
\section{News from the Plateau de Bure Interferometer}

\subsection{Weather conditions and observing}

Following the successful installation and commissioning of the New
Generation Receivers (NGRs) for the 3\,mm and 1.3\,mm atmospheric
windows last autumn, regular observing was resumed on January 18th,
2007 when the array in its D configuration. We switched to the A
array on January 28 and moved to a configuration intermediate between
A and B (keeping the three longest baselines) on March 7. The B
configuration was scheduled from March 18 until March 30 only, because
of very poor weather conditions. The C configuration was then
available until April 24, when the interferometer was brought back
into its D configuration. Many projects requesting the C
configuration, notably at 1mm, could not be finished yet and will be
deferred to the end of the summer/beginning of the upcoming winter
semester. Since May 22nd the array is observing with 5 antennas in D
configuration. The current antenna maintenance period is foreseen to
end in October.

All in all, the observing conditions were quite mediocre due to the
unusually warm winter and humid spring in Europe.

As far as A-rated projects are concerned, we look forward to bring
many of these to completion before the end of the current summer
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
upcoming proposal deadline have to be resubmitted. In any case, {\bf
all} projects will be kept on the schedule until the end of the summer
semester (i.e. until November 30). Investigators, who wish to check the
status of their project, may consult the interferometer schedule on
the Web at ../PDBI/ongoing.html. The page is updated daily.

The global VLBI session, which includes the array in the 3\,mm
phased-array mode, started on time on May 10. The interferometer,
equipped with a new maser and the new receiving system successfully
participated for two nights.

\subsection{Installation of the 2\,mm band}

It is foreseen to equip all 6 antennas with 2\,mm mixers 
still in  the current summer period. The 2\,mm band will be offered to the
community in the following Call for Proposals on a shared-risk basis
only, since delays in the installation cannot be excluded and the overall 
performance of the receivers is not known yet.

\section{Call for Observing Proposals for the Plateau de Bure Interferometer}

\subsection{Conditions for the next winter session} 

Based on our experience in carrying out configuration changes in 
winter conditions with
limited access to the observatory, we plan to schedule four
configuration changes next winter. We therefore ask investigators to
submit proposals for any of the 4 primary configurations of the six
antenna array.

A preliminary configuration schedule for the winter period is outlined
below. Adjustments to the provisional configuration planning will be
made according to proposal pressure, weather conditions,  installation
of the 2\,mm receiver band and other contingencies.   Due to the
installation of the new generation receivers last fall, the C
configuration was not available in December, and this spring not all
of the projects requesting the C configuration could be worked off.
These projects are now postponed for the C configuration to be
scheduled at the end of the summer semester and at the beginning of 
the upcoming winter. We therefore may again  have less
time available in C configuration this winter than in regular previous
years.  The configuration schedule given below should be taken as a
guideline, in particular when the requested astronomical targets
cannot be observed during the entire winter period (sun
avoidance circle of radius 45$^\circ$.).

\vskip 4mm
\begin{center}
\begin{tabular}{cc}
\hline\hline
Conf & {Scheduling Priority Winter 2007/2008} \\
\hline
C & December\\
A & December -- January\\
B & February -- March\\
C & March -- April\\
D & April -- May\\
\hline
\end{tabular}
\end{center}
\mbox{}\\[1ex]

We strongly encourage observers to submit proposals for the new set of
AB configurations that include 730 and 760 meter baselines.  For these
proposals we ask to focus on bright compact sources, possibly at high
declination.

We invite proposers to submit proposals also for observations at
3\,mm. When the atmospheric conditions are not good enough at 1.3\,mm,
3\,mm projects will be observed: in a typical winter, 20-30\% of the
time used for observations is found to be poor at 1.3\,mm, but still
excellent at 3\,mm.

Proposals requesting the 2\,mm band should be submitted on a
shared-risk basis only.

All applications under this call for proposals will have to take into
account that  the new receivers cannot be operated simultaneously at
more than one frequency band. Investigators will therefore have to make it
clear whether a request is made for one (e.g.,\ 3\,mm) or more (e.g.,\
3\,mm and 1\,mm) frequency bands. 

