5.1. Data preparationΒΆ
Some data preprocessing has to be done in order to have reliable inversions with Hazel. The process takes the following steps:
Data normalization: the data has to be normalized to the local quiet Sun continuum. This is sometimes slightly difficult because the nearby Si I line has strong wings and one should use that pseudocontinuum. The very first step would be to remove large scale variations of the continuum, so that it is as flat as possible (perhaps removing fringes if you have any). Then, if you are only inverting He I, you have to deal with the presence of the nearby Si I line because the He I multiplet is in the wings of this photospheric line. In this version of the code you can use SIR to simuultaneously invert the photospheric and chromospheric lines, so you do not have to worry for decontaminating from the Si I line. You can even do that with the telluric contamination nearby. If the data is off-limb, things are typically easier because there is no continuum but sometimes there is some stray-light that can give you a headache. In this case, the input should be normalized by the peak emission.
Wavelength calibration: the data has to be wavelength calibrated. How to do it depends on whether you want an absolute calibration of velocities or not. If you want such absolute scale, the best is to do the wavelength calibration using telluric lines and then transform everything to the Sun using the relative velocity between the observed region and the Earth. If not, maybe using some weak surrounding photospheric lines is enough.
Computation of the boundary condition and heliocentric angle: every pixel should be labeled with its heliocentric angle (this is important for observations close to the limb, where mu is changing fast) and its boundary condition. So, you need to get a map of heliocentric angles together with your map of observed Stokes profiles. Concerning the boundary condition, it is enough to compute the ratio between the continuum intensity at every pixel and the average at the same heliocentric angle.
Rotation of the reference system: it is important to understand which is the reference direction for positive Stokes Q in the observations. Note that the output of the code depends on the \(\gamma\) angle, which exactly defines this positive Q direction. Two possibilities appear. The first one is to set \(\gamma\) in the code so that you understand which is the reference direction in the code and then rotate the Stokes Q and U data so that the reference direction for Stokes Q is aligned with that of the code. The second possibility is to keep the data as it is and then put the appropriate value of \(\gamma\) in the code to make both reference directions equal. This is usually not difficult, but it requires to understand which is the reference direction for the telescope, which is sometimes difficult to get. It is always a good advice to have the scattering geometry in mind and try to adapt it to your observations. See Basic Equations for more information.