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Astronomical Data Analysis Software and Systems XIII ASP Conference Series, Vol. 314, 2004 F. Ochsenbein, M. Al len, and D. Egret, eds.

Developments in the Starlink AST Library - an Intelligent WCS Management System
D.S. Berry The Starlink Project, Rutherford Appleton Laboratory, Didcot, Oxfordshire, United Kingdom. OX11 0QX. Email: dsb@ast.man.ac.uk Abstract. Recent developments in the AST library for managing WCS information are described. These include support for spectral coordinate systems, and compliance with FITS WCS papers I, II and III.

1.

The Starlink AST Library · Home page at http://www.starlink.ac.uk/ast/. · Provides a comprehensive range of facilities for attaching world coordinate systems to astronomical data, for retrieving and interpreting that information and for generating graphical output such as coordinate grids based upon it. · Can read and write WCS information stored in the form of FITS-WCS headers, but also provides its own richer format. · Ob ject-oriented design implemented in pure ANSI C with interfaces for C, FORTRAN and Java (via JNI). · Has built-in intelligence for identifying flavours of celestial, spectral and other coordinate systems and determining how to transform between them, allowing general purpose code to be written which makes no assumptions about the nature of the coordinate systems. · Includes a flexible and versatile "tool-kit" for creating and modifying collections of coordinate frames interconnected by arbitrarily complex transformations. · Easy-to-use graphical facilities allow the production of annotated grids. Graphics are drawn via a simple "driver" module which AST calls to draw lines, strings, markers, etc. AST includes a driver for PGPLOT; drivers for other graphics systems (e.g. Tcl/Tk, Java/Swing, etc.) can easily be (and have been) written. An example of code which draws a coordinate grid is shown on this poster.

2. 2.1.

What's New in AST V2.0 - Highlights Only

· A new class called SpecFrame has been added which represents positions within an electro-magnetic spectrum . SpecFrame can convert spectral axis values between any of the coordinate systems or rest frames listed in FITS-WCS paper III, using any appropriate units. 412 c Copyright 2004 Astronomical Society of the Pacific. All rights reserved.

An Intelligent WCS Management System

413

· The astConvert method, which automatically finds a Mapping between any two coordinate Frames, now takes account of the "Units" string in the two Frames, so long as they use the conventions for described units included in FITS-WCS paper I. 2.2. V3.0 - Highlights Only

· Support for FITS-WCS papers I, II, III & IV has been extended significantly. The current status is shown in Tables 1 and 2. · Includes new documentation giving the details of AST's implementation of FITS-WCS papers I, II and III. · The SkyFrame class (which represents coordinate systems on the sky) now supports ICRS and helio-ecliptic coords. · The Mapping class (which represents coordinate transformations) has a new method to determine the rate of change of any Mapping output with respect to any Mapping input. · Several new sub-classes of Mapping have been added: PolyMap: Performs a general N-dimensional polynomial transformation. GrismMap: Models the dispersion in a grism or prism. ShiftMap: Shifts each axis by a specified constant. · Simplification of complex Mappings has been extended. 3. Coming Soon · New XML formats for WCS information, including IVOA Space-TimeCoordinate format. · Support for coordinate systems representing time, including automatic identification and conversion between different temporal systems. · Increased coverage of FITS-WCS papers.

414

Berry

Table 1.

Support within AST V3.0 for FITS-WCS features.
Status Ignored (except for the SIRTF-specific "SIP" code which is interpreted correctly when reading a FITS header, but is never written out to a new FITS header). Supported Supported Supported Supported

Feature Axis distortions (as described in paper IV)

Alternate axis descriptions User-defined fiducial points Choice of PCi ja or CDi ja matrix Deprecated WCS conventions (e.g. keyword CROTAi, EPOCH, PCiiijjj, the NCP pro jection, etc.) Alternate keyword formats for inclusion in tables Units in comment strings Uncertainties in the coordinates "WCSDEP" convention Use of longitude pro jection parameters to store LONPOLE and LATPOLE Common, non-standard features

Unsupported Unsupported Unsupported Unsupported Supported Support for reading (but not writing) various common, non-standard feature such as "TNX" and "ZPX" pro jections is included. More can easily be added as requested.

An Intelligent WCS Management System Table 2.
Keyword WCSAXESa CRVALia CRPIXja CDELTia CROTAi CTYPEia

415

Support within AST V3.0 for FITS-WCS keywords.
Status Supported Supported Supported Supported Supported (only written when producing a "FITS-AIPS" header) Coordinate/algorithm/distortion Supported (all coordinate systems [intype cluding helio-ecliptic] and algorithm codes listed in papers I, I I and I I I, with the exception of the "STOKES" and "COMPLEX" values defined in paper I, the "CUBEFACE" value defined in paper I I, and the "-TAB" algorithm defined in paper III) Units of coordinate values Supported (including automatic unit conversion if the units strings conform to paper I conventions) Transformation matrix Supported Transformation matrix Supported Algorithm numerical parame- Supported ter Algorithm textual parameter Unsupported Coordinate version name Supported Random error Unsupported Systematic error Unsupported Coordinate rotation (celestial Supported axes) Coordinate rotation (celestial Supported axes) Frame of reference (celestial Supported (all systems, except ecliptic axes) coordinates are currently assumed to be FK5) Coordinate epoch (celestial Supported axes) Coordinate epoch (celestial Supported (only written when producing axes) a "FITS-AIPS" header) Time of observation Supported Spectral reference frame Supported (all systems, except CMBDIPOL) Spectral reference frame fixed Unsupported during observation Observation position Supported Average date of observation Physical velocity of source for SPECSYS=SOURCE cases Optical velocity of source for SPECSYS=SOURCE cases Velocity w.r.t. standard of rest Line rest frequency Line rest wavelength in vacuum Descriptive axis label Supported Supported Supported Unsupported Supported Supported Supported Description WCS dimensionality Value at reference point Pixel of reference point Increment at reference point Rotation at reference point

CUNITia

PCi ja CDi ja PVi ma PSi ma WCSNAMEa CRDERia CSYERia LONPOLEa LATPOLEa RADESYSa

EQUINOXa EPOCH MJD-OBS SPECSYSa SSYSOBSa OBSGEOX/Y/Z MJD-AVG VSOURCEa ZSOURCEa VELOSYSa RESTFRQa RESTWAVa CNAMEia