Acta Photonica Sinica, Volume. 53, Issue 10, 1053408(2024)
Performance Test of Fabery-Perot Etalon for Radial Velocity Measurement(Invited)
Radial velocity measurement based on high resolution spectrum of stellar is crucial to exoplanet detection. To meet the requirement of detection of small rocky type exoplanet which is similar to earth, the astronomic high resolution spectrographs need advanced wavelength calibration technology to remove the errors of instrument to reach the precision of 1 m/s or better. The white light-illuminated Fabery-Perot Etalon (FPE) is a new type of wavelength calibrator, which can supply the precision of wavelength reference at 0.1 m/s/day theoretically. Recent years, wavelength calibration for astronomic high resolution spectrograph using Fabery-Perot etalon attracts great attentions due to its high precision, flexible parameters customization for different spectrographs, ease of use and low cost. We developed one calibrator based on an air-spaced Fabery-Perot etalon inside a temperature well controlled vacuum vessel. The free spectral range, fineness and working wavelength of etalon is 30 GHz, 37 and 500~700 nm, respectively. The fineness of the calibrator reduces to about 11 due to the effects of the near-field distribution of its feeding fiber. The material of spacer is ULE 7973. A 500 nm long pass filter and a 700 nm short pass filter were used to filter out the light from a halogen tungsten lamp, the emission of which is similar to that of 2 800 K blackbody. The temperature variation coefficient close to etalon inside the Vacuum vessel is about 12 mK/℃.To test the performance of this calibrator, we feed it into the calibration system of the fiber-fed High Resolution Spectrograph (HRS) at Xinglong 2.16 meters telescope. This spectrograph is the main stellar radial velocity facility of China. The resolution is about 50 000 with the working wavelength range from 360~900 nm. The temperature and air pressure of the environment of the spectrograph are well controlled. After the upgradation in the year of 2021, the spectrograph now has 2 feed fibers, one for observation, another for simultaneous reference. Accordingly, the calibration system of spectrograph has 2 units, unit A and unit B, each of them has inputs of Flat lamp, ThAr lamp, LFC and a spare port. With the equipment of two-channel simultaneous calibration system, the drifts of HRS itself during the testing can be eliminated by feeding the reference channel with a calibrator source such as ThAr lamp, Laser Frequency Comb (LFC) or FPE calibrator. After the FPE calibrator was settled in the room of HRS, the vacuum-thermostatic control system was turned on. After 30 hours of stabilization, the calibrator was fed to HRS and a series of performance tests were conducted. With the single-channel exposures of the FPE calibrator, its parameters and spectral line profiles can be analyzed. With the series of the two-channel exposures of LFC-FPE, the stability of the FPE calibrator can be analyzed. Utilize the spectral lines produced by the FPE calibrator to recalibrate the HRS, and verify the performance of the FPE wavelength solution by the LFC equipped at HRS. For the sake of extraction of HRS data, exposures of bias, flat field, ThAr, and LFC were captured according to the HRS operation manual. The experimental data from HRS in this paper were processed by an independently developed data reduce pipeline based on the Python language.From the 1D spectra of the FPE calibrator by HRS, it can be seen that the spectral lines are dense and evenly distributed, with no obvious overlaps between the lines, and the local flux of the spectral lines is very uniform. Analysis with the wavelength solution of HRS reveals that the wavelength coverage of this calibrator is from 500~685 nm, and the flux at 500 nm is relatively weak, while the flux at 685 nm is much stronger, with a difference of approximately 10 times. Furthermore, the measured frequency spacing between lines is about 29.87 GHz. All above parameters meet the expectation of design. By comparing the profiles of measured FPE spectral lines with the simulated FPE lines, which is obtained by convolving the FPE theoretical transmission spectral lines with the thorium lines by HRS as its profiles, it can be seen that the profiles and Full Widths at Half Maximum (FWHM) of the measured spectral lines and the simulated spectral lines are well consistent, which reveals that the lines of the FPE calibrator can not be resolved by the spectrograph. The results of simultaneous calibration test of LFC-FPE shows that the FPE spectral lines still exhibited significant drifts at the beginning of the test, and became stable after more than 40 hours. Comparing with LFC-LFC tests which performed at intervals, it can be found that the stability of stabilized FPE lines is better than 0.61 m/s over 5 hours. Within its operating wavelength range, the FPE calibrator has 5 425 evenly spaced transmission peaks, which is much more than the 949 thorium lines provided by the ThAr lamp in the same wavelength range. After comparison the performances of the wavelength solutions of ThAr lamp and FPE calibrator by the LFC, it can be found that the standard deviation of the residuals of FPE solution is nearly half that of Thar solution, which reveals a higher calibration accuracy of the FPE calibrator.Utilizing the astronomical high-dispersion spectrograph HRS and the simultaneous calibration technology with LFC, the characteristics of the new wavelength calibrator developed based on FPE were studied. The wavelength range, flux distribution, line spacing, line width, and short-term stability of the spectral lines all met the design expectations. Wavelength calibration experiments on the HRS indicate that the FPE calibrator can provide higher calibration accuracy compared to the traditional ThAr lamp. The dense lines, wide spectral coverage, and good line stability indicate that white light illuminated FPE is an excellent wavelength calibration light source that can improve the accuracy of wavelength calibration and radial velocity measurement for astronomical high-dispersion spectrographs. Due to its flexible parameter customization, it is very suitable for various astronomical spectrographs with different resolutions and working ranges. Further testing and research are needed for the characteristics of long-term drifts and the performance of calibration, which are highly dependent on the stability of the working environment. Replacing a high-power white light source to improve the uniformity of the total spectral flux of the calibrator, the performance of the calibrator can be further enhanced.
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Yanting LU, Huiqi YE, Liang TANG, Yang ZHAI, Kai ZHANG, dong XIAO. Performance Test of Fabery-Perot Etalon for Radial Velocity Measurement(Invited)[J]. Acta Photonica Sinica, 2024, 53(10): 1053408
Category: Special Issue for“the 40th Anniversary of Institute of Modern Optics Nankai University”
Received: May. 28, 2024
Accepted: Sep. 6, 2024
Published Online: Dec. 5, 2024
The Author Email: YE Huiqi (hqye@niaot.ac.cn)