Call for Observing Proposals for the Plateau de Bure Interferometer

Conditions for the next winter session

Based on our experience in carrying out configuration changes 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 of the 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, the installation of the NGRx and other contingencies. E.g., depending on proposal pressure, the time in the A configuration may be extended. Due to the installation of the new receivers, the foreseen December session in the C configuration may be postponed and merged with the March-April session. We therefore may have less time available in C configuration this winter than in previous years. The configuration schedule should be taken as a guideline, in particular when the requested astronomical targets cannot be observed during the entire winter period (45$^\circ$ sun avoidance circle).

Conf	Scheduling Priority Winter 2006/2007
C	December
A	January - February
B	February - March
C	March - April
D	April - May

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 3mm. When the atmospheric conditions are not good enough at 1.3mm, 3mm projects will be observed: in a typical winter, 20-30% of the observing time is found to be poor at 1.3mm, but still excellent at 3mm. All applications under this call for proposals will have to take into account that the NGRx cannot be operated simultaneously at more than one frequency. Investigators will therefore have to make it clear whether a request is made for one (e.g., 3mm) or two (i.e. 3mm and 1mm) frequency bands. Proposals that need observations in the two bands will have to be duly justified.

Call for Proposals

Proposal category

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

3mm:

Proposals that ask for 3mm data only.

1.3mm:

Proposals that ask for 1.3mm data only.

dual freq.:

Proposals may ask for observations at 3mm and 1.3mm. Note, that these will NOT be simultaneous since the new generation receivers cannot be operated simultaneously in the two frequency bands. Proposals should justify the need of both bands and make it clear which band is priority.

time filler:

Proposals that have to be considered as backup projects to fill in periods where the atmospheric conditions do not allow mapping, or potentially, to fill in gaps in the scheduling, or periods when only a subset of the standard configurations will be available.

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 and more generally all non-standard observations. These proposals will be carried out on a ``best effort'' basis only.

SD:

Science demonstration proposals that use the improved figures of the new receivers. Up to 30% of the available observing time might be given to this category. Please note that the science demonstration proposals will not be observed should the expected sensitivity not be reached at the beginning of the observing session.

The proposal category will have to be specified on the proposal cover sheet and should be carefully considered by proposers.

Configurations of the six-antenna array

The six-element array can be arranged in the following configurations:

Conf	Stations
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

The general properties of these configurations are:

$\circ$

A alone is well suited for mapping or size measurements of very compact objects. It provides a resolution of $0.8''$ at 100GHz, $\sim$0.35$''$ at 230GHz.

$\circ$

B alone yields $\sim$1.2$''$ at 100GHz and, in combination with A provides an angular resolution of $\sim$1.0$''$ at 100GHz. In addition, because it contains long, intermediate and some short baselines, it could still be used in a tapered mode when a project is observed in marginal weather conditions despite some loss of sensitivity (for backup projects). It is mainly used for relatively strong sources.

$\circ$

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 100GHz, $\sim$1.5$''$ at 230GHz) and with B for higher resolution ($\sim$1.7$''$ at 100GHz, $\sim$0.7$''$ at 230GHz). C alone is also well suited for snapshot and size measurement experiments.

$\circ$

D alone is best suited for deep integration and coarse mapping experiments. This configuration provides both the highest sensitivity and the lowest atmospheric phase noise.

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 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: ../IRAMFR/ARN/aug05/aug05.html) for further details.

Signal to Noise

The installation of the new generation receivers on the Plateau de Bure corresponds to a significant change of the receivers, transmission lines and backend. Although the new receivers will ultimately provide large gains in sensitivities and bandwidths as compared to the present receivers, they have not yet been tested under real observing conditions. IRAM will do its utmost so that the performances of the interferometer equipped with the new generation receivers will be as close as possible to those which are expected. However, it cannot guarantee these performances for the coming observing session. We consider therefore two categories of proposals that will be rated separately: 1) science demonstration proposals ($\sim 30$% of the total time) that can be observed only with the improved noise temperatures that the new receivers should normally provide. Please check the SD bullet if you apply for this category. 2) regular proposals that can be done with the current receiver noise temperatures (i.e. the same as last year). Both types of proposals may take advantage of the extended baselines and of the increased useful bandwidth. Conservatively, we plan to limit the science demonstration proposals to about 1/3 of the available time. In the case the expected sensitivity will not be reached at the beginning of the observing session, the science demonstration proposals will not be observed.
The sensitivity calculations can be made by using equation (1) below.

