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Astron. Astrophys. 333, 732-740 (1998)

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3. Solar observations

3.1. Methods

The weakness of the Li i feature in the solar spectrum makes observations and analysis at high spatial resolution challenging. Spectroscopic observations of the Li i resonance doublet at 671 nm in the Sun were made with the 0.48 m Swedish Vacuum Solar Telescope (SVST) on La Palma in October and November 1995. The Littrow spectrograph described by Scharmer et al. (1985) was used with an extra mask inserted in the spectrograph housing to block reflections from the Littrow lens. The spectrum was registered by a 10-bit Kodak Megaplus 1.6 CCD camera operating synchronously with a similar camera that imaged the slit-jaw. A spectrograph slit of 35 µm width was used, corresponding to 0:003. This gives a nominal spectral resolution of [FORMULA]. All observations discussed here were made of quiet photosphere at solar disk centre ([FORMULA]) on November 2, 1997. The exposure time was 150 ms.

The first part of reductions were made with routines written for the purpose in the ANA interactive data processing language (Shine et al. 1988, Hurlburt et al. 1997).

Flat-fielding of the spectral frames is a problem since there is no continuous light source suitable to stand in for the Sun. The method introduced by Lites et al. (1990) employs the summing up of defocused exposures taken while the telescope scans over the solar disk. A mean spectrum is constructed for each frame by summing over the spatial direction. The resulting spectral frames are then divided with their mean spectra to get a flat field. The problem with this procedure is that unwanted systematic vertical structures cannot be removed and can even be created, e.g. by dust specks. To partially take care of this problem, a reference spectrum was made using several different grating settings so that each spectral region was recorded using different parts of the detector. Comparing the reference spectrum with the mean spectrum (from flat-field exposures) disclosed low-spatial-frequency variations which could then be corrected for. Apart from the flat-field and dark corrections, the spectra were corrected for geometrical distortions and put on a common intensity scale with the mean continuum intensity for an observational sequence as reference.

After all these treatments and corrections, close inspection of spectral frames, with the spectral lines and the continuum variations removed by division with a continuum mask and the mean spectrum, still showed a low-frequency fringe pattern with an amplitude of about 0.5 %. This is the same order of magnitude as the relative depth of the lithium line. The fringes was removed by division with a sine-wave pattern whose parameters were found by a combination of least-square fitting and eye estimates. At the same time, the spectral frames were corrected to have the same choice of continuum level as that of Brault & Müller (1975).

The resulting spectra differ from the output of a perfect instrument due to image degradation by the atmosphere and the telescope, and scattered light in the spectrograph after the slit. The impact of the former may be assessed by measuring the continuum contrast in the spectra and looking at the simultaneous slit-jaw image, the importance of the latter can be deduced by comparing strong spectral lines with a solar atlas of high spectral resolution and purity. The strong Ca I line at 672 nm was observed to have a residual intensity of 0.41 of the continuum level compared to 0.35 in the digital issue of the solar atlas of Delbouille et al. (1973). If this difference is interpreted as the result of the addition of uniform stray light, the level of this stray light is about 10 % of the "true" continuum level. This is apparently not unusual for single-pass instruments though it seems that to diagnose the origins of such straylight can be difficult (Gulliver et al. 1996).

Seventeen of the best spectra were chosen for further analysis (from now on using routines written in the IDL data processing language), they consist of [FORMULA] arrays corresponding to [FORMULA]. The pixelsizes in the spectral and the spatial direction oversamples the diffraction-limited spatial resolution ([FORMULA]) and the nominal spectral resolution ([FORMULA]). The RMS intensity contrast of the spectra, as measured along the full length of the slit in a continuum spectral region, is around 6 %. This is a quality measure on the spectra that shows that the current observations do not rival the best ever achieved with this instrument, probably those of Lites et al. (1989), but are still good.

It is not possible to examine the Li i feature in each spectral row since the S/N there is not high enough to allow reliable continuum determinations for such weak lines. Thus, the spectra were binned for each frame according to continuum intensity so that the variation of line properties with photospheric brightness may be investigated. For sufficiently strong and unblended lines, simple integration, without any assumption on line shape, is the best way to get equivalent widths. For the weakest lines, a Gaussian profile was fitted to the central part of the line feature. The equivalent width of the Li i feature measured in this way - the profile is fitted to the strongest and least blended doublet component - will not be equal to the true equivalent width for the whole doublet. But since the line is on the linear part of its curve of growth, the measured value will be closely proportional to the "real" equivalent width which corresponds to what we get from theory. Fig. 2 shows the binned spectra for one frame together with one single spectral row.

[FIGURE] Fig. 2. The upper panel shows the mean spectrum from one exposure with the lines that are included in the study marked. The lower panel shows the binned spectra in the region around the Li i feature. One single unbinned spectrum is included for reference. Li i 1 marks the main doublet component which is the one that is measured. The continua that are fitted to the spectra are shown

In the further analysis of the data, the binned spectra with less than 20 single contributing spectra were discarded in order to decrease the noise. These represent the extremes of the continuum intensity range.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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