200 nm WFE on a faint guide star

Yesterday we tested closing the loop on a faint R=13 mag star.  Ambient light from computer monitors in the test tower was too bright and so we closed the loop from the “control room” in the chem. lab.  It’s a good thing we got all the computers, desks, and high-speed ethernet links so that our control room is all set up and we could darken the “dome”!

We closed the loop on this R=13 star with the conservative 75th %ile seeing we’ve been using, 0.8” FWHM (r_0~15cm) and 33 mph winds (~15 m/s).  The frame rate was 200 Hz to get enough counts on the WFS with the pixels binned by 2.  Our resulting Strehl was 13% in z’-band which is about 200 nm rms wavefront error.  (The correction for our simulated turbulence not having high-order modes, and the correction for the double-pass on our DM in the test tower have a negligible contribution at this level).  Here is the image in z’-band:

For comparison, typical Keck AO wavefront control on H~7-9 mag NGS is also around 200 nm, or ~50% Strehl in H-band (result from 1000 Keck images from my dissertation).  So we are quite happy with this image quality!

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