructor based on telemetry from the AGWS and ASM (to compute the turbulence profile), and knowledge of the NGS positions. The input and output ports of the NGLAO Reconstructor Server are listed in Table 10-33 [TBA].