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%APN3_PROCEEDINGS_FORM%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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% TEMPLATE.TEX -- APN3 (2003) ASP Conference Proceedings template.
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\documentclass[11pt,twoside]{article}  % Leave intact
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\begin{document}   % Leave intact

%-----------------------------------------------------------------------
%			    Paper ID Code
%-----------------------------------------------------------------------
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%%%% ID=P2-4

%-----------------------------------------------------------------------
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\title{Chandra FITS Dictionary}
%\titlemark{ }

%-----------------------------------------------------------------------
%		          Authors of Paper
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%                  R.\ M.\ Crutcher\altaffilmark{1}}
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%                 61801}
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%          \affil{University of Kentucky}
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\author{S. Winkelman, A. Rots, K. Michaud, S. Blecksmith}
\affil{Chandra X-Ray Center/Smithsonian Astrophysical Observatory}

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\contact{Sherry Winkelman}
\email{swinkelman@cfa.harvard.edu}

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\paindex{Winkelman, S.}
\aindex{Rots, A.}     % Remove this line if there is only one author
\aindex{Michaud, K.}     % Remove this line if there is only one author
\aindex{Blecksmith, S.}     % Remove this line if there is only one author

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% Use the "et al." form in the case of seven or more authors, or if
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\authormark{Winkelman, Rots, Michaud, Blecksmith}

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\keywords{databases, archives, observatory: operations, interoperability, X-ray}

%-----------------------------------------------------------------------
%			       Abstract
%-----------------------------------------------------------------------
% Type abstract in the space below.  Consult the User Guide and Latex
% Information file for a list of supported macros (e.g. for typesetting 
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\begin{abstract}          % Leave intact
% Place the text of your abstract here - NO BLANK LINES
Well-defined data and metadata specifications are fundamental for the
operation of large observing facilities.  We have collected the
complete body of FITS keywords, as used in Chandra data products, into
a dictionary database that provides easy access to their use and
meaning.  We present an account of the functionality of the
dictionary, as well as a description of the database design and
details of the tools which display the dictionary. 
\end{abstract}

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\section{Introduction}

Well-defined data and metadata specifications are fundamental for the
operation of large observing facilities.  For the Chandra Data Archive
almost all data products are in FITS format, following a stringent set
of conventions concerning the data formats and header construction.
The header keywords form the core of the metadata for our archive, therefore
a proper understanding of the rules and conventions that govern their
meaning and usage is essential to a correct interpretation of the
data.

We have collected the complete body of FITS keywords, as used in
Chandra data products, into a dictionary database that provides easy
access to their use and meaning.  As such, it provides metadata on the
metadata.  This is of obvious interest to our users, but also, and
particularly, for facilitating multi-mission data analysis.  In
addition, such dictionaries are invaluable as an aide to projects and
missions that are in the process of defining their metadata
conventions.  Finally, the Virtual Observatory also has a clear need
for electronic access to metadata definitions.

\section{The Dictionary}

The Chandra X-ray Center (CXC) FITS dictionary was designed to provide
general definitions of the approximately 300 keywords.  In addition,
the dictionary also provides more detailed definitions of keywords as %FO: used 
they are used in the more than 250 distinct types of FITS files
produced in the processing of Chandra data.  All definitions include a
detailed definition, a descriptive string describing the keyword, and
a datatype.  To complete the definition, the format, units, maximum
and minimum values, allowed values, default values, and notes on usage
of a keyword are included where appropriate.

\newpage
In order to accommodate the various needs of users, the CXC FITS web
interface gives users three forms of access to the dictionary.

\begin{description}

\begin{figure}
\epsscale{0.8}
\plottwo{P2-4_f1-1.eps}{P2-4_f1-2.eps}
\caption{Snapshots of the search and browse screens.}
\label{P2-4:fig1}
\end{figure}

\begin{figure}
\epsscale{0.8}
\plottwo{P2-4_f2-1.eps}{P2-4_f2-2.eps}
\caption{Snapshots of the generate header screen and a sample header.}
\label{P2-4:fig2}
\end{figure}

\item{Search by keyword:} This page returns general definitions that
apply to all CXC data files. Wildcards can be used in the search. If
an exact match cannot be made, close matches will be returned. A list
of filetypes using the keyword is also returned.  See
Figure~\ref{P2-4:fig1} for a snapshot.

