Figure 8:
IRAM 30-m telescope current beam pattern calculated from
Eq. (16) and the parameters of Table 1. The heavy lines show the
case of a Gaussian approximation of the main beam (Eq. (23)). The accuracy
of the profiles is 1 dB beyond the main beam area. The beam
patterns are shown on logarithmic scale (dB). For each wavelength the
profile is normalized to 1/2 width of the full beam (see Table 2) |

The calculation of the beam pattern requires a knowledge of the
diffraction pattern . Without entering into lengthy
calculations (see Minnett Thomas 1968; Goldsmith 1987), the
diffraction pattern of the 30-m telescope is obtained with sufficient
accuracy, *when compared to measurements,* from the approximation
= , with calculated from the expressions given by Abramowitz
Stegun (1972) and the reduction factor , 0.22, 0.27
for the 1st, 2nd, and 3rd sidelobe. In this calculation the beam width (FWHP)
of the Airy pattern is (Table 1) at
*u* = 1.62. As evident from Fig. 8, at the level of the 3rd
sidelobe the diffraction pattern and the error pattern have similar
intensities. The sidelobe structure and the error beam seen in
Fig. 8 are not observed in regular pointing scans made with the
30-m telescope because the sensitivity of the procedure is only -10 dB to
-15 dB. The sidelobe structure is also not seen in the composite Moon
scans (Figs. 3, 4) since this detail is lost in the
convolution (Eq. (18)).

In many applications the diffracted beam is approximated
by a Gaussian main beam without sidelobes

(23) |

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