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The next generation of giant ground and space telescopes will have thelight-collecting power to detect and characterize potentially habitableterrestrial exoplanets using high-contrast imaging for the first time. Thiswill only be achievable if the performance of Giant Segmented Mirror Telescopes(GSMTs) extreme adaptive optics (ExAO) systems are optimized to their fullpotential. A key component of an ExAO system is the wavefront sensor (WFS),which measures aberrations from atmospheric turbulence. A common choice incurrent and next-generation instruments is the pyramid wavefront sensor (PWFS).ExAO systems require high spatial and temporal sampling of wavefronts tooptimize performance, and as a result, require large detectors for the WFS. Wepresent a closed-loop testbed demonstration of a three-sided pyramid wavefrontsensor (3PWFS) as an alternative to the conventional four-sided pyramidwavefront (4PWFS) sensor for GSMT-ExAO applications on the new ComprehensiveAdaptive Optics and Coronagraph Test Instrument (CACTI). The 3PWFS is lesssensitive to read noise than the 4PWFS because it uses fewer detector pixels.The 3PWFS has further benefits: a high-quality three-sided pyramid optic iseasier to manufacture than a four-sided pyramid. We detail the design of thetwo components of the CACTI system, the adaptive optics simulator and the PWFStestbed that includes both a 3PWFS and 4PWFS. A preliminary experiment wasperformed on CACTI to study the performance of the 3PWFS to the 4PWFS invarying strengths of turbulence using both the Raw Intensity and Slopes Mapsignal processing methods. This experiment was repeated for a modulation radiusof 1.6 lambda/D and 3.25 lambda/D. We found that the performance of the twowavefront sensors is comparable if modal loop gains are tuned.

Three-sided pyramid wavefront sensor. II. Preliminary demonstration on the new CACTI testbed