A Model for the Stray Light Contamination of the UVCS Instrument on SOHO

TL;DR

This paper develops a detailed model to quantify and suppress stray-light contamination in the UVCS instrument on SOHO, improving the accuracy of solar corona observations by accounting for diffraction and scattering effects.

Contribution

It introduces a comprehensive model of stray-light suppression that considers diffraction and scattering, aligning well with laboratory and in-flight measurements.

Findings

Model accurately predicts stray-light fractions for UVCS.

Stray-light levels depend on mirror micro-roughness and aperture effects.

Applicable to multiple spectral lines beyond Lyman alpha.

Abstract

We present a detailed model of stray-light suppression in the spectrometer channels of the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. The control of diffracted and scattered stray light from the bright solar disk is one of the most important tasks of a coronagraph. We compute the fractions of light that diffract past the UVCS external occulter and non-specularly pass into the spectrometer slit. The diffracted component of the stray light depends on the finite aperture of the primary mirror and on its figure. The amount of non-specular scattering depends mainly on the micro-roughness of the mirror. For reasonable choices of these quantities, the modeled stray-light fraction agrees well with measurements of stray light made both in the laboratory and during the UVCS mission. The models were constructed for the bright H I Lyman alpha emission line, but they are applicable to other spectral lines as well.