urn:nasa:pds:context:instrument:soho.lasco 1.0 1.7.0.0 Product_Context urn:nasa:pds:context:instrument:lasco.soho 2016-10-01 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model urn:nasa:pds:context:instrument_host:spacecraft.soho::1.0 instrument_to_instrument_host Cook, J.W., LASCO Handbook for Scientific Investigators, Version 1.0, Naval Research Laboratory, 1994. reference.COOK1994 LARGE ANGLE SPECTROMETRIC CORONAGRAPH Imager not applicable not applicable Instrument Overview =================== A full discussion of the Large Angle Spectrometric Coronagraph is provided in the LASCO Handbook for Scientific Investigators, Version 1.0 (Cook 1994, [COOK1994]), which can be found in the documents directory of this archive. Excerpts for the instrument are provided here. LASCO Overview ============== 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. 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. C1 is an internally occulted coronagraph, while C2 and C3 are externally occulted instruments. C1 has a narrow passband Fabry-Perot interferometer tuned to hot coronal emission lines. C1 has not been used regularly since June 24, 1998, thus telemetry since then has been re-allocated to the C2 and C3 coronagraphs. Each of the three telescopes has a filter wheel, a polarizer wheel, and a shutter, and each uses a front-side illuminated, 1024x1024 pixel CCD. The pixel scale of the telescopes are 5.6 ''/pixel in C1, 11.8 ''/pixel in C2 and 56 ''/pixel in C3. The quantum efficiency of the CCD is about 0.3-0.5 in the 500 to 700 nm spectral region. The full discussion of the CCD camera is outlined in Ch 8 of Cook (1994) [COOK1994]. Telemetry, Encoding and Compression =================================== LASCO is basically telemetry limited in its operations. After taking and processing an image, the data is passed to a 2 Mbyte telemetry buffer. Its storage capacity is approximately 10 compressed images, and with the low spacecraft telemetry rate (4.2 Kbps) results in a transmission time of about 60 minutes for a full 1024x1024, 2 bytes per pixel image. (Note that the individual pixel intensities from the camera analog-to-digital converters are actually represented by only 14 bits, leaving a factor of 4 headroom in the 16 bits which are reserved in the data format.) Thus, image compression is desirable. The average compression factor is about 10. Thus on average, the readout time is about 6 minutes, which allows around 200 images each day to be transmitted. A number of image processing techniques are included in the flight software, from simple square root through transform encoding. The types of processing and compression utilized are determined by ground command and executed through stored sequences, with parameters stored in tables. After a camera image is stored in a 2 Mbyte image buffer, the appropriate algorithms are applied. Additional compression is obtained by transmitting only the pixels that are not obscured by the occulting disk or the aperture stop. Time resolution can be traded against field coverage to further reduce the data download requirement.