Polarization Calibration of the Chromospheric Lyman-Alpha SpectroPolarimeter for a 0.1 % Polarization Sensitivity in the VUV Range. Part I: Pre-flight Calibration

The Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) is a sounding rocket experiment designed to measure for the first time the linear polarization of the hydrogen Lyman- α line (121.6 nm) and requires a 0.1 % polarization sensitivity, which is unprecedented for a spectropolarimeter in the vacuum UV (VUV) spectral range. A unique polarization calibration experiment was conducted under vacuum conditions to estimate the response matrix of the instrument. For this purpose, a custom-made light source was designed to inject Lyman- α light with a known linear polarization state into the spectropolarimeter. Two methods were employed to change the orientation of the linear polarization input: one by rotating the light-source itself (direct method), the other by rotating a half-waveplate located after the light-source’s polarizers (waveplate method). The spurious polarization, scale factor, and azimuth error terms of the response matrix were successfully estimated from the polarization calibration measurements. However, it was found that the direct method could not provide an accuracy better than 0.1 % on the spurious polarization terms, whereas their required tolerance was < 0.017 % . On the other hand, the waveplate method determined these terms with only a ∼ 0.04 % accuracy due to residual cross-talk between polarization and intensity. Nevertheless, the polarization calibration confirmed the very low spurious polarization level of the instrument, which will also be confirmed with the flight data. The resulting response matrix deviated from an ideal one, and possible causes of the deviation are discussed by considering the polarization properties of the optical components.