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. NCAR has approved funding for this instrument. It will replace the aging Mauna Loa Mk4 K-coronameter in early 2013.

Science Goals

• To understand the formation of coronal mass ejections (CMEs) and their interaction with surrounding coronal structures and related activity (e.g. flares, prominence eruptions and shock waves)
• To identify Earth-directed CMEs (e.g. halos) in realtime
• To determine the density distribution of the corona over solar cycle time scales
• To measure the radial brightness gradients beyond 1.5 Rsun in magnetically open regions

Progress Report

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 great enough 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 3 solar radii.

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 pixel deep well area array detector with high frame rate (150 Hz) will be used to rapidly collect these photons over a 6 solar radii full FOV at a pixel size of less than 6 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. 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.

The K-coronagraph successfully completed its Conceptual Design Review (CDR) on May 13, 2010. A panel of coronagraph and optical experts from the National Solar Observatory, the Naval Research Lab and the Earth Observing Lab/NCAR reviewed the K-coronagraph conceptual design. Please see Tech Note #8 to retrieve information presented at the CDR. The panel was pleased with the progress and provided recommendations on scattered light testing, objective lens material and polishing, filter designs and design paths for future upgrades. The panel will reconvene in May 2011 for the K-coronagraph Preliminary Design Review (PDR).

Considerable progress has been made since the CDR. The two most critical items in the initial design phase of a coronagraph are the detector and the primary objective lens. Since the CDR, the design of the primary objective lens has moved from the conceptual stage to a detailed specification for the lens. A preliminary optical design has been completed. The selected detector, the PhotonFocus MV-D1024E, has been purchased and has undergone rigorous testing. Preliminary tests show that a modified 14-bit analog-to-digital converter (ADC),which records the top 12 significant bits, can be sufficiently characterized to remove bit-errors and provide noise levels low enough to meet or exceeed the science requirements (see Tech Note 16 for a list of science requirements). Simulations are also being conducted to identify the most efficient and effective algorithms to remove aerosol noise during realtime data acquisition. Ferroelectric Liquid Crystals (FLCs) have been purchased and are being tested in the optics lab to determine the ideal configuration for the polarization modulation. HAO is on target to meet the work required for the Preliminary Design Review in May 2011.

The High Altitude Observatory and the COSMO K-coronagraph project have funded a new solar dome at the National Center for Atmospheric Research (NCAR) Mesa Lab. The new dome is the same size (16.5 feet in diameter) as the large dome at MLSO and will house the HAO solar-pointed spar, which is a duplicate of the MLSO spar. This new facility will be used to test and calibrate the COSMO K-coronagraph before it is deployed to MLSO in early 2013. The facility is able to host up to 4 solar telescopes and may also be used for Education and Outreach programs.