
\section{News from the Plateau de Bure Interferometer}

\subsection{System upgrades}

During the last few month, all Bure antennas were equipped with dual
polarization receivers operating in the 2\,mm band from 129\,GHz to
174\,GHz (see also the contribution by Roberto Neri in the IRAM
Newsletter No.~70). At the same time, the 1.3\,mm receivers were
upgraded with new triplers that now allow to observe up to a frequency
of 267\,GHz.

These upgrades came along with a complete exchange of the computer
system on Plateau de Bure (real-time system, antenna control
computers, ...)  where the former HPUX and OS9 systems were replaced
by  64-bit CPUs operated under LINUX. This hardware upgrade also
required major modifications of the corresponding antenna control and
data acquisition software in the real-time system.  After some initial
debugging, regular observing was resumed in December.

The new 2\,mm receivers and the extended tuning range of the 1\,mm band
were successfully tested and commissioned during the Christmas period,
the new frequency windows are offered to the community in the
following Call for proposals.

\subsection{Weather conditions and observing}

The current winter semester has been suffering from extremely poor
weather conditions so far and only quite a limited number of 1\,mm
projects could be finished by now. We moved the array into its
C-configuration on November 4  and switched to the A configuration
(that provides baselines up to 760m) on January 28.  It is planned to
move to the B configuration around end of February and again  to the C
configuration by end of March. The switch back to the most compact
configuration D is foreseen before the end of April. According to
these plans, it will not be possible to complete projects requesting
deep integrations using the compact configurations before the end of
the current observing period.

As far as A-rated projects are concerned, we still hope to bring many
of these to completion before the end of the current winter
semester. B-rated projects are likely to be observed only if they fall
in a favorable LST range. We remind users of the Plateau de Bure
interferometer that B-rated proposals which are not started before the
end of the winter period have to be resubmitted.

Global VLBI observations, which include the array in the 3\,mm
phased-array mode, are planned from May 8 to 13, 2008.

Investigators, who wish to check the status of their project may
consult the interferometer schedule on the Web at
../PDBI/ongoing.html. This page is updated daily.

\signed {Jan Martin WINTERS}

\section{Call for Observing Proposals on the Plateau de Bure Interferometer}
\subsection{Conditions for the next summer period} 
\noindent

As every year, we plan to carry out extensive technical work during
the summer semester, including the regular maintenance of the
antennas.   Further work will also be done on the reduction of the sun
avoidance circle.  During this period, regular scientific observations
will therefore mostly be carried out with the five element array. We
plan to start the maintenance at the latest by the end of May and to
schedule the D configuration between June and October.

We strongly encourage observers to submit proposals that can be
executed during summer operating conditions. To keep the procedure as
simple as possible, we ask to focus on:

\begin{itemize}
\item[$\circ$] observations requesting the use of the 2\,mm and 3\,mm receivers
\item[$\circ$] circumpolar sources or sources transiting at night between 
June and September,
\item[$\circ$] observations that qualify for the 5D, 6D, and 6C configurations
\end{itemize}

\subsection{Proposal category}
Proposals should be submitted for one of the four categories:
\begin{description}
\item[{\sc 1.3mm:}\/] Proposals that ask for 1.3\,mm data.
3\,mm receivers can be used for pointing and calibration purposes, but cannot
provide any imaging. Proposals requesting the extended tuning range 
(256-267\,GHz) will be carried out on a 
``best effort'' basis only during the summer semester.
\item[{\sc 2mm:}\/] Proposals that ask for 2\,mm data. 3\,mm receivers can be 
used for pointing and calibration purposes, but cannot
provide any imaging. 
\item[{\sc 3mm:}\/] Proposals that ask for 3\,mm data.
\item[{\sc time filler:}\/] Proposals that have to be considered as
background projects to fill in periods where the atmospheric
conditions do not allow mapping, to fill in gaps in the
scheduling, or even to fill in periods when only a subset of the standard
5-antenna configurations will be available. These proposals will be
carried out on a ``best effort'' basis only.
\item[{\sc special:}\/] Exploratory proposals: proposals whose
scientific interest justifies the attempt to use the PdB array beyond
its guaranteed capabilities. This category includes for example
non-standard frequencies for which the tuning cannot be guaranteed,
non-standard configurations and more generally all non-standard
observations. These proposals will be carried out on a ``best effort''
basis only.
\end{description}
The proposal category will have to be specified on the proposal cover
sheet and should be carefully considered by proposers.

