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ADIDNAME = NSSD0238;

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PACKAGE_TYPE =		RP;
REVISION_NUMBER =       0;
SUBMISSION_DATE =       1995-12-04;
REVISABLE =             YES;   
RELEASABLE =            NO;

TITLE = 		"SOHO CDS Instrument Description";

DESCRIPTION = "Description of the Solar and Heliospheric Observatory Mission's
Coronal Diagnostic Spectrometer (CDS) Instrument";

REVISION_COMMENT = 	"Original Version";

BEGIN_GROUP = ORIGINATOR;
  NAME = 		"Luttermoser, Dr. Donald Gerard";
  AFFILIATION =  	"Applied Research Corporation/NASA/GSFC";
  EMAIL = 		( Internet, "lutter@orpheus.nascom.nasa.gov" );
  PHONE = 		"+1 301 286 2371"
  FAX =   		"+1 301 286 0264"
  POSTAL_ADDRESS =	( "SOHO/EAF, Building 26, Room G-1"
			, "NASA/Goddard Space Flight Center"
			, "Code 682.3"
			, "Greenbelt, MD 20771 USA"
			);
END_GROUP = ORIGINATOR;

BEGIN_GROUP = REVISER;
  NAME = 		"Luttermoser, Dr. Donald Gerard";
  AFFILIATION =  	"Applied Research Corporation/NASA/GSFC";
  EMAIL = 		( Internet, "lutter@orpheus.nascom.nasa.gov" );
  PHONE = 		"+1 301 286 2371"
  FAX =   		"+1 301 286 0264"
  POSTAL_ADDRESS =	( "SOHO/EAF, Building 26, Room G-1"
			, "NASA/Goddard Space Flight Center"
			, "Code 682.3"
			, "Greenbelt, MD 20771 USA"
			);
END_GROUP = REVISER;

BEGIN_GROUP = REVISER;
  NAME = 		"Thompson, Dr. William T.";
  AFFILIATION =  	"Applied Research Corporation/NASA/GSFC";
  EMAIL = 		( Internet, "William.T.Thompson.1@gsfc.nasa.gov" );
  PHONE = 		"+1 301 286 2040"
  FAX =   		"+1 301 286 0264"
  POSTAL_ADDRESS =	( "SOHO/EAF, Building 26, Room G-1"
			, "NASA/Goddard Space Flight Center"
			, "Code 682.3"
			, "Greenbelt, MD 20771 USA"
			);
END_GROUP = REVISER;

BEGIN_GROUP = REVISER;
  NAME = 		"Pike, Dr. David";
  AFFILIATION =  	"Rutherford Appleton Laboratory";
  EMAIL = 		( Internet, "cdp@astro1.bnsc.rl.ac.uk" );
  PHONE = 		"+44 1235 82 5835"
  FAX =   		"+44 1235 44 6509"
  POSTAL_ADDRESS =	( "Space Science Department"
			, "Rutherford Appleton Laboratory"
			, "Chilton, Didcot"
			, "Oxfordshire, OX11 0QX, United Kingdom"
			);
END_GROUP = REVISER;

BEGIN_GROUP = REVISER;
  NAME = 		"Harrison, Dr. Richard A.";
  AFFILIATION =  	"Rutherford Appleton Laboratory";
  EMAIL = 		( Internet, "harrison@solg2.bnsc.rl.ac.uk" );
  PHONE = 		"+44 1235 44 6884"
  FAX =   		"+44 1235 44 6509"
  POSTAL_ADDRESS =	( "Space Science Department"
			, "Rutherford Appleton Laboratory"
			, "Chilton, Didcot"
			, "Oxfordshire, OX11 0QX, United Kingdom"
			);
END_GROUP = REVISER;

BEGIN_GROUP = REVISER;
  NAME = 		"Poland, Dr. Arthur I.";
  AFFILIATION =  	"NASA/GSFC";
  EMAIL = 		( Internet, "poland@pal.gsfc.nasa.gov" );
  PHONE = 		"+1 301 286 7076"
  FAX =   		"+1 301 286 0264"
  POSTAL_ADDRESS =	( "LASP"
			, "NASA/Goddard Space Flight Center"
			, "Code 682.1"
			, "Greenbelt, MD 20771 USA"
			);
END_GROUP = REVISER;

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PROJECT_NAME =    "Solar and Heliospheric Observatory (SOHO)";
MISSION_NAME =    "Solar and Heliospheric Observatory (SOHO)";
INSTRUMENT_NAME = "Coronal Diagnostic Spectrometer (CDS)";

