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\title{\textbf{Practice Work Session P3 (1.5h)}\\Preparation of observation}
\author{G.\ Duvert,\\
        Laboratoire d'Astrophysique de Grenoble (LAOG)}
\date{\textbf{Euro Summer School}\\
  \emph{Observation and Data reduction with the Very Large Telescope
    Interferometer} \\
4-16 June 2006, Goutelas (France)}
\maketitle

\begin{abstract}
  This Practice work applies the knowledge of the two previous
  practice sessions to the preparation of an observation. Here we will
  use ASPRO to simulate the data that could be taken on the the
  object during real observations, and estimate how accurately we
  could fit a model in the observations.
\end{abstract}

\section{The Source Model}
 We plan to observe Achernar, for which spectroscopic observations
 predict a large rotational velocity and hence an oblate ellipsoid
 shape. Interferometric observations of Achernar at VLTI 
 with the VINCI instrument (2-telescopes) and small (35 cm) apertures
 have already been presented by Kervella in session S1. 

 We will use the full aspro interface. 

 We model the star with the first model described by Kervella in 
 his presentation (see local website ``http://192.168.8.150'')
\section{Source Position}
Use CDS to retrieve the coordinates of Achernar\footnote{In case of
  network problems, Achernar is EQ 2000.0 1:37:42.8466 -57:14:12.327,
  MV 0.5 MJ 0.790 MH 0.860 MK 0.880}.  	  	
\section{Observatory}
We want to observe Achernar with the VLTI, 3 Telescopes, around 1 to 2.2 microns
(AMBER). (Why?)
\section{Observation Period}
Starding with today 7 June, find the best period of year to observe
Achernar with the VLTI.
\section{Best Baseline Set}
Now, we want to find the best baseline set (3 of the 4 Unit Telescopes
of the VLT in our case) to explore the hypothetical peculiar structure
of Achernar. 
Find the best coverage (or coverage/wavelength) that samples the
regions of the fourier transform of the source's brightness
distribution where the signature of the elliptical shape of the object
is strongest.
\section{Resulting visibilities}
Now, let's restrict ourselves to 4 hours of observation (-2 to +2 hour
angle); shorten the integration time from 90 min to 60 min since
nowadays this is more representative of the speed of the
VLTI+AMBER. Select the ``fixed delay'' of the 3rd delay line to avoid
unobservable positions due limited delay line strokes\footnote{This
  is automatically optimizes in the ``simpler'' version of
ASPRO dedicated to VLTI, but must be taken into account in the general
all-purpose version used here}.

Each time we create a new plot with ``PLOT UV COVERAGE'', the corresponding
list of uv points, defining the spatial coverage of the full
observation (super synthesis) is written in a file (named
``oipt\_psf.uvt'' by default). 
This file will be further used as a ``template table''
to be filled with visibilities obtained from a model of the object.

Use the same model as before (values are alredy filled in) to populate a uv
table (by default named ``model'', lets rename it as ``model1''). This corresponds to the list of
visibilities and other interferometric observables that would be
produced by the real (however ``perfect'') interferometer.

We can explore the various columns of this table with the
``UV~EXPLORE'' Panel. Look particulary to the variation of $V^2$ as a
function of baseline length and baseline angle.

\section{Model refinement}
Use figure in slide 30 of Kervella's presentation to refine the model,
and recompute the visibilities in a new file, ``model2''. ``Explore'' the
UV table as before. Do we see differences?

\section{Expected accuracy}
At that point, we could add the expected ``noise'' introduced by the
focal instrument (AMBER) depending on the observational mode selected
(AMBER has 3 different spectral resolutions), and evaluate what
precision we can expect on the model's parameters, with regards to the
precision on the measurements obtained in the given observing time.
However this is best done with the simpler interface of ASPRO
dedicated to VLTI observations and will be done in P5.

\section{The ESO observation preparation software}
Have a look at the ESO observation preparation software VISCALC, available
under the ``exposure time calculators section'' of the ``Proposal
Preparation and Submission'' section at ESO website: {\em
  http://www.eso.org/observing/etc/}. The ESO product is targeted more
closely on the VLTI instruments, and knows about delay line (DL) regions of
shadowing (when for example the carry of one DL intersects the beam
of another) , and about pointing limitations of each telescope due to
shadowing by another, or by the UT domes.

Perform the same selections as with ASPRO with Achernar and compare results. 

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