CALSPEC: Wide Field Camera 3 Infrared Grism Spectrophotometry

Abstract The collections of spectral energy distributions (SEDs) in the Hubble Space Telescope (HST) CALSPEC database are augmented by 19 infrared (IR) SEDs from Wide Field Camera 3 (WFC3) IR grism spectra. Together, the two IR grisms, G102 and G141, cover the 0.8–1.7 μm range with resolutions of R = 200 and 150, respectively. These new WFC3 SEDs overlap existing CALSPEC Space Telescope Imaging Spectrograph (STIS) standard star flux distributions at 0.8–1 μm with agreement to ≲1%. Some CALSPEC standards already have near-IR camera and multi-object spectrogragh (NICMOS) SEDs; but in their overlap region at 0.8–1.7 μm, the WFC3 data have better wavelength accuracy, better spectral resolution, better repeatability, and, consequently, better flux distributions of ∼1% accuracy in our CALSPEC absolute flux SEDs versus ∼2% for NICMOS. With the improved SEDs in the WFC3 range, the modeled extrapolations to 32 μm for the James Webb Space Telescope flux standards begin to lose precision longward of the 1.7 μm WFC3 limit, instead of at the 1.0-μm-long wavelength limit for STIS. For example, the extrapolated IR flux longward of 1.7 μm for 1808347 increases by ∼1% for the model fit to the data with WFC3, instead of just to the STIS SED alone.