\subsection{Creating an accurate astrometric calibration}

In some cases the approximate astrometric calibration derived in the
previous stage will be adequate.  However, it is possible to use
additional astrometric reference stars to refine it.  Assuming that
you already have an approximate astrometric calibration proceed as follows.

\begin{enumerate}

  \item Query the {\sf USNO at ESO} catalogue to find the objects that
   overlay the JKT image, following the second part of the recipe in
   Section~\ref{RETRIEV_RECIP}.  Alternatively, you can load the example
   local catalogue {\tt ngc1275usno.tab}.

  \item Click on the {\sf Image-Analysis} button on the menu-bar along the
   top of the main window.  Choose the {\sf Astrometry calibration} item
   and then {\sf Fit to star positions} from the next menu.  The {\sf Fit
   astrometry reference positions} dialogue box (Figure~\ref{ASTROM_R_RADEC})
   should appear.

  \item Import the catalogue of reference stars by clicking on the {\sf
   Grab} button and choosing the appropriate catalogue.

  \item Click on the {\sf Clip} button to remove reference stars which fall
   outside the image.

  \item Delete all the catalogue objects which correspond to galaxies,
   nebulae, blended double-star images \emph{etc}, none of which make
   good astrometric reference objects.  The procedure to delete an object
   is:

  \begin{itemize}

    \item click on the symbol for the object in the main window, so that
     the object is highlighted in the catalogue window,

    \item click on the {\sf Delete} button and then confirm that the object
     is to be deleted.

  \end{itemize}

  \item When you are happy that you have removed all unsuitable objects,
   clear the catalogue markers: click on the {\sf Graphics} menu in the
   main window's menu-bar and choose {\sf Clear}.

  \item Click on the {\sf Centroid} button in the {\sf Fit astrometry
   reference positions} dialogue box to refine the pixel positions for all
   the reference stars.  New markers should be drawn over the JKT image
   showing the revised positions of the reference stars.

  \item Click on the {\sf Fit/Test} button and the revised calibration
   using all the reference stars will be created.

   If any unsuitable objects remain amongst the reference stars, or the
   fit is in some other way unsatisfactory, then repeat the steps above
   to remove the offending objects and repeat the fit.

   When the fit is acceptable click the {\sf Accept} button.

  \item Finally, save a version of the image with the revised calibration.
   Click on the {\sf File} menu button in the main window, choose the {\sf
   Save as} item and use the resulting dialogue box to save the image as a
   new file, perhaps called {\tt ngc1275jktast.sdf}.

\end{enumerate}

