News from the Plateau de Bure Interferometer

After the extension of the baselines last year, the Plateau de Bure Interferometer will see yet another major upgrade with the planned installation of the new generation of SIS receivers. These receivers, the result of a 6-year development effort at IRAM, are designed to operate in single-side band mode in one of any of the four band frequencies covering the 3mm (Band 1), 2mm, 1.3mm (Band 3) and 0.8mm atmospheric windows. They will offer dual polarization capabilities (horizontal/vertical, each) with an IF bandwidth of 4 GHz. In view of the progresses made recently in the lab, the current plan is to install two channels (Band 1 and Band 3) for the upcoming winter scheduling period (2006/2007), and to have the full set of bands operational at the Plateau de Bure at the end of 2007. With the present correlator, it will only be possible to correlate two 1 GHz wide sections (one per polarization) within the 4 GHz bandwidth. Correlation of the full 4 GHz band on both polarizations will have to await the completion of a new correlator, presently under development. Finally, an important difference to the current receivers is that simultaneous dual-frequency observing will not be possible with the new generation receivers of the Plateau de Bure Interferometer. The installation of the new generation receivers (NGRx) on the Plateau de Bure corresponds to a large system change, including transmission lines, software, and backends, which will result in unprecedented gains in sensitivity and bandwidth. To provide the new receivers for the coming winter period, IRAM is working on a very tight and intense schedule and calculated risks have to be taken in this transitional period. In view of recent work and tests done in the receiver laboratory, we are confident that the change to the new generation receivers will occur as planned. We have therefore decided to reissue a new call for proposals for the Plateau de Bure Interferometer which takes into account these recent developments and will supersede the call which was distributed in July. The new deadline for submission has been extended to September 11th, 2006 at 17:00 CEST (UT + 2 hours). In order to incorporate the potential risks associated with this significant and difficult change, the new call for proposals will include 1) science demonstration proposals ($\sim 30$% of the total time) which will need the improved sensitivities of the new generation receivers and 2) regular proposals which will be based on the current receiver noise figures. For both types of proposals the advantage of the extended baselines and of the increased useful bandwidth should be taken into account. 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.

Pierre COX

The new generation receivers

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 band can be connected to the IF transmission at any time. Because of this reason and the pointing offsets, 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 for phase calibration and/or time-shared alternate frequency observations. The two IF-channels (one per polarization), each 4 GHz wide (total 8 GHz) are transmitted by optical fiber to the central building. At present, that 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 IF 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.

New PdBI Receiver Specifications
	Band 1	Band 3
RF coverage$^1$	83-116	201-256
$\rm T_{rec}$$^2$	40-55	40-60 (LSB)
$\rm T_{rec}$$^2$		50-70 (USB)
$\rm G_{im}$$^3$	$\rm -10 \, dB$	$\rm -12 -8$ dB
RF range in LSB	83-104$^4$	201-244
RF range in USB	104-116	244-256

$^1$ Based on nominal RF frequency being converted to the center of the IF at 6 GHz. The total coverage extends 2 GHz more either side. The upper edge for Band 3 currently is limited by the triplers and should be raised to 267 GHz in 2007.
$^2$ Typical laboratory values.
$^3$ Estimated values based on measured junctions at the image frequency. Better values are expected at the band center.
$^4$ Transition between LSB and USB for Band 1 is flexible. The value will be fixed when all mixers are installed.
The receiver specifications will ultimately increase the sensitivity of the PdBI for spectral (single line) observations by factors 2 and 3 at 100 GHz and 230 GHz, respectively. Never such a gain has been planned in a single step at the Plateau de Bure since the opening of the interferometer. The expected factors in sensitivity result from gains in the receiver noise, from a better rejection of the image sideband at 230 GHz and from the possibility to observe with two orthogonal polarizations simultaneously. They assume that the performances obtained from the first front-end units in the laboratory will apply to all the units and will not degrade on the Plateau de Bure, due to unexpected phase instabilities or to data transmission problems. This, of course, cannot be guaranteed, especially during the first months of operation. In order to operate this major change at the Plateau de Bure, regular observing will be suspended during the receiver installation and testing period from October to November.

Weather conditions and observations

All in all, the weather conditions have been very good at the Plateau de Bure last winter with long periods of excellent phase stability and low atmospheric opacity. The interferometer recorded a 50-60% observing efficiency in January and February but only a very low efficiency of 30% in March. To optimize the observing efficiency with respect to the sun avoidance constraints of A-rated projects and due to some delays with the installation of the prototype-type NGRx at the beginning of the last winter semester, the configuration schedule of the interferometer was slightly adjusted. The array was moved to the most extended new configuration A (including stations E68 and N46) in mid January, moved to the new B configuration in mid February and to the C configuration in the second half of March. Because of the weather conditions, the array was rearranged to the most compact configuration (D) at the end of April. The Global VLBI observations from May 4 to 8, 2006 could not be joined by the Plateau de Bure array because of the irreparable breakdown of the CNRS maser 5 days before the start of the VLBI session. Most of the A-rated projects could be completed before the end of the winter period, although a few projects requesting the D configuration had to be moved into the current summer period. We have also invested observing time on a number of B projects, and even on a few targets of opportunity. Since last December, a total of 57 different projects has successfully been scheduled for observations. Concerning projects that have been started shortly before the end of the winter period, we plan to bring these to completion in the next few months. A few deep integration and low-resolution observations of sources in the Orion-Taurus region had to be suspended because of sun avoidance constraints and are now deferred to the end of the summer semester. Finally, we would like to remind users of the Plateau de Bure Interferometer that B-rated summer proposals which were not started by the proposal deadline, should be resubmitted. Investigators, who would like to check the status of their project, may consult the interferometer schedule on the Web at ../IRAMFR/PDB/ongoing.html.