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Astron. Astrophys. 317, 942-948 (1997)

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2. Observations

The observations consist of spectropolarimetric measurements from plage regions near disc centre of the Sun obtained with the Gregory Coude Telescope (Kneer et al. 1987) at the Observatorio del Teide/Tenerife on August 25 and September 3, 1992, under good seeing conditions. A Stokes V polarimeter, consisting of a [FORMULA] retarder and two crossed calcite rods, was mounted behind the entrance slit of the Czerny-Turner spectrograph. This gives two separate beams and spectrograms for right and left circularly polarized light, i.e. for [FORMULA] and [FORMULA] from which the Stokes I and V components can be derived. The width of the entrance slit of the spectrograph was 60 µ m ([FORMULA] 0[FORMULA]5). The spectrograph allows the simultaneous imaging of two separate spectral bands onto the same detector by means of a spectrum cutter. Spectral regions near 615 nm were selected to combine the Fe I  617.3 nm line either with Fe I  615.1 nm or with Fe II  614.9 nm. A Peltier-cooled CCD detector from Wright Instruments Ltd with a Thomson 1024x1024 chip (THX 31156) was used. Short exposure times of 0.1-0.2 s were chosen to obtain high spatial resolution.

Fe I  617.3 has an equivalent width [FORMULA] = 0.67 pm. It is a pure Zeeman triplet with Lande factor [FORMULA]. Fe I  615.1 nm ([FORMULA] = 0.45 pm) possesses a complicated Zeeman pattern with [FORMULA] = 1.833, while Fe II  614.9 ([FORMULA] = 0.35 pm) has [FORMULA] with two [FORMULA] components at the same position as the [FORMULA] components. The Fe I lines are from the same multiplet, No. 62. To construct models of small-scale magnetic flux tubes, Kneer et al. (1996) have exploited that these lines are sensitive to temperature variation due to the high ionization degree of iron in the solar atmosphere. On the other hand, the combination of the Fe II line of low magnetic sensitivity with the Fe I  617.3 line with large Landé factor has proved fruitful for separating magnetic and non-magnetic line broadening (cf. Kneer et al. 1996).

The reason for observing plages was that the chance of imaging magnetic elements onto the slit is high there. During the observations, the slit-jaw images in white light, H [FORMULA], and Ca K were monitored and recorded. The spectrograms in [FORMULA] were taken from regions with high chromospheric Ca emission, but otherwise at random. In addition several dark CCD frames and some 20 flat field spectrograms from disc centre with varying telescope positions were obtained.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998