Keck I AO Description Notes

More about Adaptive Optics at W. M. Keck Observatory

Optics Bench Overview:

Like most AO system, the incoming light from the natural guide star is split in two parts by an IR transmissive dichroic (see the Optics Bench Overview):
the visible and the near infrared (NIR) channels. The wavefront distortion is the same at all wavelengths, but corresponds to a smaller fraction of a wave for the NIR domain compared to the visible. For the same correction applied to the light, the image quality will be better at longer wavelengths. As a consequence, the visible part of the light is used for wavefront sensing and the NIR part is sent towards the science instrument or the interferometric tunnel.

The analysis of the light is made using a wavefront sensor. A real time computer gets the information from the wavefront sensor and given the telescope pupil geometry and other system parameters, sends the voltages to apply to the actuators of the tip-tilt (TT) and deformable mirrors (DM).

The three main modules of the AO system are the wavefront sensor, the real time computer, the tip-tilt mirror and DM. We give below a short description of these three modules :

wavefront sensing :
The pupil image is used to retrieve the phase of the wavefront by mean of a ``Shack-Hartmann wavefront sensor''. The telescope pupil image is sampled into 20x20 sub-pupil elements using a lenslet array of 20x20 elements. The lenslet array creates an image of each wavefront element onto a 2x2 pixel area of a 64x64 pixel CCD. The CCD frame is read and transfered to the real time computer.
The maximum frame rate for the CCD is 672 Hz and the wavefront is analyzed every ~ 1.5 ms in that optimal case. The frame rate can be decreased down to 55 Hz when observing faint targets (V magnitude of about 13.5).
Each sub-pupil image is always formed onto 4 CCD pixels. Note that the size on the sky of each individual pixel can be changed between 0.6, 1.0 and 2.4 arcsec by changing the lenslets to provide a 1.2, 2.0, or 4.8 arcsec diameter field . Also, the field stop can be changed and get any of these three values.
control software :
the real time computer is part of the WF controller. It gets the intensity for every CCD pixel. It then computes the total intensity per sub-aperture and the image centroid position for every sub-aperture. Starting from the centroid value for all sub-apertures, the phase of the total wavefront is retrieved.
The accuracy in determining the right wavefront phase at a given moment will be given by the signal to noise in every sub-aperture.
Given the geometry of the telescope (remember that Keck telescope has a serrated hexagonal pupil) and various calibration files for the wavefront sensor, the WFC computes a system matrix that compensates for pupil rotation and is used to transform the wavefront measurements from the WFS into voltage commands for the tip-tilt and DM actuators. The amplitude of the voltage applied to the tip-tilt and DM is constrained by the values of the system gains, namely TT gain and DM gain.
adaptive optics mirrors :
Tip-tilt mirror and deformable mirror (DM) are driven by actuators. Each of the 3 tip-tilt mirror actuators and the 349 DM actuators are receiving new position commands at a rate given by the WFS frame rate. An interferogram of the DM gives a real time view of the DM shape and let the operator know how the system is performing.

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Last update : 01/25/2001 by Keck AO Team