The polarimetry mode of CanariCam will make use of a (composite) single half wave retarder and a Wollaston prism, thereby delivering dual-beam polarimetry across nearly the full field of view. Both components are being built under contract by Cleveland Crystals. Both components are AR coated with the same coating, optimized to maximize the transmission through the 10 micron atmospheric window whilst balancing the ordinary and extraordinary rays. Each of the two components is discussed in more detail below.
A 'true' half wave retarder (aka half wave plate or HWP) was no practical due to the very small thickness of such a plate. Hence, two crystals which have a net retardance of half a wave at 10 microns was designed and built. The predicted efficiency of the HWP is optimized for 10 microns to span the entire 10 micron atmospheric window. The design drawing of the HWP allowed Cleveland Crystals to produce the composite HWP shown below. The HWP will soon be mounted into the HWP holder.
The Wollaston prism has recently been completed at Cleveland Crystals. It was built to this design drawing, and will be mounted into the Wollaston holder. An image of the prism is shown below, and two further images may be seen here and here.
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Chris Packham; March 05, 2004
