Saturday, September 20, 2008

Closed Loop!!!

Finally. Today was a very long day. Lots of lab work, but we finally installed the lenslet array, put in the deformable mirror and closed the loop on an aberrated source. We're not done but this was a major step in getting the AO system aligned... Damn I'm tired. Got to bed around 2am then came in at 7 to take some people from the AMOS conference through the building with Mike and JD. Now I'm home on O'ahu. Weird to think that early this morning I was 'closing loop'.

A not-quite pupil image. It's starting to converge.
Jeff being jeff.
Lenslet array! A bunch of small lenses cut and glued in to place to form a nice symmetric pattern to sense the curvature of a wavefront in small sections (that pupil chopped up into little bits).
Lenslets.
Lightpath! Kind of hard to see but the simulator in the lower right launches light off a fold onto the deformable mirror in the bottom middle. There's actually a flat in there in this shot. The DM is on the left of the flat. The DM sends the light through a lens and a fold mirror on to a membrane. A beamsplitter picks off 1% of the light to the 'science camera' off on the far left. The rest goes off the membrane to another powered mirror that puts the pupil onto the lenslets (cable in the upper left).
No arrows.
Already one casualty... a chipped beam-splitter. We replaced the cube 50/50 beam-splitter for a different on-axis one and moved the science camera to the far table. This 99%/01% beam-splitter actually works fine broken since the beam is only 1", not 2. Somebody dropped it and chipped the corner. $600. Not so cool. Glad it wasn't me!
A tilted wavefront has been sensed! The left side shows flux in the lenslet - something like 400-800 photons per channel read out at 2kHz. The right side shows the wavefront curvature/derivative signal.
We put in the phase screen on the motor and saw this 'star' dance!!! An aberrated 'star' point-spread function is a bunch of speckles. On the sky, if you average over say 30 seconds, you get a nice fat gaussian about as wide as the speckles. That's your turbulent PSF. Tons of speckles, lots of 'turbulence' in this one.
Watch Nick Kaisers's aberrated wavefront simulation videos... He took a flat wavefront, put it through atmospheric turbulence, then focused it using a telescope of varying sizes.

http://www.ifa.hawaii.edu/~kaiser/

A small telescope's PSF in normal turbulence - you only catch a few 'patches' of turbulence so the star looks more or less like a point.
video

A big telescope's PSF in normal turbulence - lots more speckles because the bigger telescope mirror catches more 'patches' of turbulence.


video

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