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<H1><A NAME="SECTION000130000000000000000"></A><A NAME="ss:framesets"></A>
<BR>
Coordinate System Networks (FrameSets)
</H1>

<P>
We saw in &#167;<A HREF="node113.html#ss:introducingconversion">12</A> how astConvertastConvert could be
used to find a MappingMapping that inter-relates a pair of coordinate systems
represented by Frames. There is a limitation to this, however, in that
it can only be applied to coordinate systems that are inter-related by
suitable conventions. In the case of celestial coordinates, the
relevant conventions are standards set out by the International
Astronomical Union, and others, that define what these coordinate
systems mean. In practice, however, the relationships between many
other coordinate systems are also of practical importance.

<P>
Consider, for example, the focal plane of a telescope upon which an
image of the sky is falling. We could measure positions in this focal
plane in millimetres or, if there were a detector system such as a CCD
present, we could count pixels. We could also use celestial
coordinates of many different kinds. All of these systems are
equivalent in their effectiveness at specifying positions in the focal
plane, but some are more convenient than others for particular
purposes.

<P>
Although we could, in principle, convert between all of these focal
plane coordinate systems, there is no pre-defined convention for doing
so. This is because the conversions required depend on where the
telescope is pointing and how the CCD is mounted in the focal
plane. Clearly, knowledge about this cannot be built into the AST
library and must be supplied in some other way. Note that this is
exactly the same problem as we met in &#167;<A HREF="node83.html#ss:framedomains">7.12</A> when
discussing the DomainDomain attribute--<SPAN  CLASS="textit">i.e.</SPAN> coordinate systems that
apply to different physical domains require that extra information be
supplied before we can convert between them.

<P>
What we need, therefore, is a general way to describe how coordinate
systems are inter-related, so that when there is no convention already
in place, we can define our own. We can then look forward to
converting, say, from pixels into galactic coordinates and <SPAN  CLASS="textit">vice
versa.</SPAN>  In AST, the FrameSetFrameSet class provides this capability.

<P>
<BR><HR>
<!--Table of Child-Links-->
<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>

<UL CLASS="ChildLinks">
<LI><A NAME="tex2html1674"
  HREF="node121.html">The FrameSet Model</A>
<LI><A NAME="tex2html1675"
  HREF="node122.html">Creating a FrameSet</A>
<LI><A NAME="tex2html1676"
  HREF="node123.html">Adding New Frames to a FrameSet</A>
<LI><A NAME="tex2html1677"
  HREF="node124.html">The Base and Current Frames</A>
<LI><A NAME="tex2html1678"
  HREF="node125.html">Referring to the Base and Current Frames</A>
<LI><A NAME="tex2html1679"
  HREF="node126.html">Using a FrameSet as a Mapping</A>
<LI><A NAME="tex2html1680"
  HREF="node127.html">Extracting a Mapping from a FrameSet</A>
<LI><A NAME="tex2html1681"
  HREF="node128.html">Using a FrameSet as a Frame</A>
<LI><A NAME="tex2html1682"
  HREF="node129.html">Extracting a Frame from a FrameSet</A>
<LI><A NAME="tex2html1683"
  HREF="node130.html">Removing a Frame from a FrameSet</A>
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