IRAF Newsletter -- Number 14 -- April 1998

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New Coordinate Transformation Tasks

Three new coordinate transformation tasks imcctran , skyctran , and wcsctran have been installed in the new IRAF V2.11 imcoords package. These tasks use the image header coordinate system to perform one or more of the following functions: 1) precess the image coordinate system ( imcctran ), 2) convert the image coordinate system from one celestial coordinate system to another ( imcctran ), 3) transform from pixel to world coordinates and vice versa ( skyctran and wcsctran ), or 4) locate objects detected in one image in another ( skyctran ). Skyctran can also be used to: 1) precess equatorial coordinates, or 2) convert coordinates from one celestial coordinate system to another. IRAF V2.11 supports equatorial, ecliptic, galactic, and supergalactic image celestial coordinate systems; earlier versions support equatorial image coordinate systems only.

The following examples demonstrate the basic functionality of the new tasks. To reproduce these examples on a local system copy the image dev$wpix to a local directory and edit in the missing EQUINOX keyword as shown below.

Precess or Transform the Image Header Coordinate System

Imcctran converts the image coordinate system from one celestial coordinate system to another. The conversion shifts and rotates the image coordinate system but leaves the pixel coordinates of the reference point and the coordinate projection unchanged. Precession is treated as a special coordinate conversion.

The following example converts the coordinate system of wpix from equatorial B1950.0 to equatorial J2000.0 and then from equatorial J2000.0 to galactic. The initial image copy avoids overwriting the header of wpix which is used in later examples.

Transform from Pixel to World Coordinates andVice Versa

The tasks wcsctran and skyctran convert from pixel to world coordinates and vice versa using the image coordinate system. Wcsctran works on images of any dimension with any valid coordinate system. The task skyctran works only on two-dimensional images with valid celestial coordinate systems.

The first two examples show how to use wcsctran to convert from pixel coordinates to world (in this case equatorial) coordinates and back again. The format and units specifications ensure that the world coordinates are written and read in hours and degrees.

The next two examples show how to use skyctran to convert from pixel to galactic coordinates without modifying the image coordinate system which in this case is an equatorial coordinate system.

The final example shows how skyctran can be used to locate objects for which only galactic coordinates are available in an image with an equatorial coordinate system. The tvmark task is used to mark the detected objects on the image display.

Locate Objects Detected in One Image in Another

Skyctran can locate objects detected in one image in another image which has a different celestial coordinate system, e.g., equatorial and galactic as shown in the following example. Starfind is an imcoords task which automatically locates stellar objects in images.

Precess or Convert Celestial Coordinates

The skyctran task is a general celestial coordinate conversion tool as well as an image coordinate conversion tool as shown in the following two examples.

Lindsey Davis

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Posted: 07May1998