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We report on a detailed study of the measurement stability for WFC3 IR grismstare and spatial scan observations. The excess measurement noise for bothmodes is established by comparing the observed and theoretical measurementuncertainties. We find that the stare-mode observations produce differentialmeasurements that are consistent and achieve $\sim\,1.3$ times photon-limitedmeasurement precision. In contrast, the spatial-scan mode observations producemeasurements which are inconsistent, non-Gaussian, and have higher excess noisecorresponding to $\sim\,2$ times the photon-limited precision. The inferiorquality of the spatial scan observations is due to spatial-temporal variabilityin the detector performance which we measure and map. The non-Gaussian natureof spatial scan measurements makes the use of $\chi^2$ and the determination offormal confidence intervals problematic and thus renders the comparison ofspatial scan data with theoretical models or other measurements difficult. Withbetter measurement stability and no evidence for non-Gaussianity, stare modeobservations offer a significant advantage for characterizing transitingexoplanet systems.

Comparing the WFC3 IR Grism Stare and Spatial-Scan Observations for Exoplanet Characterization