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The Gemini Planet Imager (GPI) entered on-sky commissioning phase, and hadits First Light at the Gemini South telescope in November 2013. Meanwhile, thefast loops for atmospheric correction of the Extreme Adaptive Optics (XAO)system have been closed on many dozen stars at different magnitudes (I=4-8),elevation angles and a variety of seeing conditions, and a stable loopperformance was achieved from the beginning. Ultimate contrast performancerequires a very low residual wavefront error (design goal 60 nm RMS), andoptimization of the planet finding instrument on different ends has just begunto deepen and widen its dark hole region. Laboratory raw contrast benchmarksare in the order of 10^-6 or smaller. In the telescope environment and instandard operations new challenges are faced (changing gravity, temperature,vibrations) that are tackled by a variety of techniques such as Kalmanfiltering, open-loop models to keep alignment to within 5 mas, speckle nulling,and a calibration unit (CAL). The CAL unit was especially designed by the JetPropulsion Laboratory to control slowly varying wavefront errors at the focalplane of the apodized Lyot coronagraph by the means of two wavefront sensors:1) a 7x7 low order Shack-Hartmann SH wavefront sensor (LOWFS), and 2) a specialMach-Zehnder interferometer for mid-order spatial frequencies (HOWFS) -atypical in that the beam is split in the focal plane via a pinhole butrecombined in the pupil plane with a beamsplitter. The original design goalaimed for sensing and correcting on a level of a few nm which is extremelychallenging in a telescope environment. This paper focuses on non-common pathlow order wavefront correction as achieved through the CAL unit on sky. We willpresent the obtained results as well as explain challenges that we are facing.

On-sky low order non-common path correction of the GPI Calibration Unit