I realized that, while we posted a photograph of the new guard ring under construction (14 Dec 2011), we haven't yet posted a photograph of the finished product. Here it is.
The major thrust for this season is to install the new, polarization-sensitive receiver, but along with that, we've had to modify the telescope dish itself.
The polarization of the Cosmic Microwave Background is incredibly weak, so we need to be very careful about any Earthly sources which might add polarization to the light from the sky. We also need to prevent the glow from the warm ground from interfering with our view from the sky. Our old ground shield did a great job of this, but it also would have added some small polarization to the light that we measured. Hence, this guard ring.
This is the telescope in the docked position. From here, we can access the equipment which normally lives in the end of the boom, including the receiver itself. Note the blue tip of the new "snout" peeking up from the end of the boom.
Along with the new "snout" around our receiver, it keeps light from leaking around the outside of the dish and into our camera. With these changes, any polarization of the microwaves that we measure will actually be coming from the sky, and not from our telescope!