Design And Stray Light Analysis Of Reflective Solar Coronagraph

The corona is the outermost layers of the solar atmosphere, extending outwardfrom the top transition region to several radius of the sun, and even farther. Solarcorona observation is a key part of the space weather forecast，and has greatsignificance to the national economy and the solar physics research.Lyman-Alpha radiation, as the main components of the chromospheres spectrumoccupies very important position in solar physics and corona physics, by LymanAlpha spectrum analysis of the observation can get main structure of solarchromospheres. For corona imaging, the most difficulty is: the brightness of thecorona is too small, thus coronagraph design need serious consideration about theinstrument pupil edge diffraction and scattering caused by primary mirror, at thesame time take the initiative to increase the internal structure such as shielding andLyot stop.This article introduces the history of coronagraph, according to Lyot’ s coronadesign criterion, introduces the basic principle of transmission solar coronagraphwith external occulter. There is a theoretical calculation about block effect and edgediffraction of external disk, and a calculation about ghost position and strength madeby objective of multiple reflections which shows that a appropriate change positionof the Lyot spot can improve the effect of shade. There is a further introduction about reflection solar coronagraph and put forwarda hypothesis about the pupil position, and the approximate calculation of the positionof the mirrors under a coaxial approximation, calculation the position of the aperturewhich is a function about the focal lengths of field mirror and third mirror, hasnothing to do with the focal length of objective mirror. Under the sameapproximation condition, the size formula of the Lyot aperture which is a function ofthree mirrors’ focal lengths and distance is calculated. The calculations have aguiding effect for reflection coronagraph design.In this paper, a reflection Lyman alpha coronagraph which diameter is40mm withfocal length is590mm has been designed, with a transfer function is close to thediffraction limitation. Analyze the stray light of the coronagraph via a model buildby optical analysis software. The edge diffraction energy from the pupil received atthe image plant. According an empirical formula, the object mirror‘s surface rootmean square (RMS) roughness values should be below0.2nm.A new sphere reflection coronagraph experimental device is designed andmanufactured, which spatial resolution of the system is17.96per millimeter. Thestray light distribution curves are got in meridional direction of the simulation modelby using the Trace pro software to build the coronagraph simulation model, andcompared with experimental data. The compared result indicates the simulationmodel is reasonable. The coronagraph system stray light point source transmittancecurves are obtained and the corresponding veiling glare index of the device is0.278.