4 Data processing

The data are available one year after the date of the observations at http://BASS2000.bagn.obs-mip.fr. The software is also available, with the user's guide, and some sets of parameters for typical observations.

The data processing consists of several steps detailed below.

4.1 Geometrical calibrations

This step determines the accurate correspondence between pixels of all channels across the solar image. The geometry is specified by the edges of channels (maximum gradient of the intensity signal). For this purpose, observations are performed in the continuum close to the line, to get similar intensities in all channels.

Control plots are produced at each step of the processing. Residual distortions can be taken into account. Plots of the geometrical components of the rectangular field versus the channel-number are used to check the accuracy. The maximum departure between results and regression lines is generally smaller than 0.2 arcsec.

4.2 Photometric calibrations

Photometric calibrations are deduced from dark current records and from flat field observations in the line profile. Pointing at the line profile instead of the continuum is necessary to remove fringes and instrumental effects. If small shifts occur with respect to geometrical calibration images (because of grating rotations), they are corrected by global translations.

In all channels, the constant-wavelength curves are approximated by second degree curves (curvature of spectral lines). A mean profile of the spectral line is obtained by averaging the signal along these curves. Wavelength overlaps of successive channels are used to determine the relative transmissions. For each channel, a 2D array is produced to specify the photometric calibration.

4.3 Wavelength calibration

The wavelength defining the zero Doppler velocity can be deduced from the line center of flat-field profiles. However, the solar rotation must be taken into account. Generally the line-center wavelength is derived from a preliminary dopplergram of the full observed field-of view (quick-look).

4.4 Dopplergrams, magnetograms and line profiles

Scans are performed across the disk, with steps between 4 $^{\prime\prime}$ and 12 $^{\prime\prime}$, according to the line. Two kinds of maps can be produced:

- Intensity maps (spectroheliograms) at a set of wavelengths, derived from spline interpolations between the channels. Various algorithms are available to improve the accuracy of spline interpolations. Line profiles (and Stokes profiles) can be restored from these maps, at any point in the field-of-view.

- Dopplergrams (and magnetograms of $B_{\parallel}$) are derived from the bisector method, at different levels in the line profile.