Wavefront Correction

Facility AO correction (AO188)

SCExAO is a booster stage for the initial AO correction provided by AO188. The table below summarizes the performance and key parameters required when putting together a proposal.

Target acquisition and Image quality: AO188
Strehl Ratio, FWHM 30-40% in H-band (median seeing), See AO188 performance
Target Acquisition and
setup overhead		 AO188 target acquisition time + 10 mn Does not include fine tuning of speckle control loop
Seeing limit ~1.2" See AO188 performance
Bright limit R=-1 Neutral Density filter can be placed in front of CHARIS for bright targets
Exclusion angle around the moon 15 degrees

ExAO primary correction (Pyramid WFS)

SCExAO's main wavefront sensor is a pyramid wavefront sensor (pyWFS), driving a 2000-actuator deformable mirror (DM). The PyWFS control loop offers 70-90% Strehl ratio in H-band in median seeing conditions. On-sky performance varies according to seeing, wind speed, and guide star magnitude. Wavefront sensing is performed in visible-to-nearIR light (from 600nm to 900nm).

High Order Wavefront Correction
Strehl >80% In median seeing in the H-band
High performance magnitude limit <8 mag (R-band) Maximum Strehl can be provided down to R=8 mag
Limiting magnitude 13 mag (R-band) The loop has been tested down to 13th magnitude and an improvement in Strehl was observed. However, below 8th magnitude there is an associated performance drop.
Wavefront sensing wavelength 600-950 nm Selectable bandpass in this range. 50-150 nm bandwidth at a time which depends on magnitude of target.
Loop speed 500Hz - 3500Hz Default speed 2 kHz used down to R=9 mag. Maximum 3.5 kHz loop speed used on bright stars and fast seeing. For R>9, the loop speed is reduced.
Loop setup time 3-5 mn This time does not include overheads from AO188 and/or coronagraph setup times.

Coronagraphic Low-Order WFS

Starlight blocked by the Coronagraph may be directed to a coronagraphic Low-Order Wavefront Sensor (CLOWFS), which keeps the star well-centered on the coronagraph mask. This mode is available with the PIAACMC and VORTEX coronagraph modes to ensure optimum starlight rejection.

Speckle Control

We offer a static speckle nulling observation mode, where a DM map is computed to create a half dark hole, and the wavefront control loop is then locking the wavefront around this new offset. This mode provides a ~2x improvement in raw contrast over one half of the field of view, but extra light (>2x) is injected on the opposite side. A speckle nulling map can be pre-carved off sky and then applied on-sky in the case targets are too faint to operate speckle nulling on. However, this is not as accurate as performing the nulling process on the star of interest
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