10.23.10

So far so good with the beam alignment, but it turns out that fixing the beam resizing reveals another problem. After rewriting some of the slope calculation code to use the new beam size, I found that identifying a poke matrix like I had been doing was producing crap. The problem now is that while the beam diameter is relatively stable, the number of lenslet spots in the WFS image can change under certain DM commands. Basically, the wavefront can change enough so that the spots move in and out of the frame entirely. This could be another consequence of using a smaller beam - increased sensitivity of the WFS centroid locations.

The number of spots in the WFS image seems to be roughly proportional to the focus mode, or the average voltage command. Applying random commands (even with a pretty large range) around a particular voltage for example doesn't really present any problems; the lenslets spots just jiggle around as you'd expect (REF video in 10.21.10). But when the center voltage is changed the total number of spots can change drastically, even if the overall beam diameter doesn't change.

The main implication of this is how to set the reference wavefront. I'm pretty much resigned to doing differential wavefront measurements after learning from AOS that I was never really using some absolute reference file. But not its no longer sufficient to just subtract the reference wavefront and apply whatever commands I want. Instead, the average command voltage has to be close to the average used to determine the reference, i.e. if commands are zeroed at a voltage of 180, then in general commands should be close to zero mean.

I don't think this should be a problem for my experiments since everything from now on will be differential measurements from that reference, so there shouldn't be any large focus biases that the PI controller has to kill. Similarly, since the disturbances are nominally zero mean, I can set the average disturbance voltage to be the same value that I use to create the reference.

However, as I'm about 2 glasses of Sailor Jerry deep at the moment this could all be nonsense. I'll sort it out tomorrow.