Flattening a deformable mirror

In the AO lab here at MPIA, we are building and testing a near-infrared wave front sensor for the four 8-meter VLT telescopes, to be used for long-baseline interferometry projects like GRAVITY.

My role is to characterize and calibrate the deforming element on our test bench, the MACAO 60-actuator bimorph mirror.

In the fall, after connecting ESO’s spare DM to its electronics rack and aligning a pupil shearing interferometer, we could clearly see the whopping trefoil figure of the mirror surface, caused by expected mounting stresses. The peak-to-valley of the trefoil is over 4 waves @ lambda = 633 nm.

Image

In order to measure the actuator influence functions and make a first attempt at building the control matrix, we needed to first flatten the figure. After a few software tricks failed, Stefan suggested we go through the actuators step by step, wiping out the bumps by trial and error.

To my surprise, this approach worked well. Below is the result after about a day’s worth of poking and tweaking half of the 60 electrodes. The surface RMS is now down to 0.03 waves, and the peak-to-valley is near 0.3 waves (overestimated in the diagram’s color bar due to some erroneous edge points). With this offset command in place, the surface is flat enough that we can begin measuring the influence functions, and form a working control matrix.Image

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