Narrow-band Filter Polarimeter

Observing with a large-aperture telescope over a large FOV is a challenging requirement. The etendue (also called luminosity or throughput) is a property of an optical system defined as the product of the solid angle through each element and the area of the element. To prevent loss of light as it passes through an optical system, the etendue of post-focus instruments must equal or exceed the etendue at the entrance aperture of the telescope. For the COSMO large-aperture coronagraph, the 1.5-m aperture and 1 degree full FOV result in an etendue of 4.23 cm² sr. For comparison, the etendue of the 4-m ATST telescope with a 5 arc minute full FOV is 0.21 cm² sr. This means that the light-gathering power of the COSMO large-aperture coronagraph will be a factor of 20 times greater than the ATST as long as it is not compromised by the post-focus instrumentation.

The COSMO Narrow-band Filter Polarimeter (NFP) is designed to perform narrow-band coronal filter magnetometry over the entire COSMO FOV. The limiting solid angle that a tunable filter can accept is set by the shift of the passband with incidence angle through the filter. The required passband for the COSMO NFP is set by the width of the FeXIII emission lines (0.14 nm). A survey of available filter technologies indicated that the incidence angle sensitivity of simple Michelson and Fabry-Perot interferometers makes them unsuitable for the COSMO NFP. However, it was determined that a wide-field birefringent filter (Lyot filter) with an aperture of 100 mm constructed with Lithium Niobate crystals would provide a tunable filter with an etendue that exceeds the COSMO requirement by a factor of about 2.

Atmospheric seeing can introduce significant noise into polarization measurements through intensity to polarization crosstalk. To mitigate this source of noise, we have adopted a dual beam design for the COSMO NFP. Polarization selection will be accomplished by the combination of a Ferro-Electric liquid crystal followed by a polarizing beamsplitter. Identical Lyot filters and detectors will be placed in each beam for simultaneous observation of orthogonal polarization states. The baseline detectors are 2048x2048 pixel HgCdTe IR arrays with 1.76 arc seconds per pixel. The Lithium Niobate birefringent filters and IR detectors are commercially available.