Use of ground-based observatories for astronomy has been greatly improved through the use of active correction systems based on subaperture wavefront sensing techniques such as Hartmann and curvature sensing. While the resulting performance significantly exceeds the performance of the unaided observatory, the use of a large number of independent subapertures to determine the necessary local corrections results in a requirement for relatively bright guidestars. This, in turn, limits the available area of the sky that supports high quality imagery of target using natural guidestar operation. The use of artificial guidestars increases the available region of the sky for viewing, but the resulting imagery is not as good as is achievable in the vicinity of a natural guidestar. The use of image-based sensing such as focus diverse phase retrieval produces higher quality wavefront sensing, but, if used as the sole WFS machine, generally requires longer processing times than can used for real-time correction. In this paper, we discuss the potential of using secondary wavefront sensing and correction systems within individual instruments to supplement the observatory active system. In particular, we present simulation results demonstrating the performance of a potential real-time focus diverse phase retrieval based WFS&C subsystem. We discuss the required observatory active correction performance, the secondary guidestar characteristics, and the processing speed requirements.
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