2.22.10

After several realignments and swapping out the mirror, I finally have the larger DM in place. I've been able to generate some pretty hot poke matrices from it; now that I know its working fo sho I've spent the last week or so updating my code and testing it out. Overall it behaves much like the smaller DM, albeit with a greater effect on the beam. I'm not sure exactly why this is, but I suppose it might be due to the larger number of actuators. Because of this I've limited it to voltages in the range [0,180] with a bias at 90V instead of the full 255.

The reconstructed actuator influence functions looked pretty solid, so I constructed a set of modes using those instead of the idealized influence functions we used for the smaller DM. The resulting modal poke matrix looks pretty good for the first 15 or so modes, but basically random after that. I'm not sure if this is due to the commands themselves being strange, or because the mirror can't reproduce high frequency shapes.

Looking at modes again has also dragged up the idea of reducing the beam size to fit in the measurement area. The current number of lenslets, around 1500, is basically an order of magnitude greater than what's found in most AO systems I've heard of, including the high fidelity HEL simulation we use to generate disturbances. The idea is that this is just causing unnecessary computational overhead since the DM's only have 31 or 61 actuators; by reducing the beam size we can utilize the WFS's partial frame mode and achieve higher frame rates (since its a CMOS sensor). This sounds dandy in theory, but in practice I've had a tough time getting a good looking, curricular beam less than around 10mm in diameter, but for the beam to fit in a 5x5 lenslet array would require the diameter to be around 2mm. The trouble is that at that size the beam isn't really circular any more, and the large intensity variations across the cross section make computing centroids from a single WFS image problematic. I don't really know what's causing this, it could be that the relay's I'm using to resize the beam may be crap for that application.

The idea of running the experiment faster is pointless anyway for a couple reasons. First, even with a 5x5 lenslet array, the WFS frame rate would still be much less than the kHz required for real real time experiments, so control would still have to be run on an event driven basis. Second, despite what's been claimed, the DM's need a finite pause between commands for them to reach steady-state. Experience generating poke matrices has shown that a pause of less than 0.08s or so between random commands results in garbage, so this is essentially the upper limit of the experiment unless there are some hardware changes...and I've had enough of those.

I'll continue looking into this a little more, but actually modeling the actuator dynamics is probably beyond what I want to do, since it would require obtaining a frequency response for each DM mode.

With the disturbance DM working, its time to move on and consider what my next move is. This week I'd like to finish modifying my simulink files to apply disturbance commands to DM61. Also, I'd like to review some of my basic adaptive filter/predictor stuff, and maybe have a nice simulink example I can "easily" port over to the experiment to play around with.

For tomorrow:
- Compare mirror step responses for DM61 and DM31 using random and single actuator commands
- Try to figure out what the extra B' is for in the eigenvalue problem for generating modes.
- Review linear predictor stuff. Start building a simple Matlab/Simulink example