The Percent Polarized data from the LASCO C3 coronagraph.

LASCO (Large Angle Spectroscopic Coronagraph) is comprised of three nested coronagraphs (C1, C2, and C3) that image the solar corona from 1.1--30 solar radii (C1: 1.1--3; C2: 1.5--6; and C3: 3--30). The inner coronagraph (C1) is a newly developed mirror version of the classic Lyot coronagraph without an external occulter, while the middle coronagraph (C2) and the outer one (C3) are externally occulted. C1 is equipped with a Fabry-Perot interferometer that can take monochromatic images over the whole field of view with a spectral resolution of 0.07 nm. By stepping the bandpass across a spectral line, line profiles and Doppler shifts can be derived from all 1024 x 1024 pixels simultaneously. With an equivalent pixel size of 5.6 arcsec, the spatial resolution of C1 is ~11 arcsec. The resolution is ~23 arcsec for C2, and 112 arcsec for C3. The calculated expected precision of the emission line measurements, for example, of the 637.4 nm Fe X line is 0.05 Km/s at 1.1 solar radii, 1.0 km/s at 2 solar radii, and 8.0 km/s at 3 solar radii, all for a 60 s exposure. All three devices use a 1024 x 1024 pixel CCD. The CCDs are passively cooled to 193 K, but can be heated above ambient temperatures to drive off condensed material. The readout rate is ~21 s for a full image, but any line on the CCD can be accessed within about 60 ms, the unwanted lines being dumped. The electronics box provides overall control, including observation programs and scheduling. An observation program can be scheduled to be run at an absolute time or just next in sequence. It can run for a certain number of iterations, an interval of time, or stop at an absolute time. The observation program tasks included, e.g., taking single images from one of the telescopes, a series of images through the three linear polarizers, and a sequence of images at a series of Fabry-Perot wavelength steps. The image processing tasks are primarily image compression, with seven different techniques being available. On average, the readout time should be ~6 minutes or about 200 images/day. This information is from the paper ``The Large Angle Spectroscopic Coronagraph (LASCO): Visible Light Coronal Imaging and Spectroscopy,'' by G. E. Brueckner et al. (Proceedings of the First SOHO Workshop, ESA SP-348, pp. 27-34, November 1992).