User Manual
Doc. Number: ESO-323064
Doc. Version: 2
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3.2.4 The Spectro-Polarimetry Unit (SPU)
The new polarimetry module (see Figure 19) for CRIRES will use polarizing gratings (PGs)
to split the incoming converging beam into left- and right-circularly-polarized beams that
continue along parallel optical axes. The choice of PGs as polarizing elements is motivated
by their different behaviour at short and long wavelengths, their small thickness, the
possibility of producing large and homogeneous samples, and their modest price. The
geometry of the periodic pattern that makes up the PGs is chosen such that infrared light
(with wavelength longer than 1 μm) is deviated, while optical light is transmitted essentially
unaltered. Thus, the PG acts as a polarizing beam splitter for circular polarization without
disturbing the operation of the AO system as described by Lockhart et al. (2014). The
polarimetric unit is very compact and is installed on CRIRES calibration slide (see Figure
16).
The polarization unit includes two circular polarisation beam-splitters for YJ and HK bands
and two beam-splitter for YJ and HK bands combined with an achromatic quarter-wave
retarder plate (QWP) for the linear polarization.
Two types of PGs are used for the CRIRES SPU:
•
HK PGs with a wavelength limits band from 1480 nm to 2540 nm
•
YJ PGs with a wavelength limits band from 960 nm to 1360 nm
The gratings are mounted on a rotating turret capable of carrying the two circular and two
linear beam-splitters each pair. Each beam-splitter unit includes two optical elements
(polarising gratings) and a rotating stage needed for beam switching. The rotation axis is
parallel to the axis of the incoming beam. It allows the positions of the two output beams to
be switched enabling calibration of the difference in throughput for the two beams.
NOTE: No Spectro-polarimetric observations are offered in P108
Figure 19: Spectro-Polarimetry Unit (SPU) view from the derotator side (left) and from the
telescope side (right). Polarization optics are mounted from the side of rotating turret.
