2 Observations

The observations were made in August 1998, between the 18th and the 24th. The telescope, a Ritchey-Chrétien for which an optical scheme is supplied in Figs. 1 and 2, operated in the so-called MTR mode (Mein & Rayrole 1985; Rayrole & Mein 1993). This mode allows polarimetric observations of the sun in up to 10 different spectral regions simultaneously. Two CCDs are available for each spectral region (see Fig. 2) to record the two orthogonal polarization signals provided by the polarization analyzer, which was situated in the Cassegrain focus of the telescope. At this position little systematic instrumental polarization is introduced (during the observations, the cooling system for the entrance and exit windows was not operating, hence an amount of instrumental polarization can be expected due to thermal stress in the glass). The analyzer consists of a fixed calcite beam splitter and two rotating quarter-wave plates (see Fig. 2). Three positions were available for each one of the plates: 0, $22.5^\circ$ and $45^\circ$ relative to a privileged direction given by the optical axis of the calcites. This setup allowed successive observation of $I\pm Q$, $I\pm U$ and $I\pm V$ (see Table 1) but it did not allow path interchange for the two signs of each polarization. Therefore, after the calcite, one of the optical paths was always assigned to the I+ Stokes signals, and the other one to the I - Stokes ones. A first wide slit was placed just before the analyzer. Opened to 12 arcsec, it limited the field of view of the observations to 12 $^{\prime \prime }$$\times$120 $^{\prime \prime }$. After the transfer optics, the two paths reached the second focus where the true slit was placed, opened to 0.5 arcsec. From there it entered the spectrograph. A mask after the predispersing grating isolated the spectral regions where the selected lines were situated. At the exit of the spectrograph were placed the 4 pairs of CCD cameras used in these observations.

 

 

Table 1: Possible polarization observing modes as a function of the position of the 2 quarter-wave plates. Plate # 1 is the nearest to the beam splitter

Plate # 1	Plate # 2	Measurement
0	0	$I\pm Q$
0	22.5	$I \pm \left(\frac{1}{2}Q-\frac{\sqrt{2}}{2}V+\frac{1}{2}U\right)$
0	45	$I\mp V$
22.5	0	$I\pm \left(\frac{1}{2}Q+\frac{\sqrt{2}}{2}U+\frac{1}{2}V\right)$
22.5	22.5	$I\pm U$
22.5	45	$I\pm \left(\frac{1}{2}U-\frac{\sqrt{2}}{2}Q-\frac{1}{2}V\right)$
45	0	$I\pm U$
45	22.5	$I\pm \left(\frac{\sqrt{2}}{2}U - \frac{\sqrt{2}}{2}Q\right)$
45	45	$I\mp Q$

Each pair observed one spectral region in I+Stokes and I-Stokes respectively. All the regions for each Stokes parameter are taken strictly simultaneously in this setup. The observed lines and the spectral dispersion in the region covered by the CCDs around them are indicated in Table 2.

 

 

Table 2: List of solar lines observed, and the spectral resolution obtained for each domain

	date	Dispersion (Å/pixel)
FeI 6301 Å  and 6302 Å	22-VIII	0.0195
FeI 6149 Å  and 6151 Å	22-VIII	0.0185
FeI 5247.0 Å  and 5247.5 Å	22-VIII	0.0065
NaD1 5890 Å	22-VIII	0.0155
FeI 5576 Å	23-VIII	0.0166

After examination of the data, we have concentrated our efforts on the analysis of the observations of August 22nd. In spite of a consistent Sahara dust-driving wind, at that date several technical problems of the telescope had been solved and an almost complete series of observations and flat field images was available. All the scans covered a sunspot of the active region NOAA-8307. Each scan consisted of between 70 and 120 steps of 1 arcsec each. The scan was done by the telescope positioning system (a mirror is scheduled to be installed which will perform this and other tasks, but it was not available yet at the moment of the observations). For each step of the scan three exposures were taken to obtain in sequence $I\pm Q$, $I\pm U$ and $I\pm V$. Each exposure spanned 300 ms. We stress the fact that all the 4 spectral domains and the two polarities were taken simultaneously. The readout of the 8 CCDs and the positioning of the analyzer for the following measure added up to 1 s per polarization and up to 3 s per scan step. This waiting time will be reduced in the future since some processes can be done in parallel (while they were done sequentially during our observations) and the CCD's readout accelerated by a judicious choice of windows.

A set of data of the spectral region around 5576 Å  comes from observations taken on August 23rd. This line is insensitive to magnetic field. It constitutes therefore a very interesting tool for the measure of the noise levels in polarization and for any polarization introduced by the instrument. Unfortunately this line was not observed simultaneously with the other spectral domains.

A series of images was taken around noon by moving the telescope in an elliptical path around the solar disc center and taking several hundred exposures. The flat field image (FFI) resulted from the combination of those images. Each one of the images still presents a residual of solar granulation, but the mean of all them is free of solar details (up to a residual of 0.6% of the intensity of the continuum), and it has been used for the reduction of the observations of the whole day as described in Sect. 3.2.

Figure 1: Optical design of the MTR mode at the THEMIS telescope. Note the position of the polarization analyser, minimizing the instrumental polarization. Only one set of CCDs (CCD1) from the MTR mode is shown in the figure. Up to 10 pairs can work simultaneously, although only 4 were used in the present observing run. At top left is a picture showing the telescope in the dome, and the bottom image shows a general view of the THEMIS telescope

Figure 2: Left: Polarization analyser diagram (the light comes from above). After the beam splitter the two images with orthogonal polarization are placed side by side. An optical system in the focus F2 is used to put them one above the other, as sketched in the central figure. Right: Close-up of the exit of the MTR mode with the 2 CCD cameras, one per path. This configuration is repeated for each spectral domain