\documentclass{article}[12pt]

\usepackage[gavo]{votutorial}


\title{Astrometric calibration using Aladin}
\author{F.~Rothmaier, M.~Demleitner, S.~Gabriel}
\titlepicture{../res/aladin-newcalib-match-new.png}

\def\icondir{./}

\begin{document}
\maketitle

% explain why it can be attractive to use Aladin for a "quick and dirty"
%  astrometric calibration instead of tools like astrometry.net
\begin{abstract}
The Aladin Sky Atlas is a very powerful Virtual Observatory
application enabling its users to visualise digitised astronomical
images, to superimpose entries from astronomical catalogues or
databases, and to interactively access related data and information from
numerous astronomical databases and archives.

Most of this functionality depends on the image having 
an astrometrical calibration. If you do not know the sky
region of an uncalibrated image, an automatic calibration tool like
\AstronomyNet{} might be a good choice. If you know, however, the
approximate
location of an image on the celestial sphere, Aladin provides a fast
and reliable method to calibrate it by using a reference image.

This tutorial explains step-by-step how to perform such an ad-hoc
astrometric calibration using Aladin.
\end{abstract}

\usedsoftware{\Aladin{}} 

\subsection*{Learn how to calibrate an image with Aladin}
\tutstep[Load an uncalibrated image into Aladin]

  \begin{itemize}
    \item Launch Aladin.
    \item Paste the URL
    $$\hbox{\url{http://docs.g-vo.org/astropics/M57RingNebula.jpg}}$$
into Aladin's \menu{Command} field on the top of the window. This
opens an uncalibrated
image of the Ring Nebula (Messier 57). (Alternatively, you can open a
web browser with \url{http://docs.g-vo.org/astropics/}, download
the file \textit{M57RingNebula.jpg} and access it via Aladin's menu:
\key{File} $\rightarrow$ \key{Load local file...}).
  \end{itemize}

\tutstep[Get a calibration source] Since our image is in a spectral
range rather close to PanSTARRS', we will use that as a reference.  You
can simply click on the PanSTARRS bookmark just below the command window
-- or use the discovery tree on the right side and look for PanSTARRS
there.  For images in other spectral ranges, you would probably want to
use a different survey.

\tutstep[Go to the rough area of your image] In this case, we know our
image shows the ring nebula M57, so type \texttt{M57} into the
\key{Command} field and hit enter; the view ill now center on M57.  For
images for which you can guess the position, type in the RA and Dec
instead.  Then zoom in until you properly see the target object (in this
case, that's about 20 arcminutes).  You can use the zoom slider on the
right side or a mouse wheel for zooming.

\tutstep[Calibrate your uncalibrated image]

  \begin{itemize}
    \item Split the main window into two sub-windows by using the middle
button of the ``multiview'' capability \icon{../res/aladin-multiview-new}.
Make Aladin display your uncalibrated image in the left and the
calibrated image in the right sub-window (to do that, drag and drop the
entries in the layer stack on the right into the panes).
    \item Click in your left panel (the uncalibrated one), then choose \key{Image} $\rightarrow$
\key{Astrometrical calibration...} in the menu. A new window opens where you 
should select the option \key{by matching stars}.
    \item If you are sure about corresponding stars in the two images,
click on a star in the uncalibrated image followed by a click on the
calibrated source. Repeat the game for at least five objects. For each
mouse click, Aladin fills the table in the calibration dialog.  Use your
common sense and the human ability to form asterisms to guide your
choice: Stars close to galaxies should be easy to pair up, or
conspicuous triangles of stars of comparable brightness.
    \item When you feel good about your objects selected, press the
\key{CREATE} button to finalise the calibration.
  \end{itemize}



\tutstep Verify that your image is calibrated
  \begin{itemize}
     \item If you hover your mouse over the image on the left, you see
that coordinates are indicated in the ``Command'' field above the main
window (before there was a ``No astrometrical reduction'' here).
     \item Click on the ``match'' icon \icon{../res/aladin-match-new} (to the right
of the ``multiview'' capability) to view the two images in the same
orientation. You should now see something along the lines of the title picture
     \item If you want to save your freshly calibrated image on disk, go to the menu 
     \key{file} and choose \key{Export planes (FITS,VOTable,...)...}.\vspace{1cm}
  \end{itemize}

Warning: if you have already overplotted a catalog over your calibrated
image, Aladin will use the closest catalog object rather than the
position you actually clicked.  It's not unlikely that's actually what
you want, but sometimes it's not\dots


\end{document}
