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Overview: Lyot filter is widely used in solar observation for spectra-scanning imaging such as GST in America, SMART in Japan, ChroTel in Germany and NVST in China. As the instability caused by environment or mechanical error when the filter works, calibration experiment at regular intervals is an important work to assure the accuracy and validity of Lyot filter. Traditional method to calibrate the Lyot filter often requires the high quality of the observation weather, which makes the calibration experiment more difficult and wastes the valuable time of telescope observation time. To cover the shortage of traditional method, this paper comes up with a new method to conduct the Lyot filter calibration experiment on-line. This method uses monochromatic imaging channel and Lyot filter scanning imaging channel simultaneously. We assume that the vibration of the intensity of the monochromatic imaging is only caused by the disturbance of the observation environment. We can correct the spectra-scanning data with monochromatic imaging data to correct the impact of environment.
We apply the calibration method in the high-resolution multi-wavelength solar imaging system to calibrate the Lyot filter in Hɑ(656.28) scanning imaging channel and correct the scanning data with TiO band(705 nm) observation data. We calculate the correction coefficient with TiO band data, and use it to correct the spectra line pictured by Hɑ scanning imaging data. The result of line curve calibration experiment shows that this method successfully eliminate the impact of the light instability on scanning curve of Lyot filter, as the RMS of scanning curve and standard line is reduced from 482 to 456 and the shape of the scanning curve is closer to the standard line. Then we get a group of data to test the center wavelength of the spectra curve. As the result shows, the true center wavelength has a bias which is about 0.025 nm after the spectra scanning data is corrected by the TiO imaging data. According to the user guide, we change the work temperature of the filter. The center calibration experiment shows, after correcting the center wavelength by setting the work temperature of Lyot filter from 41.805 ℃ to 42.43 ℃, the difference between the idea center and the true center of the filter is reduced form about 0.025 nm to less than 0.005 nm. The center wavelength is well corrected after the calibration experiment.
As the result shows, the instability of light source caused by disturbance of observation environment is reduced and the efficiency of the calibration experiment is increased.
The principle structure of Lyot filter
Visible light to near-infrared image of different solar atmosphere layer observed by high-resolution multi-wavelength solar imaging system
Transmittance curve of the Lyot filter
Calibration method process
(a) Calibration data before correction; (b) Calibration data after correction
Correcting coefficient
Pre-filter transmittance curve
Scanning observation line
Correcting coefficient
Calibration data after correction
(a) Calibration data before correction; (b) Calibration data after correction