White-light K-coronagraph

The white-light K-coronagraph is an instrument to measure white light coronal polarization brightness (pB) due to electron scattering of photospheric light by the K-corona. It will reside on the solar pointed spar in the small dome.

The design of the K-coronagraph follows from the desire to provide high cadence observations of CME formation and early acceleration at a cadence and sensitivity greater than those currently available from the Mauna Loa Mk4 K-coronameter.

In order to monitor all phases of CME acceleration, the K-coronagraph must provide high signal-to-noise pB measurements over an entire FOV starting only a few hundredths of a solar radius above the limb and extending outward to 2.5 solar radii. Signal-to-noise is limited by photon noise, atmospheric seeing and scintillation and by aerosols.

Even the lowest scattered light coronagraph designed to observe close to the limb of the Sun has a background light level, which is bright compared to the K-corona. To minimize instrumental scattered light, a Lyot coronagraph with a super polished singlet objective lens is used. Sufficient photons must be collected to be able to detect the faint outer corona against the background scatter. A newly designed 2048x2048 pixel deep well area array detector with high frame rate (150 Hz) will be used to rapidly collect these photons over a 5 solar radii full FOV at a spatial resolution of 5 arcseconds. The product of the telescope aperture times the wavelength pass band must be wide enough to provide the required photons. A wide pass band is not compatible with occulting close to the limb of the Sun, requiring the use of an objective lens far larger than needed simply to meet spatial resolution requirements. A balance between aperture size and pass band is achieved by using a 20-cm diameter objective and a 25-nm pass band.

Seeing and scintillation can introduce noise into a polarization measurement if the instrument uses a polarimeter to encode the polarization signal into a linear combination of successive camera frames. This cross-talk effect is minimized by operating at a high frame rate and by utilizing dual beam polarimetry. The two beams detect polarization with opposite signs so that their difference detects polarization without variations in intensity masquerading as signal. Aerosols can be strongly polarized and saturate detector pixels. Real-time vision processing of camera data will be used to identify and remove these artifacts from the images.