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<H2><A NAME="SECTION000171000000000000000"></A><A NAME="ss:foreignencodings"></A>
<BR>
The Foreign FITS Encodings
</H2>

<P>
As mentioned previously (&#167;<A HREF="node156.html#ss:nativeencoding">16.1</A>), there are a
number of conventions currently in use for storing WCS information in
FITS headers, which we call <SPAN  CLASS="textit">encodings</SPAN>. Here, we are concerned
with those encodings defined by software systems other than AST, which
we term <SPAN  CLASS="textit">foreign encodings</SPAN>.

<P>
Currently, AST supports six foreign encodings, which may be selected
by setting the EncodingEncoding attribute of a FitsChanFitsChan to one of the
following (character string) values:

<P><DL>
<DT><STRONG>DSS</STRONG></DT>
<DD>
<BR>
This encoding stores WCS information using the convention developed at
the Space Telescope Science Institute for the Digitised Sky Survey
(DSS) astrometric plate calibrations.  DSS images which use this
convention are widely available and it is understood by a number of
important and well-established astronomy applications.

<P>
However, the calibration model used (based on a polynomial fit) is not
easily applicable to other types of data and creating the polynomial
coefficients needed to calibrate your own images can prove
difficult. For this reason, the DSS encoding is probably best viewed
as a ``read-only'' format. It is possible, however, to read in WCS
information using this encoding and then to write it back out again,
so long as only minor changes have been made.

<P>
</DD>
<DT><STRONG>FITS-WCS</STRONG></DT>
<DD>
<BR>
This encoding is very important because it is based on a new FITS standard
which should, for the first time, address the problem of celestial coordinate
systems in a proper manner, by considerably extending the original FITS
standard.

<P>
The conventions used are described in a series of papers by
E.W.Greisen, M.Calabretta, <SPAN  CLASS="textit">et. al.</SPAN>, often referred to as the
``FITS-WCS papers''. They are described at
<TT><A NAME="tex2html45"
  HREF="http://fits.gsfc.nasa.gov/fits_wcs.html">http://fits.gsfc.nasa.gov/fits_wcs.html</A></TT>. Now that the first two papers
in this series have been agreed, this encoding should be understood by any
FITS-WCS compliant software and it is likely to be adopted widely for FITS
data in future.  For details of the coverage of these conventions provided
by the FitsChan class, see Appendix&nbsp;<A HREF="node220.html#ss:fitswcscoverage">G</A>.

<P>
</DD>
<DT><STRONG>FITS-IRAF</STRONG></DT>
<DD>
<BR>
This encoding is based on the conventions described in the document
``World Coordinate Systems Representations Within the FITS Format'' by R.J.
Hanisch and D.G. Wells, 1988.<A NAME="tex2html46"
  HREF="footnode.html#foot3389"><SUP><IMG  ALIGN="BOTTOM" BORDER="1" ALT="[*]"
 SRC="/usr/share/latex2html/icons/footnote.png"></SUP></A> It is employed
by the IRAF data analysis facility, so its use will facilitate data
exchange with IRAF. This encoding is in effect a sub-set of the current
FITS-WCS encoding.

<P>
</DD>
<DT><STRONG>FITS-PC</STRONG></DT>
<DD>
<BR>
This encoding is based on a previous version of the proposed new FITS WCS
standard which used <TT>PCjjjjiii</TT> and <TT>CDELTj</TT> keywords to describe
axis rotation and scaling. Versions of AST prior to V1.5 used this scheme
for the FITS-WCS encoding. As of V1.5, FITS-WCS uses <TT>CDi_j</TT>
keywords instead.<A NAME="tex2html47"
  HREF="footnode.html#foot3394"><SUP><IMG  ALIGN="BOTTOM" BORDER="1" ALT="[*]"
 SRC="/usr/share/latex2html/icons/footnote.png"></SUP></A> The FITS-PC encoding is included in AST V1.5 only to allow
FITS-WCS data created with previous versions to be read. It should not,
in general, be used to create new data sets.

<P>
</DD>
<DT><STRONG>FITS-AIPS</STRONG></DT>
<DD>
<BR>
This encoding is based on the conventions described in the document
``Non-linear Coordinate Systems in AIPS'' by Eric W. Greisen (revised 9th
September, 1994).<A NAME="tex2html48"
  HREF="footnode.html#foot3396"><SUP><IMG  ALIGN="BOTTOM" BORDER="1" ALT="[*]"
 SRC="/usr/share/latex2html/icons/footnote.png"></SUP></A> It is currently employed by the AIPS
data analysis facility, so its use will facilitate data exchange with
AIPS. This encoding uses <TT>CROTAi</TT> and <TT>CDELTi</TT> keywords to
describe axis rotation and scaling.

<P>
</DD>
<DT><STRONG>FITS-AIPS++</STRONG></DT>
<DD>
<BR>
Encodes coordinate system information in FITS
header cards using the conventions used by the AIPS++ project.
This is an extension of FITS-AIPS which includes some of the
features of FITS-PC and FITS-IRAF.
</DD>
</DL>

<P>
For more detail about the above encodings, see the description of the
Encoding attribute in Appendix&nbsp;<A HREF="node216.html#ss:attributedescriptions">C</A>.

<P>

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