Preliminary assembly of the AGW mechanical structure for the polarimeters. Note that two of 3m diameter flanges that couple the AGW tothe telescope are on top of each other in this picture.
Polarimeter flanges delivery to LBT from AIP.
Concerns regarding the throughput of the conceptual design version basically led to a new design based on a combination of two combined true zero-order wave-plate retarders and a Foster prism.
Instead of using a classical half-wave retarder, we rotate the entire polarimeter package in steps of 45°, i.e. obtain linear polarization without any retarding element in the optical path. This will bypass all device-specific achromatic troubles and will - at the same time - allow for a throughput in the 95-98% range. Optimum throughput for circular polarization is achieved with a combined λ/2+λ/4 zero-order retarder plate which is rotateable in steps of 45° around a fixed instrument frame.