Atmospheric Infrared Transmittance Measurement

Atmospheric transmittance and atmospheric infrared radiation are important contents in the fields of research ground-based space target detection,ground-based space remote sensing,and astronomical research and other fields.The ability of infrared detection system to receive target radiation is directly related to the atmospheric transmissivity and the atmospheric background radiation limit.The intensity of the atmospheric background radiation determines the sensitivity limit of the infrared detection system,which will direct affects the system design.In infrared astronomical research,they are also important index to reflect the astronomical observing conditions and performances of the observatory stations.Therefore,it is great significant for studying the atmosphere infrared transmission characteristics and the infrared radiation characteristics.Ground-based infrared astronomical observation,the background of atmospheric infrared radiation can not be neglected,and its characteristics and changing laws must be mastered.A measurement system of mid-infrared atmospheric radiation is designed for this purpose.In the process of using this system try to measure atmospheric infrared radiation,it is found that the instrument radiation is very strong,the atmospheric radiation is very weak.The two types of radiation signals are fully coupled.The background signal of the instrument radiation will greatly occupy the full-well electrons of the infrared detector and reduce system dynamic range.The instrument radiation also varies with ambient temperature.All of these lead to the accuracy of the system to measure atmospheric radiation very low.In order to improve the measurement accuracy,improve the dynamic range of the system,peel off the influnence of instrument radiation on the atmospheric radiation signals,the calibration method of the system is carefully studied.The ternary calibration model method is used to calibrate the system,and the whole telescope system is subjected to steady low-temperature refrigeration and the temperature control.Through these keymeasures,the stability of the system is greatly improved,and the high precision measurement of atmospheric infrared radiation is realized finally.On the basis of realizing high-precision measurement of atmospheric infrared radiation,the atmospheric infrared radiation at different zenith angles is scan measurement.Based on the radiative transfer equation solution,by fitting the radiance with the zenith angle,the average atmospheric transmittance within the in-band can be obtained.For this reason,the average atmospheric transmittance in the M' band(4.605~4.755 um)of the Ali Observatory,the Delingha Observatory and the Huairou Observatory was measured using mentioned above the measurement system.The measurement results show that the zenith average atmospheric transmittance in Ali,Delingha and Huairou are gradually reduced in the M' band,and the weighted average of the zenith average transmittance is 0.805,0.758 and 0.650 respectively.The infrared atmospheric transmittance in three places has fluctuations,the fluctuation of Huairou is small,only 0.073,indicate that the transmittance change of Huairou is the most stable among the three places.The transmittance of Ali is not much,it is0.081.The fluctuation of Delingha is relatively larger,the values of fluctuations reached 0.250,indicating that its transmittance is very unstable.The transmittance fluctuation has nothing to do with the strongness and weakness of transmittance,and the place with low transmittance may be relatively stable,but in contrast,the best infrared astronomical observation effect is in Ali.For comparison,the average transmittance simulated by MODTRAN software is respectively 0.846,0.794,and0.623,respectively.The simulated results can basically correspond to the measured results at a certain moment,indicating that the transmittance measured by the method is reliable.This transmittance measurement method does not require obtaining complex atmospheric meteorological parameters,which is relatively simple and convenient.It is convenient to make a general assessment for the infrared atmosphere transmittance of the Astronomical Observatory during the stage of astronomical site selection.This transmittance measurement method indirectly obtains the zenith average transmittance by measuring the atmospheric infrared radiance at different zenith angles.When use the system to measure the atmospheric infrared radiance,the uncertainty of the radiance measurement results will influence final fit transmittance result.Therefore,in the process of analyzing the theoretical error of the average atmospheric transmittance,the measurement error of atmospheric infrared radiation is first analyzed,including random error,the calibration error and the extrapolation error.The result shows that the measurement error of this system in the calibration region is mainly composed of the radiometric calibration error and random error,whose value is 2.4719%,0.0790% respectively;and whose composite error is2.4732%.For most of some best astronomy sites,the atmospheric radiant intensity is far lower than the radiant intensity at calibration,so extrapolation measuring is necessary when using this system to measure atmospheric infrared radiation.The estimation result of extrapolation error suggests that extrapolation measurement can lead to higher measurement error.Therefore,in order to improve the precision of atmospheric background radiation measurement,the standard radiation sources with lower radiant intensity are indispensable.After comprehensively considering various types of errors,the theoretical error of the atmospheric radiation measurement is12.64%.The theoretical error of acquisition the zenith average atmospheric transmittance is less than 10% by the error transmission analysis method.Therefore,in the process of analyzing the theoretical error of the average atmospheric transmittance,the measurement error of atmospheric infrared radiation is first analyzed,including random error,the calibration error and the extrapolation error.The result shows that the measurement error of this system in the calibration region is mainly composed of the radiometric calibration error and random error,whose value is 2.4719%,0.0790% respectively;and whose composite error is 2.4732%.For most of some best astronomy sites,the atmospheric radiant intensity is far lower than the radiant intensity at calibration,so extrapolation measuring is necessary when usingthis system to measure atmospheric infrared radiation.The estimation result of extrapolation error suggests that extrapolation measurement can lead to higher measurement error.Therefore,in order to improve the precision of atmospheric background radiation measurement,the standard radiation sources with lower radiant intensity are indispensable.After comprehensively considering various types of errors,the theoretical error of the atmospheric radiation measurement is 12.64%.The theoretical error of acquisition the zenith average atmospheric transmittance is less than 10% by the error transmission analysis method.