MK I K-Coronameter White Light Polarization Brightness Scans

The first K-coronameter (later known as MK I) was an internally occulted white light coronameter to record the polarization brightness (pB) of the Thomson scattered K-Corona continuum emission. The brightness of the polarized light was measured with a sensitive photoelectric polarimeter using electro-optic modulation of the light beam and a half-wave plate made of two quartz plates whose optical axes were crossed and had thicknesses that differed by half a wave length for 500 nm. These rotated at half the speed of a 360 degree scan around the corona (i.e. 8 minutes). Following the aperture stop the beam was modulated using a quarter-wave plate that included a ‘Z-cut’ crystal of ammonium dihydrogen phosphate (ADP) and a polaroid. The retardation of the beam was proportional to the voltage applied to the crystal. The voltage frequency, and the modulation of the light, was 50 Hz. A cone with a small hole drilled along its axis isolated the region of the corona for scanning around the Sun. A complete coronal scan took 4 minutes. Measurements were acquired every 5 degrees around the Sun. The half-angle of the cone could be varied to scan the corona at different heights from 1.125 out to 2 solar radii (as measured from Sun center). It was decommissioned in 1978. The K-Coronameter took daily scans of pB intensity at 3 heights in the corona. It did not take images of the corona. Daily scans at 1.5 solar radii are available as digitized synoptic maps in fits and gif format covering the later mission from June 1965 until January 1968. Hard copy daily scans early in the mission (September 1956 through November 1960) are available in pdf format. Digital synoptic maps from the MK II instrument are available from 1969 to 1978, superseding the MK I maps.