17.4 Accurate Position Measurements with the IRAM Interferometer

Let us start with two general and simple remarks. First, the phase equation giving the angular offset $\theta$ in Sec.17.2 shows that higher position accuracy (namely smaller values of the angular offset) is achieved for smaller values of the fringe spacing $\lambda/B$. (This was demonstrated above in the case of the least squares analysis of the u,v data.) Thus, for astrometry it is desirable to use long baselines and/or to go to short wavelengths. However, the latter case implies that the phases are more difficult to calibrate especially at mm wavelengths where the atmospheric phase fluctuations increase with long baselines. Sensitivity is always important in radio astrometry. For a point-like or compact source the sensitivity of the array varies directly as D² (n(n- 1))^0.5 where D is the antenna diameter and n is the number of antennas. Thus the detection speed varies as D⁴ n(n-1) and big antennas are clearly advantageous. Comparison of the IRAM 5-element array with one of its competitors, OVRO with 6 x 10.4 m, gives a ratio of detection speed of 1 over 0.35 in favor of the Plateau de Bure array. (Note also that the sixth antenna in the Bure array will increase the detection speed by 50%.) In addition, the large dishes of the IRAM array are good to perform quick baseline and phase calibrations; this is another clear advantage of the IRAM interferometer in astrometric observations.