Image Processing Reference
Figure 8.3 Diagram illustrating how different types of LC materials relate to optical
equivalents in terms of wave plates.
created from the individual image frames by summing them together into a single
image. The image quality can be significantly improved by determining the cen-
troid of the image and combining or stacking the images such that the image cen-
troids are coaligned. This technique, known as “shift and add,” has been used suc-
cessfully by many groups (Labeyrie 1970; Bagnuolo 1984; Ribak 1986). The
image quality can be improved further by carefully selecting the image frames used
to reconstruct the images. This technique was referred to as the “lucky strike”
method, since it relies on the rare ideal alignment of the atmosphere to produce near
diffraction-limited images (Dantowitz et al. 2000; Tubbs 2003). By combining
these specially selected images, the image quality improves significantly as shown
in Fig. 8.4.
The drawback to the summing of individual frames is that noise builds up
within the images, degrading their quality. What is desired is the ability to create
centroid-aligned images with the long integration times that a tip-tilt compensated
optical system can provide. This would require the ability to measure the move-
Figure 8.4 Time sequence of the position of a focal-plane image showing the transla-
tion in image position due to atmospheric tip and tilt (left). By capturing each image,
then determining and aligning the centroid of each frame, a single tip-tilt-removed
image is obtained (right).