SALT/PFIS Observer's Guide

3.4 Polarimetry

The polarization optics employ a "wide-field" design, in which a polarizing beamsplitter in the collimated beam takes the central half of the field and splits it into two orthogonally polarized fields, the "ordinary" (O) and "extraordinary" (E) beams. One (or two) waveplates can be inserted into the beam, right after the field lens in the collimator, and will modulate the polarization state with time. The difference between the intensities of the O and E images as a function of time yields the polarization.

For the polarimetric modes, only the central 4 arcminute portion of the focal plane is used (accomplished using a short slit for spectroscopy or a special mask blocking the upper and lower quarter of the field of view for imaging - see below). Frame transfer operations are not possible with polarimetry, because the E and O beams use up space on both sides of the frame transfer boundary.

3.4.1 Waveplate Modes

The waveplate modulators are to be used in three modes, linear, circular, and "all-Stokes". For ease of operation, the waveplates are in the same order in all modes, half-wave first. Table 1 gives the waveplate angle progression for each mode. The angle shown is that between the waveplate optic axis and the beam-splitter polarization axis, which is ? degrees to the dispersion direction.

Linear Circular All Stokes
1/2 wave 1/4 wave
0 -
45 -
22.5 -
67.5 -
11.25 -
56.25 -
33.75 -
78.75 -
1/2 wave 1/4 wave
0 +45
0 -45
22.5 -45
22.5 +45
45 +45
45 -45
67.5 -45
67.5 +45
1/2 wave 1/4 wave
0 0
22.5 33.75
45 67.5
67.5 101.25
90 135
112.5 168.75
135 202.5
157.5 236.25

3.4.2 Efficiency


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