Wollaston prisms are extensively used in focal plane astronomical
instruments working at visual, and recently also at IR wavelengths
(e.g. Packham et al. 1996).
Their main practical advantage is that they allow
a straightforward implementation of a polarimetric mode in multimode
imager/spectrometers because a Wollaston prism
inserted in the pupil plane of a focal reducer
produces simultaneous images in two perpendicular polarization
states on different parts of the array (see e.g. Fig. 3 (click here) of
Fosbury et al. 1989).
However, at least two separate measurements with the
polarization axis rotated by 
are required
to derive the first 3 elements of the Stokes vector, i.e. to determine
the polarization degree and angle.
The most common and reliable method to complete the
polarimetric measurement is to use a rotating 
plate
before the Wollaston to modify the
orientation of the polarization axis relative to the prism.
This well known system may be difficult
to implement in cryogenic instruments
working in the IR for the following reasons.
- A rotating 
retarder inside the cold dewar requires
a complex mechanical design, especially in a multimode instrument
where a mechanism for inserting/removing the 
plate from
the beam is also necessary.
- Adding a 
plate before
the dewar window and telescope focal plane may significantly
increase the thermal background and produce ghosts.
A simpler method could be obtaining measurements at different position angles, i.e. rotate the whole instrument with respect to the telescope axis. This could be an effective solution for imaging-polarimetry but cannot be used for long slit spectro-polarimetry.
A convenient alternative could be a fixed device which simultaneously produces polarized images at 0, 45, 90 and 135 degrees on four different parts of the array yielding therefore imaging or spectro-polarimetric measurements from a single exposure and without moving anything. This paper describes a simple combination of two Wollastons and two prisms (wedges) which satisfies the above requirements. Parameters of devices optimized for imaging and spectro-polarimetry at visual and IR wavelengths are also presented in Sect. 3.
Figure 1:
Left: schematic representation of a WeDoWo device and ray-tracing.
To prevent vignetting at the prisms interface
the wedge angle is chosen to ensure that all
rays (including those from the field edges) always travel above/below
the optical axis.
Center: illustration of how this device creates four polarized images
(at 0, 90, 45 and 135 degrees) of a stellar field.
Right: illustration of how the WeDoWo creates four long slit polarized spectra
(at 0, 90, 45 and 135 degrees) of two stars