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+.help ccsystems Mar00 astcat
+.ih
+NAME
+ccsystems -- list and describe the supported coordinate systems
+.ih
+USAGE
+help ccsystems
+.ih
+CATALOG ACCESS COORDINATE SYSTEMS
+
+Astcat supports several catalog access coordinate systems and manages all the
+required transformation between them based on input from the user and
+information in the catalog configuration file.
+
+.ls The Input Coordinate System
+The input coordinate system specifies the coordinate system
+of the region center coordinates used to extract data from a
+catalog. The input coordinate
+system is normally defined by the user. If undefined, the input coordinate
+system defaults to the query coordinate system.
+.le
+.ls The Query Coordinate System
+The query coordinate system specifies the coordinate system of the coordinates
+used to formulate a catalog query. The query coordinate
+system is normally defined in the catalog configuration file.
+If undefined the query coordinate system defaults to the catalog
+coordinate system. Coordinates in the input coordinate system
+must be transformed to the query coordinate system in order to formulate
+a correct query.
+.le
+.ls The Catalog Coordinate System
+The catalog coordinate system is the coordinate system of the astrometric
+catalog. The catalog coordinate system is normally defined in the catalog
+configuration file. If undefined it defaults to J2000.
+.le
+.ls The Output Coordinate System
+The output coordinate system is the coordinate system of the output
+catalog produced by the catalog query. Normally the output coordinate
+system is defined by the user. If undefined it default to the catalog
+coordinate system. Catalog coordinates are transformed from the catalog
+coordinate system to the output coordinate system before they are
+written to the output file.
+.le
+
+.ih
+IMAGE ACCESS COORDINATE SYSTEMS
+
+Astcat supports several image access coordinate systems and manages the
+required transformation between them based on input from the user, and
+information in the return image headers and the image survey configuration
+file.
+
+.ls The Input Coordinate System
+The input coordinate system specifies the coordinate system of the region
+center coordinates used to extract data from an image survey. The input
+coordinate system is normally defined by the user. If undefined, the input
+coordinate system defaults to the query coordinate system.
+.le
+.ls The Query Coordinate System
+The query coordinate system specifies the coordinate system of the coordinates
+used to formulate an image survey query. The query coordinate system is
+normally defined in the image survey configuration file.  If undefined the
+query coordinate system defaults to J2000.  Coordinates in the input coordinate
+system must be transformed to the query coordinate system in order to formulate
+a correct query.
+.le
+.ls Image Coordinate System
+The image coordinate system is the coordinate system stored in the header
+of the fits image returned by an image survey query. The image coordinate
+system type is specified in the image survey configuration file. The
+currently recognized types are "fits", "dss", and "none". The default
+image coordinate system type is none. The dss wcs is always in the
+J2000 system. The fits headers may be in any supported coordinate systems,
+but the most common system is J2000.
+.le
+.ls the output coordinate system
+The output coordinate system is the coordinate system of the final image
+stored in the image header in FITS format.  The output coordinate system
+may be the same as the image coordinate system but encoded differently
+as when a dss coordinate system is converted to its fits equivalent,
+or different as when an image without a coordinate system is assigned
+one by the user.
+.le
+
+.ih
+CELESTIAL COORDINATE SYSTEMS
+
+The astcat package supports the equatorial ("fk4", "fk4-noe", "fk5", "icrs"),
+ecliptic, galactic, and supergalactic celestial coordinate systems. In most
+cases and unless otherwise noted users can input their coordinates in
+any one of these systems as long as they specify the coordinate system
+correctly.
+
+Considerable flexibility is permitted in how the coordinate systems are
+specified, e.g. J2000.0, j2000.0, 2000.0, fk5, fk5 J2000, and fk5 2000.0
+all specify the mean place post-IAU 1976 or FK5 system. Missing equinox and
+epoch fields assume reasonable defaults. In most cases the
+systems of most interest to users are "icrs", "j2000", and "b1950"
+which stand for the ICRS J2000.0, FK5 J2000.0 and FK4 B1950.0 celestial
+coordinate systems respectively. The full set of options are listed below:
+
+.ls equinox [epoch]
+The equatorial mean place post-IAU 1976 (FK5) system if equinox is a
+Julian epoch, e.g. J2000.0 or 2000.0, or the equatorial mean place
+pre-IAU 1976 system (FK4) if equinox is a Besselian epoch, e.g. B1950.0
+or 1950.0. Julian equinoxes are prefixed by a J or j, Besselian equinoxes
+by a B or b. Equinoxes without the J / j or B / b prefix are treated as
+Besselian epochs if they are < 1984.0, Julian epochs if they are >= 1984.0.
+Epoch is the epoch of the observation and may be a Julian
+epoch, a Besselian epoch, or a Julian date. Julian epochs
+are prefixed by a J or j, Besselian epochs by a B or b.
+Epochs without the J / j or B / b prefix default to the epoch type of
+equinox if the epoch value <= 3000.0, otherwise epoch is interpreted as
+a Julian date.  If undefined epoch defaults to equinox.
+.le
+.ls icrs [equinox] [epoch]
+The International Celestial Reference System where equinox is
+a Julian or Besselian epoch e.g. J2000.0  or B1980.0.