\subsection{Configurations}
Configurations planned for the summer period are:
\vskip 4mm 
\begin{center}
\begin{tabular}{|l|c|}
\hline
Name & Stations  \\
\hline
5Dq & W08 E03 N07 N11 W05  \\
6Dq & W08 E03 N07 N11 N02 W05  \\
6Cq & W12 E10 N17 N11 E04 W09  \\
\hline
\end{tabular}
\end{center}
\vskip 4mm 
Part of the projects will be scheduled at the end of the summer period
when the six-element array is expected to be back to
operation. Projects that should be observed with a subset of the
five-element array, will be adjusted in uv-coverage and observing
time.\\

The following configuration sets are available:
\vskip 4mm 
\begin{center}
\begin{tabular}{|c|p{4.3cm}|}
\hline
Set & Main purpose \\
\hline
D  & Detection + ``low'' resolution mapping at 1.3mm \\
CD &  3.5$''$ resolution mapping at 3\,mm \\
C  & Detection at low-DEC, 2.8$''$ resolution mapping at 3\,mm \\
\hline
\end{tabular}
\end{center}
\vskip 4mm 
Finally, enter {\sc any}\/ in the proposal form if your project
doesn't need any particular configuration. 

\subsection{Receivers}

All antennas are equipped with dual polarization
receivers for the 3\,mm, 2\,mm, and 1.3\,mm atmospheric windows.  The
frequency range is 80\,GHz to 116\,GHz for the 3\,mm band, 129\,GHz to
174\,GHz for the 2\,mm band, and 201 to 267\,GHz for the 1.3\,mm band.

\vskip 4mm 
\begin{center}
\begin{tabular}{lccc}
\hline
                   & Band 1  & Band 2       & Band 3   \\
\hline
 RF range$^*$      & 80--116  & 129--174 & 201--267 \\
 $\rm T_{rec}$ LSB &  40--55  & 30--50   & 40--60   \\
 $\rm T_{rec}$ USB &  40--55  & 40--80   & 50--70   \\
 $\rm G_{im}$/[dB] &  -10     & -12 ... -10 &-12 ... -8 \\
 RF LSB            &  80--104 & 129--168 & 201--267 \\
 RF USB            & 104--116 & 147--174 &          \\
\hline
\end{tabular}
\end{center}
\mbox {} \hskip 2mm $^*$ center of the 4-8\,GHz IF band
\vskip 4mm 

Each band of the new receivers is dual-polarization 
with the two RF channels of one band observing at the same
frequency (common LO).  The different bands are not co-aligned in the
focal plane (and therefore on the sky). The mixers are
single-sideband, backshort-tuned; they can be tuned USB or LSB, both
choices being available in the central part of the RF band. The
typical image rejection is 10\,dB. Each IF channel is 4~GHz wide
(4-8~GHz). Only one frequency band can be connected to the IF
transmission lines at any time. Because of this reason and due to the
pointing offsets between different frequency bands, only one band can
be observed at any time. One of the two other bands is in stand-by
mode  (power on and local oscillator phase-locked) and is available,
e.g., for pointing. Time-shared observations between different
frequency bands can not be offered yet.

The two 4~GHz wide IF-channels (one per polarization) are transmitted
by optical fibers to the central building.  At present, the 4\,GHz
bandwidth can be processed only partially by the existing correlator,
through a dedicated IF processor that converts selected 1~GHz wide
slices of the 4-8~GHz first IFs down to 0.1-1.1~GHz, the input range
of the existing correlator.  Further details are given in the section
describing the correlator setup and the IF processor.

