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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