\item{Browse the database:} This page allows the user to browse the
database by specific filetype. The keyword definition that is returned will be
specific to its use in that filetype. For keywords that contain an
index (eg., HDUCLASn), each value for the filetype is returned.  For
keywords that come in groups (eg., TTYPEn, TFORMn, TUNITn, etc.), a
link is provided to a table giving the full group definition for the
filetype.  See Figure~\ref{P2-4:fig1} for a snapshot.

\item{Generate schematic FITS headers:} This page generates a
schematic FITS header for a selected filetype. The schematic includes
the keywords, default values, FITS comments, datatypes, and formats
for each HDU contained in the header.  See Figure~\ref{P2-4:fig2} for a snapshot.

\end{description}

\section{The Database}
\begin{figure}
\epsscale{0.8}
\plotone{P2-4_f3.eps}
\caption{Diagram of the relationship (one-to-one or one-to-many)
between the columns used to join tables in the database.  The dotted
lines are links between the usage\_note table and the rest of the
database.  The dashed lines are links between the reference table and
the rest of the database.}
\label{P2-4:fig3}
\end{figure}

The CXC follows a stringent set of conventions regarding header
construction. A %%FO (the 'A' added)
filetype is composed of one or more Header-Data Units (HDU).
Each HDU's header contains a set of components.  These
components consist of fixed sets of keywords.  In addition, each
filetype may contain additional keywords. There are also ordering
requirements for header components and keywords.  As a result, the
dictionary database became quite involved. The database layout is loosely
based on the keyword database design used by the Space Telescope
Science Institute.  See Figure~\ref{P2-4:fig3} for a diagram of the
database structure.  The tables fall into three categories: those
which define the keywords to varying degrees of specificity; those
which provide locations of keywords within headers; and those which
provide references for the information within the database.

\section{Conclusion}

The database and dictionary interface are mostly mission-independent,
possibly requiring minor revisions to one table in the database and
several queries in the CGI script to reflect a mission's
categorization of its FITS files.  As such, we offer our package to
any project that has a need for this facility.

\acknowledgments
This work is supported by HEASARC grant 16613384 and NASA contract NAS8-39073
(CXC).
%-----------------------------------------------------------------------
%			      References
%-----------------------------------------------------------------------
% List your references below within the reference environment
% (i.e. between the \begin{references} and \end{references} tags).
% Each new reference should begin with a \reference command which sets
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%
% EXAMPLE:  \reference Hagiwara, K., \& Zeppenfeld, D.\  1986, 
%                Nucl.Phys., 274, 1
%           \reference H\'enon, M.\  1961, Ann.d'Ap., 24, 369
%           \reference King, I.\ R.\  1966, \aj, 71, 276
%           \reference King, I.\ R.\  1975, in Dynamics of Stellar 
%                Systems, ed.\ A.\ Hayli (Dordrecht: Reidel), 99
%           \reference Tody, D.\  1998, \adassvii, 146
%           \reference Zacharias, N.\ \& Zacharias, M.\ 2003,
%                \adassxii, \paperref{P7.6}
% 
% Note the following tricks used in the example above:
%
%   o  \& is used to format an ampersand symbol (&).
%   o  \'e puts an accent agu over the letter e.  See the User Guide
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%      for a full list of journal macros
%   o  \adassvii is a macro that expands to the full title, editor,
%      and publishing information for the ADASS VII conference
%      proceedings.  Such macros are defined for ADASS conferences I
%      through XI.
%   o  When referencing a paper in the current volume, use the
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%      the paper ID code for the paper you are referencing.  See the 
%      note in the "Paper ID Code" section above for details on how to 
%      determine the paper ID code for the paper you reference.  
%
\section*{Links} %%% FO \begin{references}
\begin{itemize}
\item[---] http://cxc.harvard.edu/contrib/arots/fits/ascfits.ps
\item[---] http://cxc.harvard.edu/contrib/arots/fits/content.txt
\item[---] http://heasarc.gsfc.nasa.gov/docs/heasarc/fits\_overview.html
\item[---] http://archive.stsci.edu/fits/users\_guide/
\item[---] http://www.dpt.stsci.edu/keyword/keyword\_design.html
\end{itemize} %% FO \end{references}

% Do not place any material after the references section

\end{document}  % Leave intact
