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. During this period, regular scientific observations will therefore mostly be carried out with the five element array. In addition to the antenna maintenance, an upgrade of the computer system on Bure will be carried out. All HPUX and OS9 systems will be replaced by LINUX PCs along with major modifications of the corresponding antenna control and data acquisition software in the real-time system. Further work will be done on the reduction of the sun avoidance circle. Finally, new receivers operating in the 2mm band will be installed on all six antennas. This new set of receivers, which will open a new frequency band at the Plateau de Bure interferometer, will be become available for the community at the end of the summer semester, after thorough testing and commissioning.

We plan to start the maintenance at the latest by the end of May and to schedule the 5D configuration between June and September. Scheduling of the 6D and 6C configurations will be tailored to progress being made in the commissioning of the 2mm NGR system.

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 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 four categories:

1.3MM:

Proposals that ask for 1.3mm 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, or eventually, to fill in gaps in the scheduling, or even 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.

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 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:

Set	Main purpose
D	Detection + ``low'' resolution mapping at 1.3mm
CD	3.5$''$ resolution mapping at 3mm

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

Receivers

Since December 2006 all antennas are equipped with a new generation of dual polarization receivers for the 3mm and 1.3mm atmospheric windows. The frequency range is 81GHz to 116GHz for the 3mm band, and 201 to 256GHz for the 1.3mm band.

Each band of the new receivers is dual-polarization (two RF and IF channels) 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 10dB. 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. The other band is in stand-by (power on and local oscillator phase-locked) and is available, e.g., for pointing. Time-shared observations between two frequency bands can not be offered for the summer (this mode is currently being tested).

The two IF-channels (one per polarization), each 4 GHz wide (total 8 GHz) are transmitted by optical fibers to the central building. At present, the 4GHz 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.

PdBI Receiver Specifications
	Band 1	Band 3
RF coverage	81-116	201-256
$\rm T_{rec}$	40-55	40-60 (LSB)
$\rm T_{rec}$		50-70 (USB)
$\rm G_{im}$	$\rm -10$ dB	$\rm -12 -8$ dB
RF range in LSB	81-104	201-244
RF range in USB	104-116	244-256

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, 35 Jy/K at 1.3mm)
• $T_{\rm sys}$ is the system temperature ($T_{\rm sys}$ = 100K below 110GHz, 180K at 115GHz, 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 (0.9 at 3mm, 0.8 at 1.3mm).
• $N_{\rm a}$ is the number of antennas (5), and $N_{\rm c}$ is the number of configurations: 1 for D, 2 for CD, and so on.
• $T_{\rm ON}$ 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 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

The interferometer operates in the J2000.0 system. For best position accuracy, source coordinates must be in the J2000.0 system; position offsets 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 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 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:

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

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. 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 (see Section ``ASTRO'' below). At 3mm, the receivers offer best performance in terms of receiver noise and sideband rejection in Q2 (i.e. the line should be centered at an IF1 frequency of 5500 MHz) whereas at 1mm best performance is obtained in Q3 (i.e. the line should be centered at 6500MHz).

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 depends 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 (February 2007 or later) 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: 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
   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   
   ...

Sun Avoidance

For safety reasons, the sun avoidance circle has been extended to 45 degrees. Please take this into account for your target sources AND for the calibrators.

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.

Data reduction

Proposers should be aware of constraints for data reduction:

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In view of the new receiver system, 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.

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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 the Science Operations Group (sog@iram.fr) if you are interested in this.

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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.

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.

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 ../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 this call 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