Since the first successful `test' at 7 mm in 1991, the IRAM 30--m telescope has participated several times in internationally organised VLBI observing runs at 3 mm (typically 1 to 2 times per year). The VLBI equipment at Pico Veleta--Spain is installed there permanently, consisting of a GPS station, a H-maser (Neuchatel Observatory, Switzerland) and a VLBA terminal and tape unit (Interferometrics, USA). The observations are made with the standard 3mm SIS receiver(s), LSB tuned with approximately 10 dB rejection.
When it became clear that the first dual channel 3 mm + 1.3 mm SIS receiver would soon be ready and installed on Antenna 4 of the Plateau de Bure Interferometer, it was decided to perform an `in-house' VLBI experiment between the two IRAM observatories in collaboration with the VLBI groups of the Max-Planck-Institute for Radioastronomy, Bonn, the VLBI group at the Yebes Observatory, and the VLBI group of the Bordeaux Observatory. For this experiment, a H-maser was borrowed from the CNRS (France), and Yebes provided a VLBA terminal and tape unit. The Bonn correlator was made available for analysing the data. The experiment started with VLBI observations at 3 mm between Pico Veleta and Plateau de Bure at the end of October, to be followed by 1.3 mm test observations as soon as possible thereafter.
The tasks were shared as follows: the VLBI team of the MPIfR-Bonn supported the Pico Veleta operations, and provided the correlator at Bonn for a quick correlation and first look at the data, the VLBI team of Yebes supported the Plateau de Bure operations, and CERGA and the Bordeaux Observatory supported the installation and supervision of the maser.
Prior to the observations, we determined the position of Bure from GPS
measurements; the drift of the maser was referenced to the 1 pulse
per second signals of the GPS. With the limited and not crucially
selected GPS data we obtained an accuracy in X,Y,Z of 40 m, the
drift rate of the maser was 
.
3 mm VLBI observations between Pico Veleta and Plateau de Bure were
made on 25--27 October. It was decided to start by using antenna
A3 which has the best
pointing and tracking performance and a SIS receiver of good
stability. The LO system was modified so that by simple switching of
the 100 MHz reference signal the antenna could participate either in
the interferometer mode for pointing and focusing or be separated for
the VLBI measurements as a single antenna running exclusively on the
maser-controlled LO system. We were able to make reliable
pointing/focus determinations within the 
minutes intervals
between VLBI recordings. Recordings were made of 28 channels (14 LSB
+ 14 USB) of 4 MHz width each.
Although the weather varied enormously, from very bad to good, we were able to make several recordings during the first night that had been scheduled. Correlation at Bonn of these data gave high S/N fringes for the sources 0528+134 (SNR = 53 - 58), 2145+067 (SNR = 28 - 154) and 3C454.3 (SNR = 42 - 99) [The sun avoidance zone of the Bure telescopes did not allow the observation of 3C273 and 3C279].
On 29-31 October we switched to 1.3 mm using antenna 4 for 2 consecutive night sessions of 6 hours each. This antenna has a dual frequency SIS receiver for 3 mm and 1.3 mm. In order to check the system we made a few VLBI measurements at 3 mm, then turning the system alternatively to 1.3 mm VLBI observations and 3 mm pointing/focus determinations (which required considerable speed in changing connections and cables).
During these two periods weather conditions varied between reasonable and good. Two times 5 h recordings at 1.3 mm have been made. Fringes at 3mm, though very noisy, have been found for the start-up measurements with Antenna 4. Fringes at 1.3 mm are still being searched for.
A repetition of the 3 mm and 1.3 mm observations is envisaged for December.
The experiment would not have been possible without the help from many engineers of the backend and receiver groups, the operators at the telescopes and the astronomers who have participated in the preparation, execution and reduction of these observations.
IRAM - Plateau de Bure and Pico Veleta
MPIfR - Bonn and Yebes - Madrid
Observatoire de Bordeaux and CERGA.
Albert GREVE