  The next generation of extreme adaptive optics (AO) must be calibrated
exceptionally well to achieve the desired contrast for ground-based direct
imaging exoplanet targets. Current wavefront sensing and control system
responses deviate from lab calibration throughout the night due to non
linearities in the wavefront sensor (WFS) and signal loss. One cause of these
changes is the optical gain (OG) effect, which shows that the difference
between actual and reconstructed wavefronts is sensitive to residual wavefront
errors from partially corrected turbulence. This work details on-sky
measurement of optical gain on MagAO-X, an extreme AO system on the Magellan
Clay 6.5m. We ultimately plan on using a method of high-temporal frequency
probes on our deformable mirror to track optical gain on the Pyramid WFS. The
high-temporal frequency probes, used to create PSF copies at 10-22 lambda /D,
are already routinely used by our system for coronagraph centering and
post-observation calibration. This method is supported by the OG measurements
from the modal response, measured simultaneously by sequenced pokes of each
mode. When tracked with DIMM measurements, optical gain calibrations show a
clear dependence on Strehl Ratio, and this relationship is discussed. This more
accurate method of calibration is a crucial next step in enabling higher
fidelity correction and post processing techniques for direct imaging ground
based systems.
