2 Instrumentation

Spatial scanning takes full advantage of the GCT's spectrograph: high spatial and spectral resolution, wide observable wavelength range (near infrared to ultraviolet), simultaneous observations of several spectral lines (if requested on one CCD chip). Spatial scanning can be performed in different ways. When changing the pointing of the telescope it is necessary to move a large mass very fast and very precisely. This would require major changes in the telescope mount and drives. The GCT (Kneer et al. 1987) has a coudé mount, therefore drift scanning requires an image derotating device to align the East-West direction of the image always perpendicularly to the entrance slit of the spectrograph. Additionally, it would be necessary to control the drift speed in order to adapt it to the exposure time. The optical arrangement of the GCT and its spectrograph is given by Stix (1991).

These problems are avoided by MISC. It is mounted in front of the spectrograph's entrance slit and allows one to move the image perpendicularly across the entrance slit. The optical system of MISC consists of two major parts. The first one consists of three mirrors, which are arranged similarly as the active surfaces of a Dove prism. Figure 2 illustrates this device and its optical path. Shifting this mirror arrangement by a distance $\Delta x$ shifts the image by $2 \Delta x$. A minor disadvantage of this setup is that the optical axes of the telescope and the spectrograph are shifted against each other. This moves the light bundle across the grating and changes slightly its illumination. This problem could be solved by an accurate mechanical and optical adjustment and considering this effect in the data reduction.

  

Figure 2: Left: image scanner seen from above. The white dashed line marks the light path through the scanning device. The Bowen compensator is indicated. Right: scheme of the light paths through the scanner for two positions

  

Figure 3: Full view of the image scanner. The darker upper part is the scanning device, the lower part is the mounting which allows easy mechanical adjustment

  

Figure 4: The MISC Stokes-V polarimeter

The second part is a Bowen compensator (Koschinsky & Kneer 1996 and references in there). This Bowen compensator consists of two $\lambda /8$ retarder plates, which can be rotated against each other. Thus it is possible to produce phase retardations between $-\lambda /4$ and $+\lambda /4$. This device is used to compensate the phase changes from reflections at the mirrors thus preserving the excellent polarimetric properties of the GCT.

The mirrors and the Bowen compensator are mounted on a carriage running on a slippage-free recirculating ball spindle, which is driven by a stepping motor. One step of the motor moves the carriage by $2.03\,\mu$m, this yields $4.06\,\mu$m image shift, corresponding to $0\hbox{$.\!\!^{\prime\prime}$}03$ in the telescopes focal plane. Figure 3 shows the complete Micro-Image-Scanner on its mounting.

For polarimetry, a Stokes-V polarimeter can be added to the setup. This polarimeter consists of an achromatic (500-900 nm) $\lambda /4$ plate and a Savart plate (two crossed calcite rods). Beam splitting and beam diameter are optimized for the use with MISC and the CCD system described below. Even though the retarder plate is achromatic it is necessary to adjust it properly for the desired wavelength. It is known that achromatic retarder plates are not perfect over their wavelength range. The retardation angle and the orientation of the fast axis are slightly varying with wavelength. A reproducible adjustment helps to optimize the calibration. It is also possible to remove the $\lambda /4$ plate from the polarimeter and move it back in place with reproducible accuracy and calibration. This allows to use retarder plates for various wavelengths or a $\lambda /2$ plate to measure the other components of the Stokes vector.

The polarimeter is placed behind the spectrograph's entrance slit. In its present configuration as a Stokes-V polarimeter it is possible to achieve a crosstalk as low as 0.5% at 770nm and 3.5% at 617nm. In combination with the scanner the crosstalk varies between 1% and 6%. The efficiency of the $\lambda /4$ plate is better than 98%. The MISC-Polarimeter is shown in Fig. 4.

Fast scanning requires a camera system which is able to read out the CCD chip and to digitize the pixel values at high speed, and high dynamics and reliable accuracy. The FlamestarII system from LaVision satisfies these requirements. Originally it was designed for laser spectroscopy, but it fits very well into our system. It uses a Thomson TH 7863 FT CCD chip with $384 \times 286$ pixels. The pixel size of $\rm 23 \times 23\,\mu m^2$ fits the spatial and spectral resolution of the spectrograph. The chip is read out and digitized with 1.8 Mpixel/s at 12 bits dynamics. The chip is Peltier cooled. Due to the high readout speed the effective dynamic range of the camera is between 10 and 11 bits. The main control of the camera is based on a 486DX2/66 PC. LaVision provides a powerfull software for controlling and macro-programming of this system.