urn:nasa:pds:context:instrument:mcdonald.struve2m1.molp 1.0 1.10.0.0 Product_Context 2020-02-18 1.0 Initial export from OLAF urn:nasa:pds:context:telescope:mcdonald.struve2m1 instrument_to_telescope Breger, M., Interstellar grain size: A look at the deviant four, Astrophysical Journal 233, 97-101, 1979. McDonald Observatory Linear Polarimeter Polarimeter The Linear Polarimeter was put into service at McDonald Observatory in 1978. The main element is a Glan-Air prism, rapidly rotating to counteract atmospheric transparency fluctuations. The Glan prism was chosen as an analyzer due to its very high throughput, achromatic nature, optical precision, and low cost. A slight disadvantage results from the presence of an air gap inside the prism and the resultant lateral image displacement (less than 0.1mm). A slight image rotation causes an instrumental polarization of 0.05% which varies slightly with wavelength, is constant in time, and can easily be subtracted out. Other sources of instrumental polarization, including the aluminized telescope mirrors, have turned out to be less than 10^-4. Tests have shown that under ideal observing conditions, the polarimeter can measure accurately to a precision on the order 10^-4. In practice, therefore, the precision of the polarimeter is limited by photon statistics. For more information about the instrument, including a schematic diagram, see Breger (1979). Breger, M., Interstellar grain size: A look at the deviant four, Astrophysical Journal 233, 97-101, 1979.