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NDF is a data format for storing arrays of numbers with associated ancillary information. It is the native data format used by Starlink packages, and is based on the Hierarchical Data System (HDS), which allows data to be stored in the form of arbitrary structures of arrays and scalars, using character, integer, logical or floating point primitives.
The following components are recognized within an NDF structure:
Starlink distributes a library of routines which provide access to all
the components of an NDF structure. The library is documented fully in
Starlink User Note 33, available from the Starlink librarian,
ussc@star.rl.ac.uk
, or from the Starlink WWW site
http://www.starlink.rl.ac.uk/. It includes facilities for doing
on-the-fly data conversion so that a wide range of non-NDF data
formats can be read and written transparently by NDF-based software. The
library is written mainly in FORTRAN, but additional C and Perl
interfaces are available. Since NDF is based on HDS, it transparently
converts NDFs written on machines which have a different byte-ordering
from that on which the data are read.
The 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. Documentation is available in Starlink User Note 210 (FORTRAN) and 211 (C), available from the Starlink librarian or from the Starlink WWW site. One of the design goals of the AST library was to make it ``stand-alone''--it does not depend on any other Starlink libraries (with the exception of the SLALIB positional astronomy library). Although it is object-based, it has procedural interfaces for both C and FORTRAN.
One of the most important AST objects is the FrameSet, which represents an arbitrary collection of coordinate Frames, with associated Mappings describing how to convert positions from one Frame to another (see Figure 1). An AST Frame represents a coordinate system and can be:
An AST Mapping is a recipe for transforming position between Frames. Sub-classes of Mapping are available which provide basic linear transformations, a wide range of spherical projections, generalised algebraic expressions, and user-defined transformations. Mappings may be combined together to produce compound Mappings (see Figure 2).
Graphical facilities within AST include production of entire annotated coordinate grids from an arbitrary Mapping (any discontinuities and singularities in the Mapping are handled transparently). Any graphics package capable of drawing straight lines and text can be used with AST. A simple interface module needs to be written for each new graphics package. AST comes with a pre-written interface for PGPLOT.
Objects can be saved and retrieved as a set of ``keyword=value'' strings from any external data system capable of storing lines of text, but special support is provided for storing AST objects in the form of FITS header cards. FrameSets can be stored and retrieved using several different popular encodings of WCS information within FITS headers, including the standard proposed by Calabretta & Greisen.
The current version of the NDF access library has been modified to include a new standard item, the ``WCS'' component, which attaches an arbitrary collection of world coordinate systems to the data.
Each NDF now has a ``current coordinate Frame'' which can be Pixel coordinates, ``Axis'' coordinates (defined by the AXIS component of the NDF), or any other coordinate Frame which has been imported into the WCS component (e.g., R. A./Dec., focal plane position, offset from the field centre, etc.). Data reduction commands report positions in the current Frame of the NDF being processed, and expect the user to supply positions in the same Frame. User commands exist to change the current Frame in any NDF.
Most of the main Starlink packages (GAIA, KAPPA, CCDPACK, POLPACK, ESP, CONVERT, etc.) have been modified to take advantage of the facilities provided by AST and the WCS component. These include:
Through these packages, the Starlink Software Collection now provides a powerful, uniform interface for importing, modifying, exporting and displaying world coordinate information.