A Novel Polarimetric Approach for Sun Center Determination and In-Flight Calibration Using Metis Coronagraph on Solar Orbiter

Abstract In both coronagraphic and total solar eclipse observations, the solar disk is not directly visible. Blocking direct light from the photosphere is essential to observe the visible solar corona, which is $10^{-5}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>5</mml:mn> </mml:mrow> </mml:msup> </mml:math> to $10^{-11}$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>11</mml:mn> </mml:mrow> </mml:msup> </mml:math> of the disk intensity. This lack of direct observation of the solar disk introduces uncertainty in determining the Sun center behind the occulter, especially in cases where instrument limitations or low signal-to-noise ratios make it challenging to apply standard astrometric approaches. We present a novel method for locating the Sun center behind the occulter during coronagraphic observations, developed using the Metis polarimetric measurements during the first close Solar Orbiter perihelion. We further suggest how this technique can enable in-flight polarization calibration. We carried out polarimetric observations of the solar corona using data from the Metis visible-light (VL) channel (580 – 640 nm). The linearly polarized brightness of the Thomson scattered corona is expected to be mainly tangential to the solar limb. By identifying pairs of such tangential polarization vectors at approximately $180^{\circ }$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>180</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:math> apart, the Sun center can be geometrically determined as the intersection point of the lines passing by these vectors. Alternatively, if the position of the Sun center is already known, the results can be further refined and potentially used to calibrate the elements of the demodulation matrix employed to derive the Stokes parameters. This article presents a novel method for detecting the Sun center behind an occulter. The approach was successfully tested, considering different distances from the Sun and off-pointing maneuvers. The discrepancy between the actual Sun center and the one estimated using this method is typically within a few pixels on the Metis VL detector, when we use coronal data with high signal-to-noise ratio. These results suggest that the method provides a valuable alternative to traditional astrometric techniques and could enable new in-flight calibration strategies for polarimetric instrumentation.