\subsection{Finding astrometric reference stars}

The purpose of this stage is to find a small number of astrometric
reference stars, imaged on the CCD frame, which can be used to define the
preliminary astrometric calibration.  Traditionally finding reference
stars is a long-winded task involving consulting printed atlases and
catalogues.  However, the on-line resources available to GAIA allow the
process to be automated and simplified.  Proceed as follows.

\begin{enumerate}

  \item You need to extract a region of the Digitised Sky Survey (DSS)
   roughly corresponding to the region imaged in the CCD frame.  The
   first part of the recipe in Section~\ref{RETRIEV_RECIP} gives exactly
   the procedure required.  Either: repeat this procedure, load the image
   that you created when previously working through
   Section~\ref{RETRIEV_RECIP} into GAIA, or load file {\tt ngc1275dss.sdf}
   into GAIA (the last is the required region, already extracted from
   the DSS).

  \item Adjust the colour table until the image appears as in
   Figure~\ref{RETRIEV_R_MAIN}.  Click on the {\sf View} menu and select
   the {\sf Colors\ldots} option.  A panel will appear.  Set the colour
   scale algorithm to {\sf Linear}, the colormap to {\sf ramp} and the
   intensity to {\sf neg}.  Then click on the {\sf Close} button.

   Set the magnification by clicking on the {\sf Scale:} button (in the
   bottom left of the control panel in the centre top of the window) and
   setting it to {\sf 2x}.

  \item Overlay the DSS image with objects selected from the {\sf USNO at
   ESO} catalogue, following the second part of the recipe in
   Section~\ref{RETRIEV_RECIP}.

  \item The next step is to choose five stars in the image to act as
   reference stars.  These stars should be:

  \begin{itemize}

    \item easily identified,

    \item reasonably bright,

    \item stars rather than galaxies (because star images have more
     precisely defined centres),

    \item isolated from other stars and galaxies (to avoid images which
     blend together),

    \item spread reasonably uniformly over the image,

    \item not very close to the edge of the image (later you will need to
     to identify the corresponding stars in the JKT image, and the two
     areas of sky are not exactly the same).

  \end{itemize}

   Figure~\ref{ASTROM_R_PAPER} shows five suitable stars in the DSS
   example image.  To select the stars: hold down the {\tt shift} key and
   click on each star in turn (without releasing the {\tt shift} key).  As
   you do so the selected star is highlighted in both the image and the
   catalogue windows.  You can add as many stars as you like, but five is
   adequate.

  \begin{figure}[htbp]
     \centering
     \includegraphics[totalheight=6in]{sc17_astrom_r_paper.ps}
     \begin{quote}
     \caption{DSS image with reference stars marked
     \label{ASTROM_R_PAPER} }
     \end{quote}
  \end{figure}

  \item Copy the selected stars to a new table dialogue box by clicking on
   the {\sf Options} menu in the catalogue dialogue box and choosing the
   {\sf Extract selected} item.  A new catalogue dialogue box listing just
   the selected objects will appear.  Henceforth you will work with this
   catalogue dialogue box.

  \item To double-check which objects you have selected: click on the {\sf
   Graphics} menu in the main window and choose {\sf Clear}.  All the
   catalogue object markers will disappear.

   Re-plot the selected objects by clicking on the {\sf Plot} button towards
   the bottom of the new catalogue dialogue box.  Label the chosen objects
   by clicking on the {\sf Options} menu in the catalogue dialogue box
   menu-bar and choosing {\sf Label all objects}.

  \item It is useful to print out a copy of the image with the reference
   stars marked, for use in the next stage of the calibration.  If you are
   using the colour table described above you will need to adjust it so
   that the star identifications are legible, typically by inverting the
   image to make the stars white against a dark background (as in
   Figure~\ref{ASTROM_R_PAPER}), rather than vice versa.

  \item Print out a paper copy of the image by clicking on the {\sf File}
   option (the rightmost option in the menu-bar along the top of the main
   window) and choose the {\sf Print...} option, followed by {\sf Image...}.
   Use the ensuing dialogue box to save the image as a postscript file,
   which you can then print.

  \item Save the catalogue of selected objects.  Click on the {\sf File}
   menu in the {\sf USNO at ESO (1)} dialogue box (the leftmost item in its
   menu-bar) and choose the {\sf Save as\ldots} option.  You should specify
   a file-type of {\tt .lis} so that the catalogue is saved in the `ASCII
   Header' format (see Section~\ref{CATS}).

\end{enumerate}

You now have a list of suitable reference stars. You can quit GAIA at this
point, but it is better to proceed directly to the next stage of the
recipe.

