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Principle of operation

The principle of operation of TurPol is illustrated in Figure 2.1. The diaphragm in the focal plane of the telescope has two apertures, one passing the star's light plus the sky background, the other one passes background light equal to that going through the stellar aperture. A rotating chopper opens and closes alternate apertures, illuminating the photo-cathode of the photo-multiplier tube consecutively with the star and background light. A field lens produces an image of the telescope exit pupil on the photo-cathode, thereby avoiding image motion due to guiding errors. This is of importance because the sensitivity of a photo--cathode is generally not uniform over its surface, and any movement would give rise to spurious intensity variations.

 
Figure: Principle of operation of TurPol.

The chopper simultaneously chops the light from two IR LEDs onto two photo-diodes. This signal is used to synchronise the electronics in such a way as to accumulate the star plus sky and sky alone signals into two separate counters. Thus, sky subtracted photometry is done.

By inserting a plane parallel calcite plate into the beam before the focal plane, polarisation measurements can be made. As can be seen from Figure 2.1, the calcite plate splits the incoming beam into two relatively displaced but parallel components, the so--called ordinary and extra--ordinary rays, which are orthogonally linearly polarised. The calcite plate is constructed and placed so that the ordinary and extra--ordinary beams pass centrally through the two apertures. Each aperture now passes the light from the star and the sky, and by measuring their relative intensities, after passing through a wave-plate, the degree and angle of polarisation of the star light can be determined. In TurPol the intensities are measured at eight rotational positions of the half wave plate.

One important advantage of this method is that the possible sky polarisation is cancelled out, albeit at the cost of having to subtract twice the sky intensity. Since the sky background can be very highly polarised and variable, this is a small price to pay. Polarimetry of faint stars in the presence of moonlight is only possible due to this cancellation method, as is the high precision ( 0.005%) obtained on bright stars.



next up previous contents
Next: Construction Up: Detailed description Previous: Detailed description



Lars Freyhammer
Wed Oct 27 18:02:24 ACT 1999