Vignetting Effect in the Soft X-Ray Telescope Onboard Yohkoh: I. Numerical Simulation

Abstract

Using a numerical simulation method, we examine the vignetting effect in the Soft X-ray Telescope (SXT) onboard Yohkoh. The off-axis variation of the effective area in the field-of-view shows that the vignetting in SXT cannot be described properly with a one-dimensional axisymmetric model. Our model assumes a response function of the X-ray mirror that is symmetric about the optical center, and an effect of occultation due to other telescope structures that is symmetric about the geometrical center; the vignetting is the result of these two contributions. We found that a rotationally non-symmetric distribution of the SXT effective area is mostly due to the offset of the optical center from the geometrical center of the telescope. The deviation from rotational symmetry due to the offset is most noticeable at the outskirts of the field-of-view, which results in the dispersion of effective area when considered as a one-dimensional distribution. The model cannot completely describe the vignetting in SXT because the fitting errors are larger than the measurement errors. We ultimately need a fully two-dimensional model for the vignetting in SXT.