Chromosphere and Prominence Magnetometer

The COSMO Chromosphere and Prominence Magnetometer (ChroMag) is a polarimeter to measure chromosphere and prominence magnetic fields using the chromospheric spectral lines of He I (587.6 and 1083 nm) and Hα (656.3 nm). Measuring prominence magnetic fields embedded within the corona provides a more complete picture of field configurations than could be provided from a coronal or chromospheric magnetometer alone. Simulations of the inversion of magnetic field data in prominences and filaments indicate that a tunable filter instrument can provide data of sufficient quality to meet the science requirements of COSMO as long as noise can be kept at a level lower than 10^-3. The tunable filter also must have a bandwidth of 0.025 nm in the visible region and 0.046 nm in the IR. These requirements are met by the ChroMag design shown below.

The ChroMag is a 20-cm Lyot coronagraph that operates with an occulting disc for prominence measurements and operates un-occulted for filament measurements. The design maintains an instrumental scattered light level small compared to prominence brightness. Even with this constraint, it is possible to use an air-spaced doublet objective lens allowing for observations over the required large spectral range.

The lens for ChroMag was chosen so that the optical path can have low scattered light and be axially symmetric to minimize telescope polarization. Signal to noise is limited by the total number of photons collected and by atmospheric seeing. High frame rate 2048x2048 pixel detectors are used in order to perform the polarization measurement rapidly compared to atmospheric seeing and to rapidly collect photons. A dual beam polarimeter is used to minimize seeing-induced cross-talk. In order to make polarimetric measurements spanning the entire solar disc at a high (<1 minute) cadence, ChroMag uses a pair of tunable Lyot filters, one for each of the dual beams. With interchangeable pre-filters to isolate spectral regions around the chromospheric lines of interest, the Lyot filters can be tuned across the spectral lines using liquid crystals on each filter stage. Only about a dozen wavelengths are needed to sample each spectral line and these can be collected in about 15 seconds/line. Scans will be repeated and can be summed to increase signal-to-noise. The ChroMag will have a full FOV of 2.5 solar radii and a spatial resolution of 2.3 arcseconds.