Research On Féry Prism Dispersing Loar Spectrometer

Measurements of solar radiation and its variation are not only important forsolar physics research, but also necessary for studies on the Earth’s climate changes,space weather forecast and other application areas. Total Solar Irradiance (TSI) andSolar Spectral Irradiance (SSI) are the two aspects of solar radiation measurements.Precise, continuous and overlapped TSI measurements at Top of the Atmosphere(TOA) have been taken out internationally, whereas the SSI measurements are stilllack of sufficient accuracy and temporal continuity, especially difficult to cover thewhole spectral range. Therefore, a spaceborne Solar Prism Spectrometer (SPS) withwide spectral coverage is suggested; a miniaturized electrical substitution radiometerESR can be used by it for high-precision absolute irradiance calibration on orbit. Thetasks of optimization and simulation of prism-dispersion optical system, design ofprecision wavelength scan and feedback system, low-noise detection of weakspectral signals, wavelength and radiation calibrations of SPS prototype and so onhave been carried out successively.Firstly, the dispersion optics of SPS covering wavelength from0.25to2.50μmis designed based on Féry prism principles. The flat-field spectrum moves as theprism rotating within±2.5°, and is scanned by four exit slits and correspondingphotodiodes of different sensitive wavelength ranges. Based on Huygens PointSpread Function (PSF) and linear system theory, a Spectral Response Function (SRF) simulation method is suggested, from which the SPS’s spectral bandpass (FWHM) isderived as being from1to41nm, and the relationship between the centralwavelength of exit slits and prism rotation angle is also deduced. A reference opticalpath composed of a concave mirror and a linear CCD is designed for precise rotationangle feedback.Secondly, the SPS’s electrical control system consisting of a embedded systemand a PC master program is developed. Precise photocurrent detection and finewavelength scanning are the bases for the SPS’s radiation and wavelength accuracyrespectively. A low-noise pre-amplifier circuit is designed, and experiments indicatethe SNRs of VNIR1and VNIR2spectral channels are greater than1000. The prism’sfine rotation is achieved by applying a reliable stepper motor driver and a high ratiogear mechanism, and the precise angle feedback is achieved by CCD image process.The rotating resolution reaches0.8″, and the angle feedback pricision reaches0.11″.The PC program is designed with debugging, measurement and calibration modes toremotely control the SPS.Finally, wavelength calibration experiments are carried out by using a mercurylamp and a high-resolution grating monochromator. The relationship betweenwavelength and CCD spot centroid are established respectively for VNIR1andVNIR2spectral channels by polynomial fittings, and the calibration uncertainties areestimated to be better than0.37nm and0.89nm respectively. The equivalence ofwavelength scanning method and prism scanning method for SRF measurement istheoretically analyzed and numerically simulated, and a632.8nm laser is applied tomeasure the SRF according to the latter method. The bandpass obtained fromscanning632.8nm laser has a deviation of less than7%from simulation results. A1000W standard irradiance lamp is used to carry out the irradiance calibrationexperiment, with a calibration uncertainty evaluated to be~3.17%. The measurementuncertainty is evaluated to be~3.18%.