Inferring spectral characteristics of the H-alpha spectral line observed by the DOT Lyot filter

Koza, J.¹, Rybak, J.¹, Gömöry, P.¹, Kucera, A.¹

¹ Astronomical Institute of the Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovakia

Abstract:
A tunable Lyot filter can serve as a spectroscopic device rendering wide-field 2-D pseudospectroscopy of solar structures and follow-up crude reconstruction of a spectral line profile at each pixel within the field of view. We developed a method of inferring of the Doppler shift, the core intensity, the core width, and the core asymmetry of the H-alpha spectral line observed by the Lyot filter installed on the Dutch Open Telescope (DOT). The spectral characteristics are inferred through the fitting of five intensity samples, separated from each other by 0.35 A, by a 4th-order polynomial, a Gaussian, and a parabola. We use the atlas H-alpha profile as a reference in estimating deviations of the derived spectral characteristics. The Gaussian is the most preferable means for measurements of the Doppler shift with deviations smaller than 1 km/s. When using the 4th-order polynomial, deviations are within the interval +-2.5 km/s, but it renders comparable deviations of the core intensity and the width as the Gaussian. The deviations are largely insensitive to the shape of the filter transmission, but depend mostly non-linearly on the Doppler shift. Therefore, they do not cancel out if the spectral characteristics are represented by their relative variations. Results can be used as corrections of spectral characteristics extracted from area-averaged H-alpha profiles acquired by the DOT Lyot filter.