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<H3><A NAME="SECTION001113100000000000000"> </A>
<A NAME="ch07:standard-keywords"> </A>
<BR>
Standard keywords
</H3>
The FITS format specifies a number of ``standard'' keywords with well
defined meaning. They can be classified in two groups namely: a) data
structure definition, and b) general description. The first group is
translated into special MIDAS descriptors for the data structure in
question. The conversion for image files is given in
Table <A HREF="node114.html#ch07:tab3">7.1.3</A>. The <TT>START</TT> descriptor of a MIDAS image
refers to the first pixel and is computed from the values <TT>CRVAL</TT>
and <TT>CRPIX</TT> for the reference pixel as given in the FITS header.
The actual reference pixel is saved in the descriptor <TT>REFPIX</TT>
which is necessary for non-linear coordinate systems. Each element of
the descriptor <TT>CUNIT</TT> has 16 character. Thus, the string
corresponding to the FITS keyword <TT>CTYPE1</TT> is stored in <TT>CUNIT(17:31)</TT> as it is viewed as a single string of type <TT>C*1</TT>.
<P>
<BR>
<DIV ALIGN="CENTER"><A NAME="10329"> </A>
<TABLE>
<CAPTION><STRONG>Table 7.3:</STRONG>
Name translation between standard FITS keywords and
MIDAS descriptors. Asterisk indicates that data value
is transformed (see text).</CAPTION>
<TR><TD>
<DIV ALIGN="CENTER">
<TABLE CELLPADDING=3 BORDER="1" ALIGN="CENTER">
<TR><TD ALIGN="LEFT">FITS keyword</TD>
<TD ALIGN="LEFT">MIDAS descriptor</TD>
<TD ALIGN="LEFT">FITS keyword</TD>
<TD ALIGN="LEFT">MIDAS descriptor</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> NAXIS</TT></TD>
<TD ALIGN="LEFT"><TT> NAXIS</TT></TD>
<TD ALIGN="LEFT"><TT> OBJECT</TT></TD>
<TD ALIGN="LEFT"><TT> IDENT</TT></TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> NAXIS</TT>n</TD>
<TD ALIGN="LEFT"><TT> NPIX</TT>(n)</TD>
<TD ALIGN="LEFT"><TT> DATE-OBS</TT></TD>
<TD ALIGN="LEFT"><TT> O_TIME</TT>(1)</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> CRPIX</TT>n</TD>
<TD ALIGN="LEFT"><TT> REFPIX</TT>(n)</TD>
<TD ALIGN="LEFT"><TT> DATAMIN</TT></TD>
<TD ALIGN="LEFT"><TT> LHCUTS</TT>(3)</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> CRVAL</TT>n</TD>
<TD ALIGN="LEFT"><TT> START</TT>(n)<SUP>*</SUP></TD>
<TD ALIGN="LEFT"><TT> DATAMAX</TT></TD>
<TD ALIGN="LEFT"><TT> LHCUTS</TT>(4)</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> CDELT</TT>n</TD>
<TD ALIGN="LEFT"><TT> STEP</TT>(n)</TD>
<TD ALIGN="LEFT"><TT> EPOCH</TT></TD>
<TD ALIGN="LEFT"><TT> O_POS</TT>(3)</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> CROTA</TT>n</TD>
<TD ALIGN="LEFT"><TT> ROTA</TT>(n)</TD>
<TD ALIGN="LEFT"><TT> EQUINOX</TT></TD>
<TD ALIGN="LEFT"><TT> O_POS</TT>(3)</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> BUNIT</TT></TD>
<TD ALIGN="LEFT"><TT> CUNIT</TT>(1)</TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> CTYPE</TT>n</TD>
<TD ALIGN="LEFT"><TT> CUNIT</TT>(n+1)</TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
</TR>
</TABLE> <A NAME="ch07:tab3"> </A> </DIV></TD></TR>
</TABLE>
</DIV>
<BR>
Tables have more complicated structures which make the translation
less direct. With respect to other keywords, images and tables are
treated in the same way. The data types of these descriptors are
checked and must correspond to the FITS standard.
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-09</I>
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