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In the previous chapters we have looked at a large variety of wavefront sensors. It is natural to wonder how they stack up against each other in real life. Unfortunately, in real life, one seldom has more than one wavefront sensor to play with. But in the winter of 1991-92, a truly unique event took place under the sponsorship of NASA/Goddard Space Flight Center. This event was a "live" wavefront sensor comparison.
Goddard's interest in wavefront sensor technology stemmed from a Hubble Project requirement to independently validate COSTAR performance. COSTAR was the optical fix needed to correct the seven waves of spherical aberration inherent in the Hubble primary mirror. COSTAR was to have the same amount of spherical aberration but of opposite sign. It would restore the Space Telescope to diffraction-limited performance.
COSTAR was built by Ball Aerospace. Ball also constructed a laboratory Hubble optical simulator. It was essentially an aberration generator that mimicked the Hubble spherical aberration. COSTAR would be placed in front of this simulator and nullify its aberration. Goddard wanted a wavefront sensor that would: 1) make sure that the aberration generator gave the correct amount of spherical aberration and the right sign, and 2) validate COSTAR performance.
To its credit, Goddard did not want a paper study comparing wavefront sensors since too much was at stake! Instead, Goddard sponsored an experimental survey of several wavefront sensor techniques. This approach allowed an evaluation of the current state of the art in a more realistic fashion. Since wavefront sensors are not off-the-shelf instruments, each system Goddard examined was a breadboard to some extent. In all, four different wavefront sensors (shown in Fig. 8.1) from four different companies participated in this study.
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