A 2 mm VLBI experiment between Metsähovi (Finland) - Pico Veleta (Spain) - SEST (Chile)

During the last 10 years, VLBI at 3mm wavelength (86 GHz) on continental and intercontinental baselines of ca 5000 to 10 000 km length (Europe - USA - Chile) has become an established observational tool. Such baselines are equivalent to 4 - 8 Mega-$\lambda$, respectively, the corresponding angular resolution being $\lambda /{\rm B} \sim 50 - 100 \mu \; {\rm arcsec}$.

Progress of mm VLBI at shorter wavelengths has been slow, not only because of the small number of sensitive and large diameter telescopes or interferometers capable of working at high frequencies, but also because of the stronger influence of atmospheric conditions (phase fluctuations). In addition to the high stability of the masers and LO-chains, the relatively small recorded bandwidth (56 to 112 MHz) limits the sensitivity at wavelengths shorter than 3 mm. Evidently, VLBI at the shorter wavelengths is restricted to the atmopheric windows at $\sim 2$ mm (i.e. 150 GHz) and $\sim 1.3$ mm (i.e. 230 GHz).

Several experiments have been made at 2 mm wavelength between Kitt Peak [KP], SEST [SE] and Pico Veleta [PV], and at 1.3 mm wavelength between PV-SE-KP and OVRO-KP-HHT-BIMA. Success so far has only been obtained with the 1.3 mm observation in 1990 between OVRO-KP on a baseline of 845 km (0.6 Mega-$\lambda$) leading to a marginal detection of 3C273 (SNR = 5), and the observation in 1995 between PV-Plateau de Bure [PdB] on a baseline of 1150 km (0.9 Mega-$\lambda$) leading to the highly significant detections (SNR = 7 to 35) of 3C273, 3C279, a few other QSOs, and SgrA*.

With the installation at the 30-m telescope of the new generation SIS receivers, the old `work-horse' 2 mm SIS receiver was no longer needed (1998). During the 2nd mm VLBI-workshop (Granada) the idea arose to use this receiver in Finland on the Metsähovi [ME] 14-m telescope for a VLBI test experiment at 2 mm (150 GHz). This radom-enclosed telescope, located at sea level, had never before been used for observations at wavelengths shorter than 3 mm; the use of a He-cooled SIS receiver being a completely new experience. Figure 4 shows the receiver installed at the vertex of this telescope. The candidate sources 3C279 and 3C273 culminate at Metsähovi only at $25^\circ$ to $30^\circ$ elevation, but the expectation of a (very) cold winter made the experiment attractive. The projected baseline PV-ME is 2000-3000 km (1.5 Mega-$\lambda$) and thus comparable to the baseline of the successful 1.3 mm PV-PdB experiment (0.9 Mega-$\lambda$). This extrapolation gave hope for a detection.

The SEST telescope has had for some time a 2 mm SIS receiver. The telescope became available as partner in this and another experiment concentrating on the baseline SE-PV. The experiment used more or less the same LO-setup as in the earlier 2 mm VLBI experiments. The baseline SE-PV is ca 9000 km (4.5 Mega-$\lambda$), the baseline ME-SE is ca 11 000 km (5.5 Mega-$\lambda$).

The observations were made in March (2 days: ME-PV) and April 2001 (2 days: ME-PV-SE and PV-SE), under moderate to poor weather conditions: warm in Finland ($0^\circ$ to $-10^\circ$ C), wet at SEST, reasonable to good at PV. The data were correlated on the recently MKIV-upgraded VLBI correlator of the MPIfR at Bonn.

The observations concentrated on the strongest sources, i.e. 3C279 and 3C273, but several QSOs were also observed. The mutual visibility of 3C279 at ME, PV, and SE is shown in Fig. 5.

Fringes with a Signal/Noise-ratio of 9.6 have been found between Metsähovi and Pico Veleta; fringes have not (yet) been found between SEST and Pico Veleta - for unknown reasons, although 3mm fringes between SE and PV were seen a few days before. Figure 6 shows the ME-PV detection of 3C279. In the near future a 2 mm VLBI experiment could be repeated, perhaps including the HHT (Mount Graham, USA), and the KP telescope. This would include the major part of the telescopes able to observe at 2 mm wavelength. The near future will also see again experiments at 1.3 mm wavelength, in particular when the phased-array PdB and PV can observe mutually in 2002. In the more distant future the LMT, CARMA, and ALMA will play and important role for mm-VLBI.

The VLBI observations involved the efforts of many persons at the observatories and at the correlator; many e-mails and telephone calls; and smoothing of the logistics by the observatory directors (S. Urpo, R. Booth, M. Grewing).

The groups participating in the experiment were:

Seppo, Prisse, Pekka, Eki, Johanni,
Ari, Jouko, (Metsähovi-SF);
Kaj (Turku-SF);
Dave, Santiago, Salvador, Juan, the telescope operators (IRAM-Spain);
John, Fredrik, Lars-G., Michel (OSO+SEST);
David, Thomas, the correlator staff (MPIfR);
Albert, Marylène, Bernard, Jean-Yves, (IRAM-FR).

A lot of equipment has been transported efficiently by Javier and Gaby and the SF colleagues. The scheduling must have been a nightmare for Clemens and Lars-Åke.

Albert GREVE, on behalf of the VLBI collaboration.

  

Figure: The `retired' IRAM 2 mm SIS receiver installed at the vertex of the Metsähovi telescope.

  

Figure: Mutual visibility of 3C279 at Metsähovi, Pico Veleta, and SEST

  

Figure: Detection of 3C279 on the $\approx 2500$ km baseline ME-PV. The graphic (red line) shows the correlated amplitude obtained with the correct delay (correlator diagnostic output).