Call for Observing Proposals on the Plateau de Bure Interferometer

Conditions for the next summer period

As every year, we plan to carry out extensive technical work during the summer semester, including the regular maintenance of the antennas. In particular, yet another antenna will be fully equipped with new aluminum panels replacing its current carbon fiber panels. 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:

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observations requesting the use of the 2mm and 3mm receivers

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circumpolar sources or sources transiting at night between June and September,

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observations that qualify for the 5D, 6D, and 6C configurations

Proposal category

Proposals should be submitted for one of the five categories:

1.3MM:

Proposals that ask for 1.3mm data. 2mm or 3mm receivers can be used for pointing and calibration purposes, but cannot provide any imaging. During the summer semester, proposals requesting the extended tuning range (256-267GHz) will be carried out on a ``best effort'' basis only.

2MM:

Proposals that ask for 2mm data. 3mm receivers can be used for pointing and calibration purposes, but cannot provide any imaging.

3MM:

Proposals that ask for 3mm data.

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.

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.

LARGE PROGRAM:

This category is offered on both IRAM instruments since the winter 2008/2009 observing period. See Section Large Observing Programs for a detailed explanation.

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

Configurations

Configurations planned for the summer period are:

Name	Stations
5Dq	W08 E03 N07 N11 W05
6Dq	W08 E03 N07 N11 N02 W05
6Cq	W12 E10 N17 N11 E04 W09

Part of the projects will be scheduled at the end of the summer period when the six-element array (likely in C-configuration) 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:

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D is best suited for deep integration and coarse mapping experiments (resolution $\sim 5\hbox{$^{\prime\prime}$}$ at 100GHz). This configuration provides both the highest sensitivity and the lowest atmospheric phase noise.

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CD is well adapted for low angular resolution studies ( $\sim 3{\farcs}5$ at 100GHz, $\sim 1{\farcs}5$ at 230GHz.

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C is appropriate for mapping, snapshot, and size measurements and for detection experiments at low declination. It provides a spatial resolution of $\sim 2{\farcs}8$ at 100GHz.

Finally, enter ANY in the proposal form if your project doesn't need any particular configuration.

Please consult the documentation An Introduction to the IRAM interferometer (http://www.iram.fr/IRAMFR/PDB/docu.html) for further details.

Receivers

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

	Band 1	Band 2	Band 3
RF range$^*$/[GHz]	80-116	129-174	201-267
$\rm T_{rec}$/[K] LSB	40-55	30-50	40-60
$\rm T_{rec}$/[K] USB	40-55	40-80	50-70
$\rm G_{im}$/[dB]	-10	-12 ... -10	-12 ... -8
RF LSB/[GHz]	80-104	129-168	201-267
RF USB/[GHz]	104-116	147-174

$^*$ center of the 4-8GHz IF band

Each band of the receivers is dual-polarization with the two RF channels of one band observing at the same frequency. The three different bands are not co-aligned in the focal plane (and therefore on the sky). Due to the pointing offsets between the 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 RF bands are presently being tested. Please contact the Interferometer Science Operations Group (sog$@$iram.fr) to discuss the feasibility in case you are interested to use this mode.

The mixers are single-sideband, backshort-tuned; they will usually be tuned LSB, except for the upper part of the frequency range in all three bands where the mixers will be tuned USB.

The typical image rejection is 10dB. Each IF channel is 4 GHz wide (4-8 GHz). The two 4 GHz wide IF-channels (one per polarization) can be processed only partially by the existing correlator. A dedicated IF processor 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.

Signal to Noise

The rms noise can be computed from

$$\sigma = \frac{J_{\rm pK} T_{\rm sys}} {\eta \sqrt{N_{\rm a... ...m a}-1) N_{\rm c} T_{\rm ON} B}} \frac{1}{\sqrt{N_{\rm pol}}}$$ (1)

where

• $J_{\rm pK}$ is the conversion factor from Kelvin to Jansky (22 Jy/K at 3mm, 29 Jy/K at 2mm, and 35 Jy/K at 1.3mm)
• $T_{\rm sys}$ is the system temperature ($T_{\rm sys}$ = 100K below 110GHz, 180K at 115GHz, 180K at 150GHz, and 250K at 230GHz for sources at $\delta \ge 20^\circ$ and for typical summer conditions).
• $\eta$ is an efficiency factor due to atmospheric phase noise and instrumental phase jitter (0.9 at 3mm, 0.8 at 2mm, and 0.6 at 1.3mm) in typical summer conditions.
• $N_{\rm a}$ is the number of antennas (5), and $N_{\rm c}$ is the number of configurations: 1 for D, 2 for CD, 1 for C.
• $T_{\rm ON}$ 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 1.6 $T_{\rm ON}$.
• $B$ is the spectral bandwidth in Hz (up to 2GHz for continuum, 40kHz to 2.5MHz 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 in the ``technical justification'' and on the Technical Sheet the one sigma noise level which is necessary to achieve each individual goal of a proposal, and particularly for projects aiming at deep integrations.

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 NULL (no Doppler tracking) can be used.

Correlator

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 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 refer to the two polarizations:

Quarter	Q1	Q2	Q3	Q4
IF1 [GHz]	4.2-5.2	5-6	6-7	6.8-7.8
input 1	HOR	HOR	VER	VER
input 2	VER	VER	HOR	HOR

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.

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.

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.

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.

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). In all three bands, 3mm, 2mm, and 1.3mm 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).

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 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 and on the selected side band (LSB or USB).

ASTRO

The software ASTRO can be used to simulate the receiver/correlator configuration. Astronomers are urged to download the most recent version of GILDAS at http://www.iram.fr/IRAMFR/GILDAS/ to prepare their proposals.

The previous 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 LINE command can be changed by the 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, 1GHz bandwidth). Default is 2006:

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

A typical session would be:

   ! choice of receiver tuning
   line xyz 93.2 lsb low 6500

   ! choice of the correlator windows
   narrow Q3 Q3             

   ! correlator unit #1, on entry 1
   spectral 1  20 600 /narrow 1   

   ! correlator unit #2, on entry 1
   spectral 2  20 735 /narrow 1   

   ! correlator unit #3, on entry 1
   spectral 4 320 300 /narrow 1   

   ! correlator unit #4, on entry 2
   spectral 4 320 666 /narrow 2   
   ...

Sun Avoidance

For safety reasons, a sun avoidance limit is enforced at 45 degrees from the sun. We are presently testing a reduced sun avoidance circle at 35 degrees and expect that projects observed in the summer semester may already profit from this reduction. This can however not be guaranteed yet and we therefore ask proposers to still take into account the 45 degrees limit for the target sources.

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.

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.

Data reduction

Proposers should be aware of constraints for data reduction:

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We recommend that proposers reduce their data in Grenoble. 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.

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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.

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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.

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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 newer versions are in general downward compatible with the previous releases. The recent upgrades of CLIC implied however 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 urge you to use the January 2007 version of CLIC.

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.

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 http://www.iram.fr/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 Introduction to the IRAM Plateau de Bure Interferometer at http://www.iram.fr/IRAMFR/GILDAS/doc/html/pdbi-intro-html and 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 this call and at http://www.iram.fr/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