Determination of Large Scale Plasma Properties of Solar Corona Using the X-Ray Telescope onboard Hinode: IV. Detailed Calibration of the Off-Axis Data

Coronal hole is important not only in the field of solar physics but also of space weather because it is known as the origin of the fast solar wind. However, the detailed mechanism of coronal heating leading to the acceleration of solar wind is still unknown to us. Theoretical modeling of the three-dimensional coronal structures is sensitive to the values of plasma properties at the base of solar corona and thus requires in advance accurate empirical description of those properties. Therefore, the study on the physical conditions of solar plasma in the coronal holes and near the off-limb areas will give us an essential clue to understand the boundary conditions on the mechanism of heating the coronal plasma. The X-Ray Telescope (XRT) onboard Hinode equips the modified grazing incidence (GI) mirror whose FOV is 34 x 34 arcmin covering the full solar disk and its neighboring regions, and as a result, the targets near the limb area are usually placed far from the center. For this reason, the imaging artefacts shown in the off-axis region should be calibrated carefully in order for the observed data to be properly interpreted.

We will introduce in this presentation the methods of how to calibrate the data taken near the periphery of Hinode/XRT full FOV, which include the restoration of the scattered light and the reduction of vignetting effect. We have analyzed over-exposed in-flight images during the solar flare to estimate the amount of scattered light inherent in the observed data. It is revealed the light scattered due to the roughness of GI mirror surface has a power-law distribution and its amount depends on the energy considered, which allows us to complete a PSF profile from the core to the scattering wing. We have also evaluated the vignetting effect in Hinode/XRT by analyzing the 2D distribution of effective area in the FOV taken from pre-launch experiments. Many interesting results on the optical characteristics will be introduced in our presentation.