\subsection{Proposal category}

Proposals should be submitted for one of the five following 
categories:

\begin{description}
\item[{\sc 1.3\,mm:}\/] Proposals that ask for 1.3\,mm data.
3\,mm receivers can be used for pointing and calibration purposes, but cannot
provide any science data. 
\item[{\sc 2\,mm:}\/] Proposals that ask for 2\,mm data. These proposals
will be observed on a shared risk basis as ``science verification''
projects to inaugurate the new receiver band should it become available
for the winter semester.
\item[{\sc 3\,mm:}\/] Proposals that ask for 3\,mm data.
\item[{\sc time filler:}\/] Proposals that have to be considered as
backup projects to fill in periods where the atmospheric
conditions do not allow mapping, to fill in gaps in the
scheduling, or even to cover periods when only a subset of the standard
6-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 of the six-antenna array}

The six-element array can be arranged in the following 
configurations:\\[1ex]
\begin{center}
\begin{tabular}{ccccccc}
\hline\hline                
Conf & \multicolumn{6}{c}{Stations} \\
\hline                        
 A &W27 &E68 &N46 &E24 &E04 &N29 \\
 B &W12 &W27 &N46 &E23 &E12 &N20 \\
 C &W12 &E10 &N17 &N11 &E04 &W09 \\
 D &W08 &E03 &N07 &N11 &N02 &W05 \\
\hline
\end{tabular}
\end{center}

\noindent The general properties of these configurations are:

\begin{itemize}

\item[$\circ$] {\tt A\/} alone is well suited for mapping or size
measurements of very compact, strong sources. It provides a resolution of
$0.8''$ at 100\,GHz, $\sim$0.35$''$ at 230\,GHz.
\item[$\circ$] {\tt B\/} alone yields $\sim$1.2$''$ at 100\,GHz and, in
combination with A provides an angular resolution of $\sim$1.0$''$ at
100\,GHz. It is mainly used for relatively strong sources.
\item[$\circ$] {\tt C\/} provides a fairly complete coverage of the
uv-plane (low sidelobe level) and is well adapted to combine with D
for low angular resolution studies ($\sim$3.5$''$ at 100\,GHz,
$\sim$1.5$''$ at 230\,GHz) and with B for higher resolution
($\sim$1.7$''$ at 100\,GHz, $\sim$0.7$''$ at 230\,GHz). C alone is also
well suited for snapshot and size measurement experiments.
\item[$\circ$] {\tt D\/} alone is best suited for deep integration and
coarse mapping experiments (resolution $\sim 5''$ at 100\,GHz). 
This configuration provides both the
highest sensitivity and the lowest atmospheric phase noise. 
\end{itemize}

\noindent The four configurations can be used in different
combinations to achieve complementary sampling of the uv-plane, and to
improve on angular resolution and sensitivity. Mosaicing is usually
done with D or CD, but the combination BCD can also be requested for
high resolution mosaics. Check the {\sc any}\/ bullet in the proposal
form if the scientific goals can be reached with any of the four
configurations or their subsets. 

Please consult the documentation on the Plateau de Bure
configurations and the IRAM Newsletter No.~63 (August 4th., 2005,
accessible on the web at   {\tt ../IRAMFR/ARN/aug05/aug05.html}) for
further details.

\subsection{Receivers}

Since December 2006, all antennas are equipped with a new generation
of  dual polarization receivers for the 3\,mm and 1.3\,mm atmospheric
windows.  The frequency range is 81\,GHz to 116\,GHz for the 3\,mm
band, and 201\,GHz to 256\,GHz for the 1.3\,mm band. For the
upcoming winter semester, the 2\,mm band may additionally 
become available, covering the frequency range 129\,GHz to 168\,GHz.

Each band of the new receivers is dual-polarization  with the two RF
channels observing at the same frequency.   The mixers are
single-sideband, with a typical image rejection of 10\,dB.  Only one
frequency band (dual polarization) can be connected to the IF
transmission lines at any time. Because of this reason and due to
pointing offsets between different frequency bands, only one band can
be observed at any time. The other band is in stand-by (power on and
local oscillator phase-locked) and is available, e.g., for pointing
and focusing.  Time-shared observations between two frequency bands
cannot be offered for the winter (this mode is currently being
tested).

