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Master sky images for the WFC3 G102 and G141 grisms

We have constructed master sky images for the WFC3 near-infrared G102 and G141 grisms using publicly available data. The method to generate the master sky images is presented. The quality of the master sky images is assessed by measuring the homogeneity of the background subtracted grism image. Data sets that had been used for constructing the master sky image show residuals of ~0.7% and ~0.5% of the sky brightness in G102 and G141, respectively. Other G141 data that were obtained several months later show larger residuals of ~1.1% of the sky brightness, which indicates a possible variability of the grism background image shape as a function of time. The average sky brightness measured in the G102 and G141 images is 0.83e/s and 1.32e/s, respectively.