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
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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.