Method for evaluating the stray-light suppression performance of an internally occulted reflecting coronagraph in the Lyman alpha band

The Lyman alpha (Lyman-<i>α</i> or <i>L</i><i>y</i><i>α</i>) radiation emitted by hydrogen is the strongest ultraviolet spectral line on the corona, which can provide important coronal information that is difficult to obtain at other wavelengths and of great significance to comprehensively study the origin of the corona. In order to observe the <i>L</i><i>y</i><i>α</i> corona and provide space weather warning in time, the stray-light suppression performance with a field-of-view (FOV) of 2.5<i>R</i>_⊙ (<i>R</i>_⊙ stands for the mean solar radius) for <i>L</i><i>y</i><i>α</i> coronagraphs should be suppressed to 10^-6<i>B</i>_⊙ (<i>B</i>_⊙ is the mean brightness of the solar disk). Since there is no <i>L</i><i>y</i><i>α</i> light source with high brightness or <i>L</i><i>y</i><i>α</i> detector with high sensitivity, it is difficult to measure the stray-light in <i>L</i><i>y</i><i>α</i> wavebands directly. On the basis of the correlation between stray-light and wavelength, an evaluation method of stray-light in <i>L</i><i>y</i><i>α</i> wavebands based on the measurement and simulation results in visible-light and ultraviolet bands is proposed. The parameters of the simulation model are modified by comparing the measurement results, and the stray-light for the internally occulted reflecting coronagraph in <i>L</i><i>y</i><i>α</i> wavebands is analyzed with the modified model. The results show that the stray-light in <i>L</i><i>y</i><i>α</i> wavebands can be suppressed to 10^-6<i>B</i>_⊙ at 2.5<i>R</i>_⊙, and this method can be used to evaluate the stray-light level of all <i>L</i><i>y</i><i>α</i> coronagraphs.