Construction

The diaphragm wheel holds up to 8 sets of double apertures or diaphragms, and the filter wheel can hold 8 filters for single channel photometry. Both wheels and the field viewing mirror are remotely controlled. The available diaphragms and filters are listed in Table 2.1

 
Table: Filters and Diaphragms.

Below the polarimeter is mounted a box with the 4 dichroic beam-splitters, the 5 field lenses with added band pass filters, the 5 PM tubes, and the power supplies for the instrument. Part of this box is cooled using a compressor circulating glycol, keeping the PMTs at about -20C. The compressor is placed on the observing floor below the telescope.

The PMTs are provided with high voltage from two power supplies placed in the control room: the UBV channels have bialkali tubes with a dark count rate of less than 10s; the RI channels have GaAs tubes with less than 20s.

Table 2.2 lists the used dichroics and filter materials to define the pass bands. Figure 2.2 shows the obtained pass bands from the combination of beam-splitter and filter convolved with the photo-cathode sensitivity curve.

 
Table: Pass band defining dichroics and filters.

 
Figure: Overall pass bands of TurPol

The polarimeter is controlled by a PC running under DOS and the interface to the instrument is through a CAMAC crate. The programme allows a quick set--up of the observing mode, definition of the integration time, and the number of integrations per observation. Sky measurements are automatically done in an optimised way, after off--setting the telescope. Each observation consists of a number of integrations each composed of 8 sub--integrations corresponding to 8 positions of the wave plate. The rotation and data acquisition is done automatically, only a start command has to be given. After each 8 sub--integrations the results are displayed on the screen. After one or more observations, a quick on--line data reduction can be made by one command. For special applications (e.g. rotating the entire instrument between sub--integrations) a manual mode can be selected, in which the START command has to be given for each sub--integration. The programme is mouse driven and user friendly.

For linear polarisation measurements, a half wave plate is used, while for circular or combined linear and circular a quarter wave plate is inserted into the beam. In this last mode, the efficiency is 0.5 for linear and 0.7 for circular polarimetry.

By removing the calcite plate from the beam --this is done with a knob on the polarimeter-- photometry can be done. The maximum time resolution in the photometric mode is about 2s, in polarimetric mode about 60s (and 5s for the simultaneously obtained photometry).