IRAF Newsletter -- Number 14 -- April 1998

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New Image Astrometry Tasks

The following image astrometry tasks have been installed in the new IRAF 2.11 imcoords package.

ccfind

Locate reference catalog objects in images

ccxymatch

Match celestial and pixel coordinate lists

ccmap

Compute plate solutions using matched coordinate lists

ccsetwcs

Update the image coordinate system using the ccmap plate solution

cctran

Transform coordinate lists using the ccmap plate solution

starfind

Automatically detect objects in images

The main function of the new tasks is to compute plate solutions for images and store them in the image headers in a FITS compatible format. The new tasks can also be used for computing accurate pixel and world coordinates for objects in images and for determining geometric parameters for images such as the plate scale and orientation. These tasks do not comprise a fully featured astrometry package. However, they have been successfully integrated into the prototype astrometry package tfinder (see article in this Newsletter) and the NOAO Mosaic reduction package mscred (see article in this Newsletter).

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

Get the Reference Object List

The new astrometry tasks assume that the user has a list of reference catalog objects already in hand. Reference coordinates may be obtained in many ways both inside and outside of IRAF. For example, the STSDAS regions task could be used to extract catalog sources appropriate for the dev$wpix field from the Guide Star Catalog. The tprint task could then be used to convert from ST binary table format to the simple text file format required by the imcoords tasks.

Locate Reference Objects in the Image

The ccfind task locates the reference objects in the image by using either the existing image header coordinate system if any, as shown in the first example, or the image coordinate system defined by the user, as shown in the second example. In both cases ccfind converts the reference coordinates from the reference coordinate system (in this case J2000.0), to the image coordinate system (in this case B1950.0), uses the image coordinate system to compute approximate pixel coordinates, and then applies a centroiding algorithm to compute final pixel coordinates. The first example requires that the image header coordinate system be complete and reasonably accurate; the second example requires that the user have apriori knowledge about the coordinates of the center of the image, e.g., from the telescope, and the image scale and orientation, e.g., from the detector.

Compute the Plate Solution

The ccmap task computes the plate solution using the matched celestial and pixel coordinate list produced by ccfind and the coordinates of the tangent point, then stores the computed solution in a database file for later use by the ccsetwcs and cctran tasks. The computed solution is defined in the coordinate system of the input coordinates, and J2000.0 is assumed by default. The task ccmap may be run interactively as shown below or non-interactively.

By default ccmap performs a standard 6-coefficient linear fit to the input coordinates. Higher order fits to the residuals from the linear fit are supported by ccmap and the plate solution evaluation task cctran . However, due to the lack of an FITS convention for representing higher order plate solutions, only the linear part of the computed plate solution is normally stored in the image header by the ccmap and ccsetwcs tasks. Users running IRAF V2.11.1 or later versions can force the entire plate solution to be recorded in the image header by setting the ccmap projection parameter to "tnx". All IRAF tasks which compute world coordinates, e.g., starfind , imexamine , wcsctran , etc., understand the "tnx" system. Users planning to export their IRAF images to another image processing system should avoid using the "tnx" projection if they are concerned about portable coordinate systems.

Update the Image Coordinate System

The ccsetwcs task creates the image world coordinate system from the computed plate solution as shown in the example below, or from a list of parameters supplied by the user.

Compute Coordinates for Program Objects

Once the plate solution is computed and stored in the image header either the new image world coordinate system or the cctran task can be used to compute celestial coordinates for program objects. For 6-coefficient linear plate solutions the two methods are equivalent; for higher order plate solutions computed prior to the release of IRAF 2.11.1 the ccmap database record must be used to retrieve the full accuracy of the plate solution. In the following examples the starfind task is used to compute pixel and world coordinates for the objects that it detects in the image, and cctran uses the plate solution to convert the starfind pixel coordinates into accurate celestial coordinates.

Lindsey Davis

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IRAF Group, National Optical Astronomy Observatories, P.O. Box 26732, Tucson, AZ 85726, Phone: (520) 318-8160, FAX: (520) 318-8360, Email iraf@noao.edu

Posted: 07May1998