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Developments in the Starlink AST library - an intelligent WCS management system

http://www.starlink.ac.uk/ast/

David S. Berry,
Centre for Astrophysics,
University of Central Lancashire,
Preston, Lancs. UK.
PR1 2HE.
dsb@ast.man.ac.uk
Tel. 01772 893564

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funded by:
The Starlink Project,
Rutherford Appleton Laboratory,
Didcot, Oxfordshire, UK.
OX11 0QX
ussc@star.rl.ac.uk
http://www.starlink.ac.uk/

Contents:

  1. The Starlink AST library

  2. What's new in AST

  3. Coming soon

  4. Support within AST V3.0 for FITS-WCS features and keywords

  5. AST in use

The Starlink AST library:

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.

Object-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 such as those in the montage above. Graphics are draw 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.

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. A SpecFrame can converts spectral axis values between any of the coordinate systems or rest frames listed in FITS-WCS paper III, using any appropriate units.

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.

V3.0 (highlights only):

Support for FITS-WCS papers I, II, III & IV has been extended significantly. The current status is shown in the two tables to the right.

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-class 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.

Coming soon:

Support within AST V3.0 for FITS-WCS features and keywords:

Feature:

Status:

Axis distortions (as described in paper IV)

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).

Alternate axis descriptions

Supported

User-defined fiducial points

Supported

Choice of PCi_ja or CDi_ja matrix

Supported

Deprecated WCS conventions (e.g. keyword CROTAi, EPOCH, PCiiijjj, the NCP projection, etc.)

Supported

Alternate keyword formats for inclusion in tables

Unsupported

Units in comment strings

Unsupported

Uncertainties in the coordinates

Unsupported

“WCSDEP” convention

Unsupported

Use of longitude projection parameters to store LONPOLE and LATPOLE

Supported

Common, non-standard features

Support for reading (but not writing) various common, non-standard feature such as “TNX” and “ZPX” projections is included. More can easily be added as requested.



Keyword:

Description:

Status:

WCSAXESa

WCS dimensionality

Supported

CRVALia

Value at reference point

Supported

CRPIXja

Pixel of reference point

Supported

CDELTia

Increment at reference point

Supported

CROTAi

Rotation at reference point

Supported (only written when producing a “FITS-AIPS” header)

CTYPEia

Coordinate/algorithm/distortion type

Supported (all coordinate systems [including helio-ecliptic] and algorithm codes listed in papers I, II and III, with the exception of the “STOKES” and “COMPLEX” values defined in paper I, the “CUBEFACE” value defined in paper II, and the “-TAB” algorithm defined in paper III)

CUNITia

Units of coordinate values

Supported (including automatic unit conversion if the units strings conform to paper I conventions)

PCi_ja

Transformation matrix

Supported

CDi_ja

Transformation matrix

Supported

PVi_ma

Algorithm numerical parameter m

Supported

PSi_ma

Algorithm textual parameter m

Unsupported

WCSNAMEa

Coordinate version name

Supported

CRDERia

Random error

Unsupported

CSYERia

Systematic error

Unsupported

LONPOLEa

Coordinate rotation (celestial axes)

Supported

LATPOLEa

Coordinate rotation (celestial axes)

Supported

RADESYSa

Frame of reference (celestial axes)

Supported (all systems, except ecliptic coordinates are currently assumed to be FK5)

EQUINOXa

Coordinate epoch (celestial axes)

Supported

EPOCH

Coordinate epoch (celestial axes)

Supported (only written when producing a “FITS-AIPS” header)

MJD-OBS

Time of observation

Supported

SPECSYSa

Spectral reference frame

Supported (all systems, except CMBDIPOL)

SSYSOBSa

Spectral reference frame fixed during observation

Unsupported

OBSGEO-X/Y/Z

Observation position

Supported

MJD-AVG

Average date of observation

Supported

VSOURCEa

Physical velocity of source for SPECSYS=SOURCE cases

Supported

ZSOURCEa

Optical velocity of source for SPECSYS=SOURCE cases

Supported

VELOSYSa

Velocity w.r.t. standard of rest

Unsupported

RESTFRQa

Line rest frequency

Supported

RESTWAVa

Line rest wavelength in vacuum

Supported

CNAMEia

Descriptive axis label

Supported


AST in use:


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Starlink: adass03/ADASS03.html (last edited 2009-09-24 12:31:12 by DavidBerry)