In the previous Newsletter, the construction of a polarimeter for measurement of circular polarization at 113.3 GHz (CN) was announced. This polarimeter was tested at the 30-m telescope during the period 22--26 February 94 by a commissionning team comprising R. Crutcher, D. Fiebieg, B. Lazareff, G. Paubert, T. Troland.
The polarimeter consists of a dielectric transmission plate, whose design was optimized by J.Lamb. The plate can be rotated between position angles of , thanks to a mechanical device designed by J.L. Pollet. The phase time used was either 1 or 2 seconds, and the blanking time 0.25 second. The polarimeter adds 10 K to the receiver noise. It was found that, by tilting the plate, the standing wave ripple could be reduced to a negligible amount.
The main purpose of the test was to investigate the polarization properties of the telescope; specifically the so-called squint. To first order, the beams of the telescope in R and L circular polarization are expected to have a slight pointing offset.
The effect has indeed been detected in several different experiments, and the clearest illustration is shown on Fig. 9. This is a map of Venus in Stokes V, the difference of intensity between the R and L signals. It shows, as expected, the difference pattern of two slightly offset beams. In the units of that map, Venus is 21000 units. The observed ``butterfly'' pattern coresponds to a 0.27 squint between the R and L beams, i.e. slightly more than 1/100 of the FWHM.
The frequency dependence of the beam squint was also investigated, to check against possible fine structure in frequency caused by standing waves. It was found to be present, reproducible, but quite small.
A detailed report in being prepared, and will soon be available on request.
Figure 9: The map of Stokes parameter V obtained on Venus.