  The next generation of giant ground and space telescopes will have the
light-collecting power to detect and characterize potentially habitable
terrestrial exoplanets using high-contrast imaging for the first time. This
will only be achievable if the performance of Giant Segmented Mirror Telescopes
(GSMTs) extreme adaptive optics (ExAO) systems are optimized to their full
potential. A key component of an ExAO system is the wavefront sensor (WFS),
which measures aberrations from atmospheric turbulence. A common choice in
current and next-generation instruments is the pyramid wavefront sensor (PWFS).
ExAO systems require high spatial and temporal sampling of wavefronts to
optimize performance, and as a result, require large detectors for the WFS. We
present a closed-loop testbed demonstration of a three-sided pyramid wavefront
sensor (3PWFS) as an alternative to the conventional four-sided pyramid
wavefront (4PWFS) sensor for GSMT-ExAO applications on the new Comprehensive
Adaptive Optics and Coronagraph Test Instrument (CACTI). The 3PWFS is less
sensitive to read noise than the 4PWFS because it uses fewer detector pixels.
The 3PWFS has further benefits: a high-quality three-sided pyramid optic is
easier to manufacture than a four-sided pyramid. We detail the design of the
two components of the CACTI system, the adaptive optics simulator and the PWFS
testbed that includes both a 3PWFS and 4PWFS. A preliminary experiment was
performed on CACTI to study the performance of the 3PWFS to the 4PWFS in
varying strengths of turbulence using both the Raw Intensity and Slopes Map
signal processing methods. This experiment was repeated for a modulation radius
of 1.6 lambda/D and 3.25 lambda/D. We found that the performance of the two
wavefront sensors is comparable if modal loop gains are tuned.
