Solar Helioshperic Observatory (SOHO) Large Angle Spectrometric 
Coronagraph (LASCO) 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), which can be found in the SOHO documents collection
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 solar radii out to very great
elongations.

The three LASCO telescopes are designated the C1, with coverage from
1.1 to 3.0 solar radii, the C2, with coverage deliberately overlapping
parts of both C1 and C3, and extending from 2.0 to 6.0 solar radii,
and the C3, which spans the outer corona from about 3.7 to 32 solar
radii.  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 arcsecond/pixel in C1,
11.8 arcsecond/pixel in C2 and 56 arcsecond/pixel in C3.  The quantum
efficiency of the CCD is about 0.3-0.5 in the 500 to 700 nanometer
spectral region.  The full discussion of the CCD camera is outlined in
Chapter 8 of Cook (1994).


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 megabyte 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 megabyte 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.

References
----------
Cook, J.W., LASCO Handbook for Scientific Investigators, Version 1.0, 
  Naval Research Laboratory, 1994.