1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
|
<!-- =defdoc regcoords regcoords n -->
<HTML>
<HEAD>
<TITLE>Spatial Region Coordinates</TITLE>
</HEAD>
<BODY>
<!-- =section regcoords NAME -->
<H2><A NAME="regcoords">RegCoords: Spatial Region Coordinates</A></H2>
<!-- =section regcoords SYNOPSIS -->
<H2>Summary</H2>
<P>
This document describes the specification of coordinate systems, and the
interpretation of coordinate values, for spatial region filtering.
<!-- =section regcoords DESCRIPTION -->
<H2>Pixel coordinate systems</H2>
<P>
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.
<P>
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 <B>global</B> or <B>local</B>
properties commands:
<PRE>
global coordsys image
circle 512 512 100
</PRE>
The <B>global</B> command changes the coordinate system for all
regions that follow, while the <B>local</B> command changes the
coordinate system only for the region immediately following:
<PRE>
local coordsys image
circle 512 512 100
circle 1024 1024 200
</PRE>
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).
<P>
<H2>World Coordinate Systems</H2>
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:
<PRE>
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
</PRE>
In addition, two mosaic coordinate systems have been defined that
utilize the (evolving) IRAF mosaic keywords:
<PRE>
name description
---- -----------
AMPLIFIER mosaic coords of original file using ATM/ATV
DETECTOR mosaic coords of original file using DTM/DTV
</PRE>
Again, to use one of these coordinate systems, the <B>global</B> or
<B>local</B> properties commands are used:
<PRE>
global coordsys galactic
</PRE>
<H2>WCS Positions and Sizes</H2>
In addition to pixels, positional values in a WCS-enabled region can
be specified using sexagesimal or degrees format:
<PRE>
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
</PRE>
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):
<PRE>
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
</PRE>
Similarly, the units of size values are defined by the formating
character(s) attached to a number:
<PRE>
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
</PRE>
For example:
<PRE>
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
</PRE>
<P>
An example of using sky coordinate systems follows:
<PRE>
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
</PRE>
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.
<H2>NB: The Meaning of Pure Numbers Are Context Sensitive</H2>
<P>
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:
<P>
<B>All pure numbers have implied units corresponding to the current
coordinate system.</B>
<P>
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.
<P>
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.
<P>
Examples:
<PRE>
circle(512,512,10)
ellipse 202.44382d 47.181656d 0.01d 0.02d
</PRE>
<P>
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.
<PRE>
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)
</PRE>
<P>
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.
<P>
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.
<!-- =section regcoords SEE ALSO -->
<!-- =text See funtools(n) for a list of Funtools help pages -->
<!-- =stop -->
<P>
<A HREF="./help.html">Go to Funtools Help Index</A>
<H5>Last updated: November 17, 2005</H5>
</BODY>
</HTML>
|
