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The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope -- III. Single Object Slitless Spectroscopy Published: 6/7/2023 7:19:12 PM Updated: 6/7/2023 7:19:12 PM

Paper abstract: The Near Infrared Imager and Slitless Spectrograph instrument (NIRISS) is theCanadian Space Agency (CSA) contribution to the suite of four scienceinstruments of JWST. As one of the three NIRISS observing modes, the SingleObject Slitless Spectroscopy (SOSS) mode is tailor-made to undertaketime-series observations of exoplanets to perform transit spectroscopy. TheSOSS permits observing point sources between 0.6 and 2.8 um at a resolvingpower of 650 at 1.25 um using a slit-less cross-dispersing grism while itsdefocussing cylindrical lens enables observing targets as bright as J=6.7 byspreading light across 23 pixels along the cross-dispersion axis. This paperofficially presents the design of the SOSS mode, its operation,characterization, and its performance, from ground-based testing andflight-based Commissioning. On-sky measurements demonstrate a peak photonconversion efficiency of 55% at 1.2 um. The first time-series on the A-typestar BD+60o1753 achieves a flux stability close to the photon-noise limit, sofar tested to a level of 20 parts per million on 40-minute time-scales aftersimply subtracting a long-term trend. Uncorrected 1/f noise residualsunderneath the spectral traces add an extra source of noise equivalent todoubling the readout noise. Preliminary analysis of a HAT-P-14b transittime-series indicates that it is difficult to remove all the noise in pixelswith partially saturated ramps. Overall, the SOSS delivers performance at thelevel required to tackle key exoplanet science programs such as detectingsecondary atmospheres on terrestrial planets and measuring abundances ofseveral chemical species in gas giants.