Quarter-wave plate polarimeters have been constructed by IRAM for measurements of circular polarization (see e.g. the March 1994 issue of the IRAM Newsletter). The main technical features of the polarimeters are briefly described below.
Each polarimeter consists of a dielectric quarter-wave plate working in transmission. It is rotated between two positions at by a motor, the switching time is , and the phase time is adjustable. From the point of view of data acquisition, it functions like other switching devices, i.e. the chopper or the wobbler, and the difference between the RCP and LCP intensities is acquired.
There are quarter-wave plates available for 113.3 GHz and 86.24 GHz. At 113.3 GHz the transmission loss is %, and its cross-polarization below 20 dB. The quarter-wave plate for 86.24 GHz has a transmission loss of %, and the cross-polarization below 26 dB. Similar plates could be fabricated for other frequencies if needed. Proposals for projects requiring the polarimeter can be submitted. They should clearly state the degree of technical performance that they demand. Besides the scientific evaluation, the acceptance and scheduling of such proposals will depend on their feasibility as judged from their requirements.