## What Problems Does AST Tackle?

Here are some of the main problems you may face when handling world coordinate system (WCS) information and the solutions that AST provides:

1. The Variety of Coordinate Systems
  Astronomers use a wide range of differing coordinate systems to describe positions within a variety of physical domains. For instance, there are a large number of celestial coordinate systems in use within astronomy to describe positions on the sky. Understanding these, and knowing how to convert coordinates between them, can require considerable expertise. It can also be difficult to decide which of them your software should support. The same applies to coordinate systems describing other domains, such as position within an electro-magnetic spectrum.

  Solution. AST has built-in knowledge of many coordinate systems and allows you to convert freely between them without specialist knowledge. This avoids the need to embed details of specific coordinate systems in your software. You also benefit automatically when new coordinate systems are added to AST.

2. Storing and Retrieving WCS Information
  Storing coordinate system information in astronomical datasets and retrieving it later can present a considerable challenge. Typically, it requires knowledge of rather complex conventions (e.g. FITS) which are low-level, often mis-interpreted and may be subject to change. Exchanging information with other software systems is further complicated by the number of different conventions in use.

  Solution. AST combines a unifying high-level description of WCS information with the ability to save and restore this using a variety of formats. Details of the formats, which include FITS, are handled internally by AST. This frees you from the need to understand them or embed the details in your software. Again, you benefit automatically when new formats are added to AST.

3. Generating Graphical Output
  Producing graphical displays involving curvilinear coordinate systems, such as celestial coordinate grids, can be complicated. Particular difficulties arise when handling large areas of sky, the polar regions and discontinuous (e.g. segmented) sky projections. Even just numbering and labelling curvilinear axes is rarely straightforward.

  Solution. AST provides plotting facilities especially designed for use with curvilinear coordinate systems. These include the plotting of axes and complete labelled coordinate grids. A large number of options are provided for tailoring the output to your specific needs. Three dimensional coordinate grids can also be produced.

4. Aligning Data from Different Sources
  One of the main uses of coordinate systems is to facilitate the inter-comparison of data from different sources. A typical use might be to plot (say) radio contours over an optical image. In practice, however, different celestial coordinate systems may have been used, making accurate alignment far from simple.

  Solution AST provides a one-step method of aligning datasets, searching for all possible intermediate coordinate systems. This makes it simple to directly inter-relate the pixel coordinates of different datasets.

5. Handling Different Types of Coordinate SystemSystem
  Not all coordinate systems used in astronomy are celestial ones, so if you are writing general-purpose software such as (say) a display tool, you may also need to handle axes representing wavelength, distance, time or whatever else comes along. Obviously, you would prefer not to handle each one as a special case.

  Solution AST uses the same flexible high-level model to describe all types of coordinate system. This allows you to write software that handles different kinds of coordinate axis without introducing special cases.