Grism Sensitivities and Apparent Non-Linearity

Recent grism observations using the NICMOS instrument on HST have extended the spectrophotometric wavelength coverage of a subset of 16 STIS standard stars to ~2∝m. These observations include the three primary WD standards that are used to establish the absolute sensitivities of the three grism modes. However in their overlap region at 0.8-1.0∝m, the ratios of the STIS and NICMOS fluxes for brightest/faintest stars disagree by almost 25%. ACS spectrophotometry for two of the same stars in the same wavelength band agrees with STIS to 2%. A comparison of the grism spectrophotometry directly with ACS F850LP and F892N photometry for eight stars again verifies the need to correct the NICMOS grism data for non-linearity. After correction, the NICMOS grism fluxes have uncertainties ranging up to ~3% at 1.7∝m. The possible effect of this newly discovered apparent non-linearity on NICMOS photometry is being investigated with new NICMOS photometric observations and by comparisons with independent IR data sets.