+Equinoxes without the J / j or B / b prefix are treated as Julian epochs.
+The default value of equinox is J2000.0.
+Epoch is a Besselian epoch, a Julian epoch, or a Julian date.
+Julian epochs are prefixed by a J or j, Besselian epochs by a B or b.
+Epochs without the J / j or B / b prefix default to Julian epochs
+if the epoch value <= 3000.0, otherwise epoch is interpreted as
+a Julian date.  If undefined epoch defaults to equinox.
+.le
+.ls fk5 [equinox] [epoch]
+The equatorial mean place post-IAU 1976 (FK5) system where equinox is
+a Julian or Besselian epoch e.g. J2000.0  or B1980.0.
+Equinoxes without the J / j or B / b prefix are treated as Julian epochs.
+The default value of equinox is J2000.0.
+Epoch is a Besselian epoch, a Julian epoch, or a Julian date.
+Julian epochs are prefixed by a J or j, Besselian epochs by a B or b.
+Epochs without the J / j or B / b prefix default to Julian epochs
+if the epoch value <= 3000.0, otherwise epoch is interpreted as
+a Julian date.  If undefined epoch defaults to equinox.
+.le
+.ls fk4 [equinox] [epoch]
+The equatorial mean place pre-IAU 1976 (FK4) system where equinox is a
+Besselian or Julian epoch e.g. B1950.0  or J2000.0,
+and epoch is the Besselian epoch, the Julian epoch, or the Julian date of the
+observation.
+Equinoxes without the J / j or B / b prefix are treated
+as Besselian epochs. The default value of equinox is B1950.0. Epoch
+is a Besselian epoch, a Julian epoch, or a Julian date.
+Julian epochs are prefixed by a J or j, Besselian epochs by a B or b.
+Epochs without the J / j or B / b prefix default to Besselian epochs
+if the epoch value <= 3000.0, otherwise epoch is interpreted as
+a Julian date.  If undefined epoch defaults to equinox.
+.le
+.ls noefk4 [equinox] [epoch]
+The equatorial mean place pre-IAU 1976 (FK4) system but without the E-terms
+where equinox is a Besselian or Julian epoch e.g. B1950.0 or J2000.0,
+and epoch is the Besselian epoch, the Julian epoch, or the Julian date of the
+observation.
+Equinoxes without the J / j or B / b prefix are treated
+as Besselian epochs. The default value of equinox is B1950.0.
+Epoch is a Besselian epoch, a Julian epoch, or a Julian date.
+Julian epochs are prefixed by a J or j, Besselian epochs by a B or b.
+Epochs without the J / j or B / b prefix default to Besselian epochs
+if the epoch value <= 3000.0, otherwise epoch is interpreted as
+a Julian day.  If undefined epoch defaults to equinox.
+.le
+.ls apparent epoch
+The equatorial geocentric apparent place post-IAU 1976 system where
+epoch is the epoch of observation.
+Epoch is a Besselian epoch, a Julian epoch or a Julian date.
+Julian epochs are prefixed by a J or j, Besselian epochs by a B or b.
+Epochs without the J / j or B / b prefix default to Besselian
+epochs if the epoch value < 1984.0, Julian epochs
+if the epoch value <= 3000.0, otherwise epoch is interpreted as
+a Julian date.
+.le
+.ls ecliptic epoch
+The ecliptic coordinate system where epoch is the epoch of observation.
+Epoch is a Besselian epoch, a Julian epoch, or a Julian date.
+Julian epochs are prefixed by a J or j, Besselian epochs by a B or b.
+Epochs without the J / j or B / b prefix default to Besselian epochs
+if the epoch values < 1984.0, Julian epochs
+if the epoch value <= 3000.0, otherwise epoch is interpreted as
+a Julian day.
+.le
+.ls galactic [epoch]
+The IAU 1958 galactic coordinate system.
+Epoch is a Besselian epoch, a Julian epoch or a Julian date.
+Julian epochs are prefixed by a J or j, Besselian epochs by a B or b.
+Epochs without the J / j or B / b prefix default to Besselian
+epochs if the epoch value < 1984.0, Julian epochs
+if the epoch value <= 3000.0, otherwise epoch is interpreted as
+a Julian date. The default value of epoch is B1950.0.
+.le
+.ls supergalactic [epoch]
+The deVaucouleurs supergalactic coordinate system.
+Epoch is a Besselian epoch, a Julian epoch or a Julian date.
+Julian epochs are prefixed by a J or j, Besselian epochs by a B or b.
+Epochs without the J / j or B / b prefix default to Besselian
+epochs if the epoch value < 1984.0, Julian epochs
+if the epoch value <= 3000.0, otherwise epoch is interpreted as
+a Julian date. The default value of epoch is B1950.0.
+.le
+
+Fields enclosed in [] are optional with the defaults as described. The epoch
+field for the "icrs" , "fk5", "galactic", and "supergalactic" coordinate
+systems is only used if the input coordinates are in the equatorial fk4,
+noefk4, fk5, or icrs systems and proper motions are used to transform from
+coordinate system to another.
+
+.ih
+SEE ALSO
+.endhelp