Photometric Calibration Of Ground-based Coronagraph

Corona is the outermost layer of the solar atmosphere.Imaging and spectral observations of the corona are important for solar atmospheric physics and space weather prediction.Coronagraph is the instrument specially designed to routinely observe the corona.In principle,physical parameters of the corona,such as plasma density and velocity,can be obtained from the observed coronal luminosity.These parameters are crucial for understanding the physics problems in the corona,such as coronal heating,solar wind acceleration and propagation of CMEs.Therefore,photometric calibrations for the original data are one of the key processes for a coronagraph to achieve scientific results and provide accurate data for space weather prediction.In this thesis,we carry out a study on the principle of photometric calibrations of coronagraph data and design a coronagraph photometer for the purpose of the photometric calibration of the ground-based spectral imaging coronagraph of the Meridian Project ?.Firstly,for the ground-based coronagraph,the weak coronal lights are seriously affected by the earth atmosphere along the transmission path before reaching the front of the primary mirror.Based on the atmospheric radiation transmission mechanism,we analysed solar irradiance measured in Weihai and Lijiang.Then the diurnal variation and annual variation of the solar irradiance in the selected passband,which is working passband of the prototype 70mm ground-based coronagraph built in Weihai.The influence of the atmospheric transmission path on solar irradiance was evaluated.These provide a theoretical basis for designing the coronagraph photometer and proves the feasibility of testing in Weihai and Lijiang.Secondly,based on the above studies and experiments,we designed a photometer that adapts to the observation and photometric calibration requirements of the ground-based coronagraph.The main parameters of the optical system of the coronagraph photometer are with a aperture of 25.4 mm,a field of view of 4° and a passband of 530±4 nm.The focal length is 100 mm.A set of baffles with decreasing apertures are set in front of the primary mirror in order to reduce the scatter lights.An attenuator is set at the front of the primary mirror and a filter is behind in order to reduce the lights and select the wavelength.The photodetector at the focus plate detect the received sunlight.In order to improve the accuracy of photometric calibration for the coronagraph,a simulated calibration system has been built in laboratory.It consists of a 532 nm narrow band laser that is used as test light source to avoid the calibration error due to beam divergence.A laser power meter and calibrated integrating sphere diode detector are used to ensure that the measurements can be transfer to the same units.The optical system transmittance and photoelectric conversion coefficient of the photometer were obtained.Compared with the observation data of wide spectrum solar direct radiometer,the variations were in agreement with each other.Finally,the developed coronagraph photometer and the prototype 70mm ground-based coronagraph were used to conduct a joint modulation test.We found that the observations of the two instruments show an inverse correlation.This can be understood because of the strong scattering effect by air in Weihai.The sunlight reach the coronagraph consists of not only the coronal lights but also the strong scattered lights.The rotation of the earth also changes the atmospheric transmission path at different times of the day significantly.The longer the atmospheric transmission path,the stronger the scattering ability,and thus the direct solar irradiance is lower as observed by coronagraph photometer while the sunlight received by the coronagraph is larger.In another case,when clouds block the sun,the scattering lights increases and that also produces such anticorrelation between the photometer and the coronagraph.Through theoretical analyses and laboratory experiments,we established the photometric calibration method for observations of ground-based coronagraph.Based on simultaneous observations of a photometer and a coronagraph,the measured coronal intensity can be normalized to the solar irradiance based the model built in this thesis.The studies set up one fundamental technique of the calibrations for the observations of the spectral imaging coronagraph.These help improve the accuracy and availability of its scientific data.For next step,we will further develop our technique and aim to invert the absolute luminosity of the corona from the observations of coronagraph,which is significant in developing space coronagraph and solar physics study.