regcoords(7)							SAORD Documentation						      regcoords(7)

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

version 1.4.2							  January 2, 2008						      regcoords(7)