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The Solar and Heliospheric Observatory (SOHO) is designed to study the internal 
structure of the Sun, its extensive outer atmosphere and the origin of the 
solar wind, the stream of highly ionized gas that blows continuously outward 
through the solar system.  The SOHO project is being carried out by the 
European Space Agency (ESA) and the US National Aeronautics and Space 
Administration (NASA) as a cooperative effort between the two agencies in the
framework of the Solar Terrestrial Science Program (STSP) comprising SOHO and 
CLUSTER, and the International Solar-Terrestrial Physics Program (ISTP), with
Geotail (ISAS-Japan), Wind, and Polar.  The original operational plan for SOHO 
involves continuously observing the Sun for at least two years from a 
permanent vantage point 1.5 million kilometers sunward of the Earth in a halo 
orbit around the Sun-Earth L1 Lagrangian point.  This spacecraft is helping 
astronomers understand the interactions between the Sun and the Earth's 
environment better than has been possible to date.  Its legacy may enable 
scientists to solve some of the most perplexing riddles about the Sun, 
including the heating of the solar corona, the acceleration of the solar wind, 
and the physical conditions of the solar interior.  It will give solar 
physicists their first long term, uninterrupted view of our nearest star.
	
The SOHO spacecraft was built in Europe by an industry team led by Matra, and 
its instruments are being provided by European and American scientists.  There
are nine European Principal Investigators (PI's) and three American ones.  
Large engineering teams and more than 200 co-investigators support the PI's in 
the development of the instruments and in the preparation of their operations
and data analysis.  NASA is responsible for the launch and mission operations.
Large radio dishes around the world which form NASA's Deep Space Network are 
being used to track the spacecraft beyond the Earth's orbit.  Mission control 
is based at GSFC in Greenbelt, Maryland, USA.  Twelve instruments are mounted 
on the SOHO spacecraft.  The following describes the Coronal Diagnostic 
Spectrometer (CDS).

CDS is a twin spectrometer which allows a detailed study of the extreme 
ultraviolet (EUV) radiation from the Sun.  With a combination of high time 
resolution and spatial resolution, and good spectral coverage, CDS can be used 
to provide diagnostic information on plasmas within the structures in the 
solar atmosphere.  CDS will detect radiation in the range 151-785 Angstroms, 
and in normal operation, one would obtain spectra and spectroheliograms
in a selection of wavelength ranges simultaneously.  By comparing the ratios
of intensities of some spectral emission lines, we can derive density and
temperature information of the plasmas being studied; by using so-called
differential emission measure techniques we can determine the distribution of
material at different temperatures and densities; by studying absolute
emission line intensities we may determine elemental and ion abundances, and
by studying line profiles we can determine flow speeds.

CDS consists of a Wolter-Schwartschild II grazing incidence telescope with
an effective focal length of 257.831 cm, a plate scale of 12.5 microns/arcsec,
a FWHM of less than 3 arcsec and a field-of-view of 4 arcmin. Light from the
telescope is reflected off a flat scan mirror and focused at a slit assembly
and fed into two different spectrometers The entire instrument can be pointed
to any target on the solar disk or solar corona.

One of the spectrometers is a normal incidence system.  Light is dispersed by
two toroidal gratings onto a 2-D detector system.  Spatial focus is achieved 
by such a system.  Thus, by selecting long, thin slits one can view images of 
the Sun dispersed in wavelength.  By interspersing exposures with movements of 
the scan mirror, one can build up images in the spectral lines detected.  The 
scan mirror step size is 2.032 arcsec (E-W direction).  There is a wide slit 
which will allow an overlapagram approach.

The normal incidence detector is the Viewfinder Detector System (VDS) which is
an intensified CCD system.  There are 2 simultaneous wavelength bands 
associated with this system: 308-381 A and 513-633 A.

The second spectrometer is a grazing incidence system.  In this, light is
dispersed from a grating at grazing incidence and recorded on four 1-D 
detectors around the Rowland circle.  Each detector records a specified range 
of the solar spectrum.  This system can view the spectrum from only one
location on the Sun's surface at a time.  Thus, to build images one must
build rastered images by interspersing exposures with movements of the
mirror (E-W) and slit (N-S motion in 1.016 arcsec steps).

The four grazing incidence detectors are MSSL spiral anode detectors placed in
a Rowland circle of radius 750 mm.  The grating ruling is 1000 lines/mm.
The four wavelength bands are: 151-221 A, 256-338 A, 393-493 A and 656-785 A.

The CDS wavelength range allows access to spectral lines emitted by plasmas
of temperature ten thousand to over 2 million Kelvins, has a spatial resolution
of a few arcseconds, a temporal resolution of one second, and a spectral
resolution of down to 0.1 Angstrom.
	
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