3.2 mm-VLBI Arrays

The wet atmosphere, even at high altitudes, has transmission windows at $\sim$3mm ($\sim$100GHz), $\sim$2mm ($\sim$150GHz), $\sim$1.3mm ($\sim$230GHz), and at shorter wavelengths, through which radio signals can propagate to the surface of the Earth. The observing facilities of mm-VLBI, summarized in Table 3.1, make use of these atmospheric windows. cm The remark 'Proposal' in Table 3.1 indicates that the array is available to observers on a competitive proposal basis. The 2-mm and 1.3-mm observations are in an experimental state. The mentioned 1.3-mm observations were experimental, and successful.

Table 3.1: mm-VLBI Arrays and Experimental Observations

Frequency(Wavel.)	Array	Telescopes	Baseline	Observing
86GHz (3.5mm)	CMVA	Table 2	$\sim$8000km	Proposal
86GHz (3.5mm)	VLBA(*)	Table 3	$\sim$8000km	Proposal
150GHz (2.0mm)	--	PV-KP-SEST		experim.
230GHz (1.3mm)	-	OVRO-KP	$\sim$500km	experim. (**)
	-	PV-PdBure	$\sim$1000km	experim. (***)

PV: Pico Veleta (Spain), KP: Kitt Peak (USA), OVRO: Owens Valley Obs. (USA), SEST: La Silla (Chile).
(*) sub-array of the VLBA array (Table 3.3); (**) [Padin et al. 1990]; (***) [Greve et al. 1995], [Krichbaum et al. 1998]

3.2.1 The CMVA Array

Figure 3.1: Observatories participating in cm and mm-wavelength VLBI. The mm-VLBI observatories of the CMVA (Table 3.2) are concentrated in Western Europe and North America. From [Felli & Spencer 1989], Schilizzi p404, with kind permission from Kluwer Academic Publishers.

$\parbox{150mm}{ \caption{Figure 3.2: Il... ...The mutual visibility is given by the overlap areas of the individual curves.}}$

The CMVA (Coordinated Millimeter VLBI-)array contains the telescopes of 12 observatories which operate together in coordinated observations for approximately 15 days per year. The telescopes are located in North-America, Europe, and Chile. The performances of the telescopes at 86GHz (3.5mm) are given in Table 3.2, the location of the telescopes is shown in Figure 3.1. The European telescopes of the CMVA are located essentially in the direction North-South, the American telescopes are located essentially in the direction East-West. As applied in other interferometers, the CMVA array uses Earth rotation to obtain uv-coverage. Some of the strong mm-VLBI sources (3C273, 3C279), which are regularly monitored (see Figure 3.9), are located at low declination (Dec = 2$^{\rm o}$, -5$^{\rm o}$) so that they can be simultaneously observed only at low local elevations, as illustrated in Figure 3.2 for 3C273 and the stations Pico Veleta - SEST - Kitt Peak - Owens Valley. The low elevation usually implies a high line-of-sight system temperature (T $_{\rm sys}$), thus a low signal-to-noise ratio, and thus a low detection sensitivity (Sect.3.5). The low elevation implies also that the uv-coverage may be incomplete, the synthesized beam asymmetric, and the final map distorted. cm contains also several telescopes expected to be available in future. These are telescopes with large collecting areas; the dedicated mm-wavelength telescopes (PdB, LMT, ALMA) are located at high altitudes.

Table: The Coordinated Millimeter VLBI Array = CMVA (86GHz)

Telescope	Country	$\cal D$	T $_{\rm sys}$	Gain	$\eta$$_{\rm A}$	Altitude
		[m]	[K]	[K/Jy]		[m]
CMVA
Effelsberg	Germany	60	350	0.13	0.13	$\sim$0
Haystack	USA	37	200	0.058	0.15	$\sim$0
Pico Veleta	Spain	30	150	0.14	0.60	3000
Onsala	Sweden	20	250	0.056	0.49	$\sim$0
SEST	Chile	15	300	0.032	0.50	2500
Amherst	USA	14	300	0.024	0.43	$\sim$0
Metsähovi	Finland	14	300	0.017	0.30	$\sim$0
(Kitt Peak(*)	USA	12	150	0.023	0.56)	2000
Owens Valley	USA	4$\times$10.4	300	0.067	0.55	1500
Hat Creek	USA	7$\times$6.1	300	0.050	0.65	1000
Expected
P. de Bure	France	6$\times$15	150	0.045	0.7	2500
GBT	USA	[100]				$\sim$0
LMT	Mex(-USA)	50				4600
Yebes	Spain	40				500
ALMA	Chile	64$\times$12	(70)	(0.03)	(0.8)	5000

(*) out of operation since July 2000.

3.2.2 The VLBA Array

The VLBA (Very Long Baseline-)array, which consists of 10 dedicated 25-m diameter telescopes (Table 3.3, Figure 3.3), is located on the North-American continent, including one antenna on Hawaii. This array observes routinely at 43GHz. The array is being upgraded for observations at 86GHz (3.5mm); a sub-array (+) is available for 86GHz VLBI observations. Some of these telescopes collaborate in CMVA observations.

Figure 3.3: The VLBA array (Table 3.3) (from [Zensus et al. 1995]).

Table: The VLBA Array [10$\times$25m antennas] (partially operating at 86GHz)

	Code	Location (USA)	Elevation (m)
	SC	St. Croix VI	16
	HN	Hancock NH	309
+	NL	N. Liberty IA	241
+	FD	Ford Davis TX	1616
+	LA	Los Alamos NM	1967
+	PT	Pie Town NM	2371
+	KP	Kitt Peak AZ	1916
+	OV	Owens Valley CA	1207
	BR	Brewster WA	255
+	MK	Mauna Kea HI	3720