regcoords(7)

💖 regcoords(7) — RegCoords: Spatial Region Coordinates 💖

okay so bestie, this is a page from the SAORD Documentation set, and it's all about how coordinate systems work when you're doing spatial region filtering. Grab a snack, we're getting into it 🍓

---

✨ 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.

So yeah — even though a value in pixels refers, by default, to the PHYSICAL coordinate system, you can absolutely say that position values should refer to the image coordinate system instead, 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). iykyk 💅

🌈 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:

• 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:

• 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:

• [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):

• 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:

• [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:

• 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. slay ✨

💀 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. not me getting an error over here 😭

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

---

and that's the whole vibe check, bestie 💜 this was version 1.4.5, dated April 14, 2011, from the SAORD Documentation girlies. periodt 💅✨🦋