The Space Telescope European Coordinating Facility. All Rights Reserved. ST-ECF Instrument Science Report WFC3-2009-18 WFC3 SMOV proposal 11552: Calibration of the G102 grism H. Kuntschner, H. Bushouse, M. Kümmel, J. R. Walsh October 16, 2009 ABSTRACT Based on the SMOV observations, the performance of the WFC3 near-IR grism G102 has been assessed. The location of the +1 st spectral order relative to the source positions in the direct imaging filters F098M and F105W are determined, field dependent trace and wavelength solutions are derived, and the absolute throughput is quantified. Similar information is derived for higher spectral orders and the -1 st order. The trace and wavelength solutions were found to be linear functions, varying smoothly over the field of view. Generally, there is good agreement (<= 1 pixel) with the calibration solutions derived during the ground calibrations in TV2 and TV3. For the +1 st order aperture corrections are provided as a function of wavelength. The cross-dispersion PSF is evaluated and agrees with the design expectations as well as ground calibration measurements. All calibrations in this ISR are given with respect to source positions derived from F098M images. Source positions obtained from F105W images show a systematic offset relative to F098M images of ∆X=-0.135±0.002 and ∆Y=-0.250±0.002 which needs to be corrected for when using the calibrations of this ISR. The +1 st order of the G102 grism shows a mean dispersion of 24.5 Å/pixel with a resolving power of R~155 at 10400 Å. The throughput of HST + WFC3 G102 peaks at 11000 Å with 41.8% and is above 10% between 8000 and 11500 Å.