Keratron
™
Onda Installation and operating manual
OPTIKON 2000
Cod. 161401EN
5-5
2015-03-27
Rev.C
Figure 5-4: Hartmann-Shack method (or S/H, for example)
Behind the
array
is placed a CCD sensor that acquires the points projected by the
lenslets. The
pattern
acquired in this manner is analyzed on a personal computer
with the help of an algorithm developed specifically for this purpose.
Figure 5-5shows the patterns of a Hartmann-Shack sensor in the case of a perfectly
flat wave front and in the case of an aberrated wave front. In the former case the
light points projected on the CCD are arranged uniformly, while in the latter case the
arrangement of the points is decidedly less uniform.
Figure 5-5: Images projected by a
lenslet
for an ideal wave front and an aberrated wave front
In the classical method of the centroids, the deviation of these points from the ideal
position gives us the measurement of the X and Y gradients of the wave front. From
the gradients the wave front itself can be reconstructed.
The problem with this method is that the deviation can only be measured provided
that the points remain within these cells.
In the Keratron
™
Onda a different method is used:
Demodulation of spatial
frequency
To the
pattern
projected by the lenslet on the
lenslet
are applied both the fast
Fourier transform (FFT)
and the corresponding antitransform
.
From this sequence
of operations is extracted the wave front’s phase, and from the wave front’s phase
we can obtain the X and Y gradients by means of a procedure called “
phase
unwrapping
” .
An advantage of this technique is an increase in the dynamics. In comparison to the
classical method, the points do not necessarily need to remain closed within the cells
(Figure 5-5), therefore, it is possible to measure larger aberrations.
