Further information may be found in Konopacky et al. 2014 “Gemini Planet Imager Observational Calibrations V: Astrometry and Distortion”
This page still to be written.
Information on astrometric performance of GPI can be found at http://www.gemini.edu/sciops/instruments/gpi/instrument-performance?q=node/12165 and in the above reference.
Distortion is small, with an average positional residual of 0.26 pixels over the field of view, and can be corrected using a 5th order polynomial. See the Correct Distortion primitive.
The distortion was calibrated in the lab by the GPI team. Instrument users should not need to rederive this calibration on their own, but should download the relevant calibration file from Gemini.
Quinn Konopacky, Jerome Maire, Sandrine Thomas, Mike Fitzgerald, Pauline Arriaga