4 Performance

To illustrate the perfomance of the system we describe some data taken from a typical oberving run. The used spectral line is FeI 630.25 nm. Using MISC with the Stokes-V polarimeter, the LaVision CCD, and the spectrographs slit width of $0\hbox{$.\!\!^{\prime\prime}$}7$ requires an exposure time of 200 ms at disk center. This exposure time provides $\approx$1000 counts per pixel in the continuum. The noise $\sigma$ is 0.22%, the achieved instrumental crosstalk is reduced to 2.6%. The length of the slit is $\approx$32$\hbox{$^{\prime\prime}$}$. Scanning a distance of $40\hbox{$^{\prime\prime}$}$ with 60 positions takes 24s and storing the data onto the hard disc takes $\approx$15s. Hence, one scan takes 39s at all and provides 13MB of spectral data. This allows to store 151 scans like this onto the disc. Taking the need for dark, flatfield an correlation scans into account, it is possible to store $\approx$120 observing scans. In this case, scanning as fast as possible provides an observing time of 78min for a single time series. Longer time intervals between two scans, binning, less positions etc. permit longer observing runs.

  

Figure 7: Four examples of physical data calculated from one scan

Figure 7 shows some images derived from the dataset of one scan taken near disk center. The upper left image is derived from the continuum near the observed spectral line. At the upper right is an image showing the minimum of the intensity in the spectral line relative to the local continuum. The Doppler image at the lower left was calculated from the wavelength of the spectral line's minimum. The Doppler velocities range from $\rm -1400\,m\,s^{-1}$ to $\rm +1400\,m\,s^{-1}$. The mean value is set to $\rm 0\,m\,s^{-1}$. An image of the magnetic flux is shown at the lower right of Fig. 7. Assuming an effective resolution element of $0\hbox{$.\!\!^{\prime\prime}$}7\times 0\hbox{$.\!\!^{\prime\prime}$}7$ the magnetic flux ranges from $\rm -2.4\ 10^{10}\,Wb$ to $\rm 1.6\ 10^{10}\,Wb$. The magnetic flux density within the resolution element ranges from $\rm -119\,mT$ to $\rm 81\,mT$ .

Similar maps, obtained with a correlation tracking device, can be found in Collados et al. (1996).