| With the further development of solar physics,putting forward the demand for higher res-olution(spatial resolution,time resolution,spectral resolution)and broader spectral range(long wave infrared)solar observation,many large ground-based optical and infrared solar telescopes was built nowadays in the world.Nevertheless,a practical problem is how to maximize the sen-sitivity of ground-based large infrared solar telescope,because the sensitivity of large infrared solar telescope is limited by the thermal radiation from the telescope and atmospheric infrared extinction and the fluctuation of atmospheric infrared radiation at the astronomy site.This will require us to research background radiation characteristics of ground-based infrared solar observation.On the one hand,researching the infrared radiation characteristics of the instru-ment;on the other hand,measuring atmospheric infrared extinction and atmospheric infrared radiation and radiation fluctuation of different sites,then finding excellent sites to place large aperture infrared solar telescopes.In this paper,based on the demand of ground-based infrared solar observation,the influ-ence of the earth atmosphere on the solar infrared radiation is analyzed,and the necessity of measuring the infrared radiation and extinction of the atmosphere is expounded;conducts in-depth study on property of instrument and atmospheric infrared radiation and transmission and its measurement methods,the infrared radiation characteristic of instrument was simulated and an infrared radiation measuring equipment with low emissivity can work in 3?5?m waveband was developed based on simulation results.Parameters of atmospheric infrared radiation and transmission influenced ground-based infrared solar observations,such as atmospheric absorp-tion,atmospheric background radiation,atmospheric radiation fluctuations were measured.(1)The influence of earth atmosphere of solar infrared radiative transfer is studied with Lowtran7 atmospheric radiative model,the atmospheric extinction(transmittance)changes with altitude is simulated,the different contributions of gas molecules to extinction and effects of altitude on extinction are studied;and then,the characteristics of solar radiative transfer in the earth atmosphere are simulated,the relationship between transmitted solar infrared radi-ation through the atmosphere and infrared radiation of the earth's atmosphere are discussed,pointed out that the measurement of atmospheric infrared radiation and extinction on the site survey of large aperture infrared solar telescope and the ground-based infrared solar observation is necessary.(2)A refrigeration equipment for the measurement of atmospheric radiation and extinc-tion characteristics has been developed,infrared radiation of instruments was simulated with Optical simulation software named Tracepro and changes rules of instrument radiation was analyzed,results show that extinction threads make instrument radiation down to 1/5 and ra-diation of instrument at-40~?C is down to 1/20 of 20~?C instrument.Because the fluctuation of atmospheric infrared radiation is faint target,considering the instrument for refrigeration;M waveband radiance was calculated by LBLRTM and represented as temperature of equiva-lent blackbody,taking temperature of blackbody as target,then the infrared radiation of target and opto-mechanical structure of instrument at image surface was simulated by Tracepro,re-sults show that the infrared radiation of-40~?C instrument is 23.6%of atmospheric infrared radiation.Considering operating temperature of opto-mechanical system,the refrigeration tem-perature of instrument was set to-40~?C and temperature control accuracy wasą0.03~?C;three stages refrigeration system with one stage cooling water circulation and two stages semicon-ductor was developed for cooling the whole instrument.A method used in the performance test of entire instrument was designed,according to the results of test,noise equivalent temperature difference(NETD)is 26.5mK for single pixel,NETD of instrument can reduce to 4.5mK based on the average of all pixels,the instrument can be used for the measurement of atmospheric radiation and its fluctuation.(3)In order to solve the problem of deficient dynamic range of single variate calibra-tion method for radiometric measurements,bivariate calibration model which allows to change exposure time was proposed;the temperature dependence of bivariate calibration model was investigated in high and low temperature controlled experiments,data of the temperature de-pendence of infrared radiation of the instrument fit well with Planck function.Thus trivariate calibration model was presented and tested in outfield experiments,results show that R~2is1.000,root mean square error(RMSE)is 4.25 adu and the relevant uncertainties in 95%con-fidence degree of a,b and d,parameters of the trivariate calibration model,are all less than0.82%.The benefit of this trivariate calibration model is that it can extend the dynamic re-sponse of the measuring instruments,and the precision of its measurement result is not affected by the variation of ambient temperature.Hence this model can be widely used and can achieve high-precision measurement result without in-situ calibration.(4)Atmospheric infrared radiance of four sites was measured precisely with trivariate cal-ibration model,the precision is better than 4.4%,then difference of atmospheric radiation in four sites during both day and night;the method for measurement of atmospheric extinction by measuring atmospheric radiation with varying zenith angle was presented based on atmo-spheric infrared radiative transfer equation and Kirchhoff law,then the method was tested by the LBLRTM simulation;spatial variation characteristics atmospheric radiation and extinction was measured through the method of radiation measurement with varying zenith angle,result-s show that atmospheric radiation increased by 67%,atmospheric transmittance decreased by33%from zenith to 60~?zenith angle at experimental location,ground-based observation will be seriously influenced by the variation of atmospheric radiation and extinction.(5)Time series of atmospheric infrared radiance of four sites were measured,measure-ment results were compared;fluctuation characteristics of atmospheric and blackbody radiation were analyzed with Allan variance method,fluctuations of atmospheric radiation are much larg-er than fluctuations of blackbody in low frequency region according to Allan variance curve;Allan variance curves of four sites during both day and night were showed,results show that d-ifference of 3 orders of magnitude between the Allan variances of four sites,difference of 2?3orders of magnitude between day and night in the same site,the influences of time-dependent Allan variance change are different in different sites,Allan variance can be used to evaluate infrared site;spatial correlation of atmospheric radiation fluctuations between different field of view of sky was researched,radiation fluctuations are strongly correlated in field of view;the effect of the radiation fluctuation on the chopping frequency of infrared astronomical ob-servations was analyzed with Allan variance,results show that the best chopping frequency of different sites is quite different,the smaller the best chopping frequency is,the more suitable for infrared solar observation..The refrigeration method,multivariate calibration method,the method for measuremen-t of atmospheric extinction by measuring atmospheric radiation with varying zenith angle and atmospheric radiation time fluctuation analysis with the Allan variance method proposed in this paper are significant for the site survey,design of large infrared solar telescopes and observa-tional mode of telescopes. |
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