Image Processing Reference
In-Depth Information
Wavefront Sensing
37
Figure 3.14
Schematic of a Shack-Hartmann wavefront sensor showing an aberrated
wavefront entering the lenslets and the displacement of the focused spots on the
sensor array.
A wavefront passing through the Shack-Hartmann lenslet array is parsed up by
the individual lenslets and focused as a spot that can be viewed on a screen, film, or
electronic camera. In the absence of aberrations in the optics, a flat wavefront pro-
vides a regular array of spots located in the focal plane on the optical axis of each
lenslet. Aberrations in the wavefront introduces a displacement of the spots, mak-
ing this test a sensitive indicator of local variations in the wavefront slopes that en-
ter the lenslet array.
The governing parameter of a Shack-Hartmann system is the ratio of the
full-aperture diameter to the size of the lenslets. In order for the Shack-Hartman to
provide optimal performance in low light conditions, the size of the lenslet must be
chosen so that the only aberration seen in the lenslet is tip or tilt. Thus, if high-order
aberrations are present, a larger number of lenslets must be used, reducing the
amount of light in each lenslet. Under bright light conditions, while only one lenslet
