11:15 〜 11:30
[PCG18-03] In situ wavelength calibration method for planteraty exploration: utilization of Fraunhofer lines
キーワード：分光器、波長校正
Accurate wavelength calibration is critical for qualitative and quantitative spectroscopic measurements of different kinds. Many spectrometers for planetary exploration are equipped with onboard calibration sources. For example, The Visible and Infrared Thermal Imaging Spectrometer-M on board Venus Express has two reference lamps [1, 2]. ChemCam, the instrument combining laser-induced breakdown spectroscopy and camera on board the Curiosity rover had a titanium plate for wavelength calibration on Mars [3].
However, such calibration sources are not always available because planetary lander missions often have strong limitations in size and mass. One example is the Raman spectrometer for MMX (RAX), a very small and lightweight (81 x 125 x 98 mm3, 1.4 kg) Raman spectrometer onboard the MMX rover [4-7]. Although RAX can measure an onboard verification target during the cruise phase to Phobos, it has no calibration target after the rover’s separation from the MMX spacecraft.
Mineral identification with Raman spectroscopy requires high precision and accuracy of wavenumber calibration. For some materials even quantitative analysis can be achieved with the Raman data such as estimating the magnesium to iron ratio (Mg#) of olivine which can constrain its crystallization history. Measuring the Mg# with an accuracy of 1% needs measurement of the peak positions of the olivine doublet at the Raman shift approximately 800 cm-1 with the accuracy as high as 0.1 cm-1 [8]. The scientific outcome of small and lightweight spectroscopic instruments can be increased by a calibration method without a reference target. In this study, we propose and validate a method for wavelength calibration using solar Fraunhofer lines observed in reflectance spectra.
To obtain the spectrum of the sunlight, we measured the light reflected by a 1% Spectralon reference standard with a breadboard model (BBM) of RAX. We extracted 29 Fraunhofer lines from the solar spectrum between 530 nm and 660 nm, such as the Na D-line at 589 nm and the H α line at 656 nm. Because some absorption lines such as one at 553 nm were strong, the lines overlapped with adjacent lines. For such lines, peak wavelength did not exactly match the literature values. To correct this effect, a superimposed spectrum was calculated based on the known peak positions and the instrument's peak function estimated with the Ne lamp spectrum. Then, the pixel numbers of absorption lines on the detector of BBM and literature values were correlated with a 3rd polynomial function. As a result, the difference in wavenumber calibration obtained with Fraunhofer lines and obtained with a conventional Ne lamp was smaller than 0.6 cm-1 in the 0-4000 cm-1 range. In the 800-900 cm-1 region, where the olivine doublet would appear, the difference was smaller than 0.2 cm-1. This result suggests that the estimation of the Mg# of olivine would be more accurate than 0.02 even when the Fraunhofer lines are used for in situ calibration.
Using this method, spectrometers for planetary exploration without calibration target, such as RAX, will be able to execute wavenumber calibration when the instruments can measure sunlit surface, yielding more accurate results for quantitative mineral analysis. Wavelength shifts can give insights to changes of the instrument’s performance even after landing. Thus, our method may help reduce the mass and size of a spectrometer module without compromising the quality of scientific data.
[1] Cardesin Moinelo, A. et al., (2010) IEEE Trans Geosci Remote Sens, 48(11), 3941–3950.
[2] Melchiorri, R. et al., (2003) Rev. Sci. Instrum., 74(8), 3796.
[3] Wiens, R. C. et al., (2012) SSR, 170(1–4), 167–227.
[4] Bertrand, J. et al., (2019) ASTRA 2019.
[5] Cho, Y. et al., (2021) EPS 73, 232.
[6] Hagelschuer, T. et al., (2019) IAC-70, 21–25.
[7] Schröder, S. et al., (2020) LPSC-51, Abstr. #2019.
[8] Kuebler, K. E. et al., (2006) Geochim. Cosmochim. Acta, 70(24), 6201–6222.
However, such calibration sources are not always available because planetary lander missions often have strong limitations in size and mass. One example is the Raman spectrometer for MMX (RAX), a very small and lightweight (81 x 125 x 98 mm3, 1.4 kg) Raman spectrometer onboard the MMX rover [4-7]. Although RAX can measure an onboard verification target during the cruise phase to Phobos, it has no calibration target after the rover’s separation from the MMX spacecraft.
Mineral identification with Raman spectroscopy requires high precision and accuracy of wavenumber calibration. For some materials even quantitative analysis can be achieved with the Raman data such as estimating the magnesium to iron ratio (Mg#) of olivine which can constrain its crystallization history. Measuring the Mg# with an accuracy of 1% needs measurement of the peak positions of the olivine doublet at the Raman shift approximately 800 cm-1 with the accuracy as high as 0.1 cm-1 [8]. The scientific outcome of small and lightweight spectroscopic instruments can be increased by a calibration method without a reference target. In this study, we propose and validate a method for wavelength calibration using solar Fraunhofer lines observed in reflectance spectra.
To obtain the spectrum of the sunlight, we measured the light reflected by a 1% Spectralon reference standard with a breadboard model (BBM) of RAX. We extracted 29 Fraunhofer lines from the solar spectrum between 530 nm and 660 nm, such as the Na D-line at 589 nm and the H α line at 656 nm. Because some absorption lines such as one at 553 nm were strong, the lines overlapped with adjacent lines. For such lines, peak wavelength did not exactly match the literature values. To correct this effect, a superimposed spectrum was calculated based on the known peak positions and the instrument's peak function estimated with the Ne lamp spectrum. Then, the pixel numbers of absorption lines on the detector of BBM and literature values were correlated with a 3rd polynomial function. As a result, the difference in wavenumber calibration obtained with Fraunhofer lines and obtained with a conventional Ne lamp was smaller than 0.6 cm-1 in the 0-4000 cm-1 range. In the 800-900 cm-1 region, where the olivine doublet would appear, the difference was smaller than 0.2 cm-1. This result suggests that the estimation of the Mg# of olivine would be more accurate than 0.02 even when the Fraunhofer lines are used for in situ calibration.
Using this method, spectrometers for planetary exploration without calibration target, such as RAX, will be able to execute wavenumber calibration when the instruments can measure sunlit surface, yielding more accurate results for quantitative mineral analysis. Wavelength shifts can give insights to changes of the instrument’s performance even after landing. Thus, our method may help reduce the mass and size of a spectrometer module without compromising the quality of scientific data.
[1] Cardesin Moinelo, A. et al., (2010) IEEE Trans Geosci Remote Sens, 48(11), 3941–3950.
[2] Melchiorri, R. et al., (2003) Rev. Sci. Instrum., 74(8), 3796.
[3] Wiens, R. C. et al., (2012) SSR, 170(1–4), 167–227.
[4] Bertrand, J. et al., (2019) ASTRA 2019.
[5] Cho, Y. et al., (2021) EPS 73, 232.
[6] Hagelschuer, T. et al., (2019) IAC-70, 21–25.
[7] Schröder, S. et al., (2020) LPSC-51, Abstr. #2019.
[8] Kuebler, K. E. et al., (2006) Geochim. Cosmochim. Acta, 70(24), 6201–6222.