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NAME

       RegCoords - Spatial Region Coordinates

SYNOPSIS

       This document describes the specification of coordinate systems, and the interpretation of
       coordinate values, for spatial region filtering.

DESCRIPTION

       Pixel coordinate systems

       The default coordinate system for regions is PHYSICAL, which means that region position
       and size values are taken from the original data. (Note that this is a change from the
       original IRAF/PROS implementation, in which the IMAGE coordinate system was the default.)
       PHYSICAL coordinates always refer to pixel positions on the original image (using IRAF LTM
       and LTV keywords).  With PHYSICAL coordinates, if a set of coordinates specifies the
       position of an object in an original FITS file, the same coordinates will specify the same
       object in any FITS derived from the original.  Physical coordinates are invariant with
       blocking of FITS files or taking sections of images, even when a blocked section is
       written to a new file.

       Thus, although a value in pixels refers, by default, to the PHYSICAL coordinate system,
       you may specify that position values refer to the image coordinate system using the global
       or local properties commands:

         global coordsys image
         circle 512 512 100

       The global command changes the coordinate system for all regions that follow, while the
       local command changes the coordinate system only for the region immediately following:

         local coordsys image
         circle 512 512 100
         circle 1024 1024 200

       This changes the coordinate system only for the region that follows.  In the above
       example, the second region uses the global coordinate system (PHYSICAL by default).

       World Coordinate Systems

       If World Coordinate System information is contained in the data file being filtered, it
       also is possible to define regions using a sky coordinate system. Supported systems
       include:

         name                  description
         ----                  -----------
         PHYSICAL              pixel coords of original file using LTM/LTV
         IMAGE                 pixel coords of current file
         FK4, B1950            sky coordinate systems
         FK5, J2000            sky coordinate systems
         GALACTIC              sky coordinate systems
         ECLIPTIC              sky coordinate systems
         ICRS                  currently same as J2000
         LINEAR                linear wcs as defined in file

       In addition, two mosaic coordinate systems have been defined that utilize the (evolving)
       IRAF mosaic keywords:

         name                  description
         ----                  -----------
         AMPLIFIER             mosaic coords of original file using ATM/ATV
         DETECTOR              mosaic coords of original file using DTM/DTV

       Again, to use one of these coordinate systems, the global or local properties commands are
       used:

         global coordsys galactic

       WCS Positions and Sizes

       In addition to pixels, positional values in a WCS-enabled region can be specified using
       sexagesimal or degrees format:

         position arguments    description
         ------------------    -----------
         [num]                 context-dependent (see below)
         [num]d                degrees
         [num]r                radians
         [num]p                physical pixels
         [num]i                image pixels
         [num]:[num]:[num]     hms for 'odd' position arguments
         [num]:[num]:[num]     dms for 'even' position arguments
         [num]h[num]m[num]s    explicit hms
         [num]d[num]m[num]s    explicit dms

       If ':' is used as sexagesimal separator, the value is considered to be specifying
       hours/minutes/seconds if it is the first argument of a positional pair, and
       degrees/minutes/seconds for the second argument of a pair (except for galactic
       coordinates, which always use degrees):

         argument      description
         -----------   -----------
         10:20:30.0    10 hours, 20 minutes, 30 seconds for 1st positional argument
                       10 degrees, 20 minutes, 30 seconds for 2nd positional argument
         10h20m30.0    10 hours, 20 minutes, 30 seconds
         10d20m30.0    10 degrees, 20 minutes, 30 seconds
         10.20d        10.2 degrees

       Similarly, the units of size values are defined by the formating character(s) attached to
       a number:

         size arguments        description
         --------------        -----------
         [num]                 context-dependent (see below)
         [num]"                arc seconds
         [num]'                arc minutes
         [num]d                degrees
         [num]r                radians
         [num]p                physical pixels
         [num]i                image pixels

       For example:

         argument      description
         -----------   -----------
         10            ten pixels
         10'           ten minutes of arc
         10"           ten seconds of arc
         10d           ten degrees
         10p           ten pixels
         0.5r          half of a radian

       An example of using sky coordinate systems follows:

         global coordsys B1950
         -box 175.54d 20.01156d 10' 10'
         local coordsys J2000
         pie 179.57d 22.4d 0 360 n=4 && annulus 179.57d 22.4d 3' 24' n=5

       At the FK4 1950 coordinates 175.54d RA, 20.01156d DEC exclude a 10 minute by 10 minute
       box.  Then at the FK5 2000 coordinates 179.57d RA 22.4d DEC draw a radial profile regions
       pattern with 4 quadrants and 5 annuli ranging from 3 minutes to 24 minutes in diameter.
       In this example, the default coordinate system is overridden by the commands in the
       regions spec.

       NB: The Meaning of Pure Numbers Are Context Sensitive

       When a "pure number" (i.e. one without a format directive such as 'd' for 'degrees') is
       specified as a position or size, its interpretation depends on the context defined by the
       'coordsys' keyword. In general, the rule is:

       All pure numbers have implied units corresponding to the current coordinate system.

       If no coordinate system is explicitly specified, the default system is implicitly assumed
       to be PHYSICAL.  In practice this means that for IMAGE and PHYSICAL systems, pure numbers
       are pixels.  Otherwise, for all systems other than LINEAR, pure numbers are degrees. For
       LINEAR systems, pure numbers are in the units of the linear system.  This rule covers both
       positions and sizes.

       As a corollary, when a sky-formatted number is used with the IMAGE or PHYSICAL coordinate
       system (which includes the default case of no coordsys being specified), the formatted
       number is assumed to be in the units of the WCS contained in the current file. If no sky
       WCS is specified, an error results.

       Examples:

         circle(512,512,10)
         ellipse 202.44382d 47.181656d 0.01d 0.02d

       In the absence of a specified coordinate system, the circle uses the default PHYSICAL
       units of pixels, while the ellipse explicitly uses degrees, presumably to go with the WCS
       in the current file.

        global coordsys=fk5
        global color=green font="system 10 normal"
        circle 202.44382 47.181656 0.01
        circle 202.44382 47.181656 10p
        ellipse(512p,512p,10p,15p,20)

       Here, the circles use the FK5 units of degrees (except for the explicit use of pixels in
       the second radius), while the ellipse explicitly specifies pixels. The ellipse angle is in
       degrees.

       Note that Chandra data format appears to use "coordsys=physical" implicitly.  Therefore,
       for most Chandra applications, valid regions can be generated safely by asking ds9 to
       save/display regions in pixels using the PHYSICAL coordsys.

SEE ALSO

       See funtools(7) for a list of Funtools help pages