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We present the wavelength calibration for the lenslet-based Integral FieldSpectrograph (IFS) that serves as the science instrument for the Gemini PlanetImager (GPI). The GPI IFS features a 2.7" x 2.7" field of view and a 190 x 190lenslet array (14.3 mas/lenslet) operating in $Y$, $J$, $H$, and $K$ bands withspectral resolving power ranging from $R$ $\sim$ 35 to 78. Due to variationsacross the field of view, a unique wavelength solution is determined for eachlenslet characterized by a two-dimensional position, the spectral dispersion,and the rotation of the spectrum with respect to the detector axes. The fourfree parameters are fit using a constrained Levenberg-Marquardt least-squaresminimization algorithm, which compares an individual lenslet's arc lampspectrum to a simulated arc lamp spectrum. This method enables measurement ofspectral positions to better than 1/10th of a pixel on the GPI IFS detectorusing Gemini's facility calibration lamp unit GCAL, improving spectralextraction accuracy compared to earlier approaches. Using such wavelengthcalibrations we have measured how internal flexure of the spectrograph withchanging zenith angle shifts spectra on the detector. We describe the methodsused to compensate for these shifts when assembling datacubes from on-skyobservations using GPI.

Gemini Planet Imager Observational Calibrations IV: Wavelength Calibration and Flexure Correction for the Integral Field Spectrograph