Image Processing Reference
Figure 3.15 The output of a Shack-Hartmann wavefront sensor as seen on a high-res-
olution CCD camera. The blue spots show a reference location; the red dots, the shift
in the array of dots due to induced tilt.
is needed to detect tip or tilt, an array of nine or more would be needed to show
defocus and so forth. Some examples of focal plane patterns for a Shack-Hartmann
array are shown in Fig. 3.15.
Modern Shack-Hartman optical wavefront sensors use large numbers of
lenslets to break up the wavefront into small segments. The overall size of these
lens arrays is also quite small, usually about the size of a quarter, requiring high
densities of the miniature lenses. The original Shack-Hartman arrays were con-
structed from crossed cylindrical lenses to make manufacturing easier. These
lenses tended to give a rectangular focal plane spot. Modern Shack-Hartman arrays
take advantage of improved micromachining capabilities and epoxy replication to
produce spherical lenslets. Imperfectly spherical lenslets induce small aberrations
that are normally not an issue in wavefront sensing.