\subsection{Signal to Noise}

The rms noise can be computed from
\begin{equation}
 \sigma = \frac{J_{\rm pK} T_{\rm sys}} {\eta \sqrt{N_{\rm a} (N_{\rm a}-1) 
N_{\rm c} T_{\rm ON} B}} \frac{1}{\sqrt{N_{\rm pol}}}
\end{equation}
where
\begin{itemize}
\item $J_{\rm pK}$\ is the conversion factor from Kelvin to Jansky (22
Jy/K at 3\,mm, 29 Jy/K at 2\,mm, and 35 Jy/K at 1.3\,mm)
\item $T_{\rm sys}$ is the system temperature ($T_{\rm sys}$ = 100\,K
below 110\,GHz,  180\,K at 115\,GHz, 180\,K at 150\,GHz, and 250\,K at 
230\,GHz for sources at
$\delta \ge 20^\circ$ and for typical summer conditions).
\item $\eta$ is an efficiency factor due to atmospheric phase noise   
and instrumental phase jitter
(0.9 at 3\,mm, 0.8 at 2\,mm, and 0.6 at 1.3\,mm) in typical summer conditions.
\item $N_{\rm a}$ is the number of antennas (5), and $N_{\rm c}$ 
is the number of
configurations: 1 for D, 2 for CD, and so on.
\item $T_{\rm ON}$ is the on-source integration time per configuration
in seconds (2 to 8 hours, depending on source declination). Because of
various calibration observations the total observing time is typically
1.5 $T_{\rm ON}$.
\item $B$ is the spectral bandwidth in Hz (up to 2\,GHz for continuum, 
40\,kHz to 2.5\,MHz for spectral line, according to the spectral correlator
setup)
\item $N_{\rm pol}$ is the number of polarizations: 1 for single polarization 
and 2 for dual polarization (see section {\it Correlator} for details).
\end{itemize}

Investigators have to specify the one sigma noise level which is
necessary to achieve each individual goal of a proposal, and
particularly for projects aiming at deep integrations.

\subsection{Coordinates and Velocities}

For best position accuracy, source coordinates must be in the J2000.0
system.

Please do not forget to specify LSR velocities for the sources. For
pure continuum projects, the ``special'' velocity {\tt NULL} (no
Doppler tracking) can be used.

Coordinates and velocities in the proposal MUST BE CORRECT: A
coordinate error is a potential cause for proposal rejection.

\subsection{Correlator}

\subsubsection{IF processor} 

At any given time, only one frequency band can be observed, but with the two
polarizations available. Each polarization delivers a 4 GHz bandwidth 
(from IF=4 to 8~GHz). The two 4-GHz bandwidths coincide in the sky
frequency scale. The current correlator accepts
as input two signals of 1~GHz bandwidth, that must be selected within
the 4~GHz delivered by the receiver. In practice, the new IF
processor splits the two input 4--8~GHz bands in four 1~GHz
``quarters'', labeled {\em Q1...Q4}. Two of these quarters must be
selected as correlator inputs. The system allows the following
choices:
\begin{itemize}
\item first  correlator entry can only be Q1 HOR, or Q2 HOR, 
or Q3 VER, or Q4 VER
\item second correlator entry can only be Q1 VER, or Q2 VER, 
or Q3 HOR, or Q4 HOR
\end{itemize}
where HOR and VER refer to the two polarizations:
\begin{flushleft}
\begin{center}
\begin{tabular}{l|cccc}
\hline\noalign{\smallskip}
Quarter  & Q1 & Q2 & Q3 & Q4 \\
IF1 [GHz]& $4.2 - 5.2$ & $5 - 6$ & $6-7$ & $6.8 - 7.8$\\
\noalign{\smallskip}\hline\noalign{\smallskip}
input 1 & HOR  & HOR  & VER  & VER  \\
input 2 & VER  & VER  & HOR  & HOR  \\
\noalign{\smallskip}\hline
\end{tabular}
\end{center}
\end{flushleft}
\bigskip

{\em How to observe two polarizations?} To observe simultaneously two
polarizations at the same sky frequency, one must select the same quarter 
(Q1 or Q2 or Q3 or Q4)
for the two correlator entries. This will necessarily result in each
entry seeing a different polarization. The system thus give access
to 1~GHz $\times$ 2 polarizations. \\

{\em How to use the full 2~GHz bandwidth?} If two different quarters
are selected (any combination is possible), a bandwidth of 2~GHz can
be analyzed by the correlator. But only one polarization per quarter
is available in that case; this may or may not be the same
polarization for the two chunks of 1~GHz. \\

{\em Is there any overlap between the four quarters?} In fact, the
four available quarters are 1~GHz wide each, but with a small overlap
between some of them: Q1 is 4.2 to 5.2~GHz, Q2 is 5 to 6~GHz, Q3 is
6 to 7~GHz, and Q4 is 6.8 to 7.8~GHz. This results from the combination
of filters and LOs used in the IF processor. \\