The two IF-channels (one per polarization), each 4\,GHz wide, 
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.

\vskip 4mm \begin{center}
\begin{tabular}{lccc}
\hline
\hline
\multicolumn{4}{c}{PdBI Receiver Specifications} \\
       & Band 1 & {\it Band 2}$^{\,*}$  & Band 3 \\
\hline
 RF coverage       & 81--116 & {\it 129--168}     & 201--256\\
 $\rm T_{rec}$ LSB & 40--55  & {\it 40--60}       & 40--60  \\
 $\rm T_{rec}$ USB & \multicolumn{1}{c}{''}&\multicolumn{1}{c}{''}& 50--70  \\
 $\rm G_{im}$      &  -10\,dB     &      &    -12 -8\,dB \\
 RF range LSB      & 81--104      &      & 201--244 \\
 RF range USB      & 104--116     &      & 244--256 \\
\hline
\end{tabular}
\end{center}
\vskip -2mm
$^*$: preliminary values

\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, 35 Jy/K at 1.3\,mm, {\it 29 Jy/K at 2\,mm})
\item $T_{\rm sys}$ is the system temperature ($T_{\rm sys}$ = 100\,K
below 110\,GHz,  170\,K at 115\,GHz, {\it 130\,K at 150\,GHz}, 200\,K at 
230\,GHz for sources at $\delta \ge 20^\circ$ and for typical winter 
conditions.)
\item $\eta$ is an efficiency factor due to atmospheric phase noise
(0.9 at 3\,mm, {\it 0.85 at 2\,mm}, 0.8 at 1.3\,mm).
\item $N_{\rm a}$ is the number of antennas (6), 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
1.4 $T_{\rm ON}$.
\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}
The interferometer operates in the J2000.0 coordinate system. For best
positioning accuracy, source coordinates must be in the J2000.0
system; position errors up to $0.3''$ may occur otherwise.

Please do not forget to specify {\tt 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 {\tt 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 is used, but with the two
polarizations available. Each polarization delivers a 4\,GHz bandwidth 
(from IF=4 to 8\,GHz). 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}. 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}

\noindent where HOR and VER refers to the two polarizations:

\vskip 4mm 
\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 & H  & H  & V  & V  \\
input 2 & V  & V  & H  & H  \\
\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 gives access
to 1\,GHz x 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 contiguous?} 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. Note that in any case, the two correlator
inputs are analyzed independently. \\

{\em Where is the selected sky frequency in the IF band?} It would be
natural to tune the receivers so 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, 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 x 512 &  20 & SSB \\
 0.078 & 1 x 512 &  40 & SSB \\
 0.156 & 2 x 256 &  80 & DSB \\
 0.312 & 1 x 256 &  80 & SSB \\
 0.625 & 2 x 128 & 160 & DSB \\
 1.250 & 1 x 128 & 160 & SSB \\
 2.500 & 2 x  64 & 320 & DSB \\
\hline
\end{tabular}
\end{center}
\vskip 3mm
\noindent Note that 5\% of the passband is lost at both ends 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 new
receiver/correlator setup possibilities. Astronomers are urged to
download the most recent version ({\bf February 2007 or later}) of GILDAS 
at {\tt ../IRAMFR/GILDAS/ } to prepare their proposals.\\

The old {\tt LINE} command has been replaced by several new
commands (see internal help):
\begin{itemize}
\item {\tt NGR\_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
   ngr_line xyz 230 lsb           

   ! choice of the correlator windows
   narrow Q1 Q3             

   ! 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 set at 45 degrees sun
distance.  Please take this into account for your target sources {\sc
and} for the calibrators. We are currently working toward a reduction
of the sun avoidance limit.

\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 data have to
be reduced  in Grenoble}. Proposers will not come for the
observations, but will have to come for the reduction.  For the time
being, remote data reduction will not be offered for projects
observed with the NGR system.
%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 the 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 NGR 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 upgrade of CLIC to handle the NGR data 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, 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 and on the help requested by the PI, 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}\\
{\bf Note however, that the documentation on 
the web has not yet been updated with respect to the new generation 
receivers. All information currently available on the new generation receiver 
system is given in this call for proposals.}\\ [5mm]

\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}


