  The Gemini Planet Imager (GPI) entered on-sky commissioning phase, and had
its First Light at the Gemini South telescope in November 2013. Meanwhile, the
fast 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 loop
performance was achieved from the beginning. Ultimate contrast performance
requires a very low residual wavefront error (design goal 60 nm RMS), and
optimization of the planet finding instrument on different ends has just begun
to deepen and widen its dark hole region. Laboratory raw contrast benchmarks
are in the order of 10^-6 or smaller. In the telescope environment and in
standard operations new challenges are faced (changing gravity, temperature,
vibrations) that are tackled by a variety of techniques such as Kalman
filtering, 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 Jet
Propulsion Laboratory to control slowly varying wavefront errors at the focal
plane 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 special
Mach-Zehnder interferometer for mid-order spatial frequencies (HOWFS) -
atypical in that the beam is split in the focal plane via a pinhole but
recombined in the pupil plane with a beamsplitter. The original design goal
aimed for sensing and correcting on a level of a few nm which is extremely
challenging in a telescope environment. This paper focuses on non-common path
low order wavefront correction as achieved through the CAL unit on sky. We will
present the obtained results as well as explain challenges that we are facing.