{\em Is the 2~GHz bandwidth necessarily continuous?} No: any combination
of two quarters can be selected. Adjacent quarters will result in a
continuous 2~GHz band. Non-adjacent quarters will result in two
independent 1~GHz bands. \\

{\em Where is the selected sky frequency in the IF band?} It would be
natural to tune the receivers such that the selected sky frequency
corresponds to the middle of the IF bandwidth, i.e.\ 6.0~GHz. However,
this corresponds to the limit between Q2 and Q3. It is therefore
highly recommended to center a line at the center of a quarter (see
Section ``ASTRO'' below). At 3\,mm and 2\,mm, the receivers offer best
performance in terms of receiver noise and sideband rejection in Q3
(i.e. the line should be centered at an IF1 frequency of 6500 MHz)
whereas at 1\,mm best performance is obtained in Q2 (i.e. the line
should be centered at 5500\,MHz).\\

\subsubsection{Spectral units of the correlator} 

The correlator has 8 independent units, which can be placed anywhere
in the 100--1100~MHz band (1~GHz bandwidth). 7 different modes of
configuration are available, characterized in the following by couples
of total bandwidth/number of channels. In the 3 DSB modes (320MHz/128,
160MHz/256, 80MHz/512 -- see Table) the two central channels may be
perturbed by the Gibbs phenomenon if the observed source has a strong
continuum. When using these modes, it is recommended to avoid
centering the most important part of the lines in the middle of the
band of the correlator unit. In the remaining SSB modes (160MHz/128,
80MHz/256, 40MHz/512, 20MHz/512) the two central channels are not
affected by the Gibbs phenomenon and, therefore, these modes may be
preferable for some spectroscopic studies.

\vskip 4mm \begin{center}
\begin{tabular}{crrc}
\hline
\hline
 Spacing & Channels & Bandwidth  & Mode \\
 \multicolumn{1}{c}{(MHz)} & & \multicolumn{1}{c}{(MHz)} & \\
\hline
 0.039 &$ 1 \times 512$ &  20 & SSB \\
 0.078 &$ 1 \times 512$ &  40 & SSB \\
 0.156 &$ 2 \times 256$ &  80 & DSB \\
 0.312 &$ 1 \times 256$ &  80 & SSB \\
 0.625 &$ 2 \times 128$ & 160 & DSB \\
 1.250 &$ 1 \times 128$ & 160 & SSB \\
 2.500 &$ 2 \times  64$ & 320 & DSB \\
\hline
\end{tabular}
\end{center}
\vskip 3mm
\noindent Note that 5\% of the passband is lost at the end of each
subband.  The 8 units can be independently connected to the first or
the second correlator entry, as selected by the IF processor (see
above). Please note that the center frequency is expressed -- as in the
old system -- in the frequency range seen by the correlator, i.e.\ 100
to 1100~MHz. The correspondence to the sky frequency depends on
the parts of the 4~GHz bandwidth which have been selected as correlator 
inputs. \\

\subsubsection{ASTRO}

The software {\tt ASTRO} has been updated to reflect these
receiver/correlator setup possibilities. Astronomers are urged to
download the most recent version of GILDAS  at {\tt ../IRAMFR/GILDAS/} 
to prepare their proposals.\\

The previous {\tt LINE} command has been replaced by several new commands
(see internal help; the following description applies to the current
receiver system). The behavior of the {\tt LINE} command can be
changed by the {\tt SET PDBI 1995|2000|2006} command, that selects the
PdBI frontend/backend status corresponding to years 1995 (old
receivers, 500 MHz bandwidth), 2000 (580 MHz bandwidth), 2006 (new
receivers and new IF processor, 1\,GHz bandwidth). Default is 2006:
\begin{itemize}
\item {\tt LINE}: receiver tuning
\item {\tt NARROW}: selection of the narrow-band correlator inputs
\item {\tt SPECTRAL}: spectral correlator unit tuning
\item {\tt PLOT}: control of the plot parameters. 
\end{itemize}
A typical session would be:

\begin{verbatim}
   ! choice of receiver tuning
   line xyz 230 lsb low 5500

   ! choice of the correlator windows
   narrow Q2 Q2             

   ! correlator unit #1, on entry 1
   spectral 1  20 520 /narrow 1   

   ! correlator unit #2, on entry 1
   spectral 2 320 260 /narrow 1   

   ! correlator unit #3, on entry 2
   spectral 3  40 666 /narrow 2   
   ...
\end{verbatim}

\vskip 4mm
\subsection{Sun Avoidance}

For safety reasons, a sun avoidance limit is enforced at 45 degrees
from the sun. Please take this into account for your target sources.

