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The LASCO Consortium for a Coronagraph on the SOHO Mission
The Solar and Heliospheric Observatory (SOHO) is an unmanned scientific space mission developed by ESA and NASA. It supports two classes of scientific investigations, centered around (1) helioseismology, or the study of the Sun's natural seismic vibrational modes, with the objective of advancing knowledge of the properties and structure of the solar interior, and (2) the processes that account for the heating and acceleration of the solar wind; more broadly, the nature and modes of evolutionary change in the Sun's outer atmosphere.
To accomplish these investigations, the SOHO spacecraft will be placed in an orbit about the first Lagrangian, or L1, libration point, that point along the Earth-Sun line at which the gravitational attraction of the spacecraft to the Earth is just balanced by its attraction to the Sun. Located approximately 1.5 million kilometers from Earth, SOHO will require a "cruise phase" of about 4 months to settle into its orbit about L1. The planned orbit allows for observations of very low-frequency helioseismologic oscillations, and for continuous observation of the Sun's outer atmosphere, but suffers some disadvantage in that the distance takes a toll on allowable telemetry data rates. The orbit is only quasi-stable, and periodic station keeping is required. The nominal lifetime for the SOHO mission is two years after arrival on station. Expendables will be carried, however, sufficient for six years of operation.
LASCO Instrument Development
The Large Angle Spectrometric Coronagraph (LASCO) is a wide-field white light and spectrometric coronagraph consisting of three optical systems having nested fields of view, that together observe the solar corona from just above the limb at 1.1 Rsun, out to very great elongations. LASCO was developed jointly by the Naval Research Laboratory (USA), the Max-Planck-Institut fur Aeronomie (Germany), The Laboratoire d'Astronomie Spatiale (France), and the University of Birmingham (UK), for flight in 1995 on SOHO.
The three telescopes comprising LASCO are designated the C1, with coverage from 1.1 to 3.0 Rsun, the C2, with coverage deliberately overlapping parts of both C1 and C3, and extending from 2.0 to 6.0 Rsun, and the C3, which spans the outer corona from about 3.7 to 32 Rsun. The inner corona instrument, C1, is a newly developed mirror version of the classic Lyot internally occulted coronagraph, while the C2 and C3 are externally occulted instruments. In addition, the C1 is fitted with an imaging Fabry-Perot interferometer, making possible spatially resolved high-resolution coronal spectroscopy in selected spectral emission and absorption lines, between 1.1 and 3.0 Rsun. High definition CCD cameras in each telescope provide detailed images with exceptional dynamic range, while large digital memories and a high-speed microprocessor support extensive on-board image processing and image data compression by large factors, that will allow transmission of up to 10 full coronal images per hour.
The reader should note that NRL is also a co-investigator institute responsible for major hardware components in the SOHO Extreme_ultraviolet Imaging Telescope (EIT) experiment, that will provide high-resolution images of the emission-line corona over the solar disk and at the limb. There is an important consequence from this: LASCO and EIT are supported by the same electronics command and data handling package. From the operational point of view, this one electronics package, the LASCO Electronics Box (LEB), supports four coronal telescopes having similar technical characteristics, C1, C2, C3, and EIT. At the command and data level, these four instruments are synchronized in highly coordinated operations, with their uplinked commands and downlinked data interleaved into a single, common signal stream.
The primary responsibilities of the four LASCO Consortium institutions for the final LASCO flight instrument are listed:
Naval Research Laboratory:
Overall system design and development
Max-Planck-Institut fur Aeronomie:
Design, fabrication, test, and calibration of the C1 optical system, including all of its optical parts, mounting cells, interior baffles, and articulated M1 mirror mount, except for the optical bench, CCD Camera, and Fabry-Perot interferometer
Laboratoire d'Astronomie Spatiale:
Design, fabrication, test, and calibration of the C2 optical system, including all of its optical parts, mounting cells, and interior baffles, except for the CCD Camera and optical bench
School of Physics and Space Research, University of Birmingham:
Design, fabrication, and test of the Coronagraph Optics Box (COB), its internal covers, and the mechanical interface to the SOHO spacecraft