$$\sigma = \frac{\JpK\Tsys } {\eta \sqrt{\Na (\Na -1) \Nc T_{\rm ON} B}} \frac{1}{\sqrt{N_{\rm pol}}}$$ (1)

where

• $\hbox{$J_{\rm pK}$}$ is the conversion factor from Kelvin to Jansky (22 Jy/K at 3mm, 35 Jy/K at 1.3mm)
• $\hbox{$T_{\rm sys}$}$ is the system temperature.
  You may use for regular proposals $\hbox{$T_{\rm sys}$}$ = 150K below 110GHz, 200K at 115GHz, 400K at 230GHz for sources at $\ge 20^\circ$
  and for science demonstration proposals $\hbox{$T_{\rm sys}$}$ = 100K below 110GHz, 170K at 115GHz, 190K at 230GHz for sources at $\ge 20^\circ$)
  
• $\eta$ is an efficiency factor due to atmospheric phase noise (0.9 at 3mm, 0.8 at 1.3mm).
• $\hbox{$N_{\rm a}$}$ is the number of antennas (6), and $\hbox{$N_{\rm c}$}$ is the number of configurations: 1 for D, 2 for CD, 2 for BC, and so on.
• $T_{\rm ON}$ 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 $T_{\rm ON}$.
• $B$ 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)
• $N_{\rm pol}$ is the number of polarizations: 1 for single polarization and 2 for dual polarization (see section Correlator for details).

Investigators have to specify the $1\sigma$ noise level which is necessary to achieve each individual goal of a proposal, particularly for projects aiming at deep integrations.

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 LSR velocities for the sources. For pure continuum projects, the ``special'' velocity 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.

Correlator

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 IF=8 GHz. The two 4-GHz bandwidths coincide in the sky frequency scale. The current (narrow-band) 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 Q1...Q4. Two of these quarters must be selected as correlator inputs. The system allows the following choices:

• first correlator entry can only be Q1 HOR, or Q2 HOR, or Q3 VER, or Q4 VER
• second correlator entry can only be Q1 VER, or Q2 VER, or Q3 HOR, or Q4 HOR

where HOR and VER refers to the two polarizations.
How to observe two polarizations? To observe simultaneously two polarizations, 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.
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 is available in that case; this may or may not be the same polarization for the two chunks of 1 GHz.
Is there any overlap between the four quarters? In fact, the four available quarters are 1 GHz wide each, but with a short 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.
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. Note that in any case, the two correlator inputs are analyzed independently.
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.

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.

Spacing	Channels	Bandwidth	Mode
(MHz)		(MHz)
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

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 is depending on the parts of the 4 GHz bandwidth which have been selected as correlator inputs.

ASTRO

The software ASTRO has been updated to reflect these new receiver/correlator setup possibilities. Astronomers are urged to download the most recent version of GILDAS at ../IRAMFR/GILDAS/ to prepare their proposals.
The old LINE command has been replaced by several new commands (see internal help):

• NGR_LINE: receiver tuning
• NARROW_INPUT: selection of the narrow-band correlator inputs
• SPECTRAL: spectral unit tuning
• PLOT: control of the plot parameters.

A typical session would be:

   ! choice of receiver tuning
   ngr_line xyz 230 usb           

   ! choice of the correlator windows
   narrow_input 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   
   ...

Sun Avoidance

For safety reasons, a sun avoidance circle is enforced at 45 degrees. Please take this into account for your sources and calibrators.

Mosaics

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

Data reduction

Proposers should be aware of constraints for data reduction:

• In general, data should be reduced in Grenoble. Proposers will not come for the observations, but may have to come for the data reduction.
• 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. Please contact us in advance.
• 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 us if you are interested in this possibility.
• 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.

Data reduction will be carried out on dedicated computers at IRAM. Remote data reduction is possible, and especially for experienced users of the Plateau de Bure Interferometer. Please contact the Interferometer Science Operations Group (sog@iram.fr) if you are interested in this possibility.

Local contact

A local contact will be assigned to every A or B rated proposal that 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.

Technical pre-screening

All proposals will be reviewed for technical feasibility in addition to the scientific review by 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.

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.

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 ../IRAMFR/PDB/docu.html
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.

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 the documentation and at ../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.

Jan Martin Winters