\subsection{Mosaics}
The PdBI has mosaicing capabilities, but the pointing accuracy may be
a limiting factor at the highest frequencies. Please contact the
Science Operations Group (sog@iram.fr) in case of doubts.

\subsection{Data reduction}
Proposers should be aware of constraints for data reduction:
\begin{itemize}
\item[$\circ$] In view of the new receiver system, {\bf you are urged
to reduce your data in Grenoble}. Proposers will not come for the
observations, but will have to come for the data reduction.  For the time
being, remote data reduction will only be offered in exceptional cases. 
Please contact your local
contact if you're interested in this possibility.
%Remote data reduction is possible, especially for experienced
%users of the Plateau de Bure Interferometer. Please contact your local
%contact if you're interested in this possibility.
\item[$\circ$] We keep the data reduction schedule very flexible, but wish to
avoid the presence of more than 2 groups at the same time in
Grenoble. Data reduction will be carried out on dedicated computers at
IRAM. Please contact us in advance.
\item[$\circ$] In certain cases, proposers may have a look at the
uv-tables as the observations progress. If necessary, and upon
request, more information can be provided. Please contact your local contact 
or PdBI's Science
Operations Group (sog@iram.fr) if you are interested in this.
\item[$\circ$] CLIC evolves to cope with upgrades of the PdBI
array. The newer versions are downward compatible with the previous
releases. Observers who wish to finish data reduction at their
home institute should obtain the most recent version of CLIC. Because
differences between CLIC versions may potentially result in imaging
errors if new data are reduced with an old package, we advise
observers having a copy of CLIC to take special care in maintaining it
up-to-date.  The recent upgrades of CLIC implied many
modifications for which backward compatibility with old PdBI receiver data has
not yet been fully checked. To calibrate data obtained with the ``old''
receiver system (up to September 2006), 
we thus urge you to use the January 2007 version of CLIC.
\end{itemize}

\subsection{Local Contact}
A local contact will be assigned to every A or B rated proposal which
does not involve an in-house collaborator. He/she will assist you in
the preparation of the observing procedures and provide help to reduce
the data. Assistance is also provided before a deadline to help
newcomers in the preparation of a proposal. Depending upon the
program complexity, IRAM may require an in-house collaborator
instead of the normal local contact.

\subsection{Technical pre-screening}
All proposals will be reviewed for technical feasibility in parallel
to being sent to the members of the program committee. Please help
in this task by submitting technically precise proposals. Note that
your proposal must be complete and exact: the source position and velocity,
as well as the requested frequency setup must be correctly given.

\subsection{Non-standard observations}
If you plan to execute a non-standard program, please contact the
Interferometer Science Operations Group (sog@iram.fr) to discuss the
feasibility.

\subsection{Documentation}

The documentation for the IRAM Plateau de Bure Interferometer includes
documents of general interest to potential users, and more specialized
documents intended for observers on the site (IRAM on-duty
astronomers, operators, or observers with non-standard programs). All
documents can be retrieved on the Internet at {\tt
../IRAMFR/PDB/docu.html}.
Note however, that not all the
documentation on  the web has already been updated with respect to the
current receivers. All information presently available on the
current receiver system is given in the {\it Introduction to
the IRAM Plateau de Bure Interferometer} at {\tt
../IRAMFR/GILDAS/doc/html/pdbi-intro-html} and in this call for
proposals.\\

\renewcommand{\finally}{Finally, we would like to stress again the
importance of the quality of the observing proposal.  The IRAM
interferometer is a powerful, but complex instrument, and proposal
preparation requires special care. Information is available in this 
call and at {\tt ../IRAMFR/PDB/docu.html}. The IRAM staff can help
in case of doubts if contacted well before the deadline. Note that the
proposal should not only justify the scientific interest, but also the
need for the Plateau de Bure Interferometer.}

\begin{latexonly}
\fullbox{3mm}{\finally}
\end{latexonly}
\begin{htmlonly}
{\bf \finally}
\end{htmlonly}

\signed {Jan Martin WINTERS}


