2 Basic principle of solar magnetograph

The principle of Zeeman splitting of a spectral line, is employed to measure the solar magnetic field. In the case of longitudinal magnetic field the $\sigma_1$ and $\sigma_2$ components of the Zeeman triplet are left and right circularly polarized. For the transverse magnetic field the $\sigma_1$ and $\sigma_2$ components are linearly polarized parallel to the field and the unshifted $\pi$ component is linearly polarized perpendicular to the field. The $\sigma_1$ and $\sigma_2$ components are separated in wavelength from the unshifted line,

$$\Delta\lambda_H = 4.67 \ 10^{-13}\, g \, \lambda^2 \, H$$ (1)

where H is the field strength in Gauss and g is the Landé factor of the spectral line, $\lambda$ and $\Delta \lambda$ are in Å units. For weak magnetic fields, the direct measurement of Zeeman splitting is not possible even with very high resolution spectrographs. Therefore the polarization properties of the Zeeman components is used to measure the magnetic field. Since the $\sigma_1$ and $\sigma_2$ components are shifted in wavelengths and oppositely polarized, choosing the left or right polarized light i.e., $\sigma_1$ or $\sigma_2$, entering the system corresponds to shifting of the lines. This principle is used in our video magnetograph. We tune the narrow band filter on the blue wing of the magnetically sensitive CaI 6122 Å line. It is found that tuning the filter at 140 mÅ away from the line center gives acceptable linearity for magnetic field measurement up to $\pm 1500$ Gauss. By switching an electro-optic variable $\lambda/4$ retarder, the field-of-view (FOV) of solar photosphere are imaged alternately in $\sigma_1$ and $\sigma_2$components. A number of images, up to 256, are added for each $\sigma_1$ and $\sigma_2$ components separately in order to achieve the acceptable signal to noise ratio. The difference between the averaged $\sigma_1$ and $\sigma_2$ images is a measure of the longitudinal magnetic field.