10.4.1 Correction for the antenna gain $\ensuremath{\mathcal{B}} _i(t)$

The antenna gain $\ensuremath{\mathcal{B}} _i(t)$ corresponds to losses due to the antenna, mainly focus (F_i) and pointing (P_i) errors coming from thermal variations of the antenna structure and surface.

$$\ensuremath{\mathcal{B}} _i(t) = P_i(t) \times F_i(t)$$ (10.17)

At Bure, we now check and correct automatically the pointing and the focus each hour. This correction is then done mainly in real time. This has improved a lot the quality of the data at 1.3mm. However, it is necessary in some cases to add a break in the amplitude (but not in the phase) fitting in order to take into account a focus error or a loss of amplitude due to pointing errors.

Note that, an error on the focus of 0.1mm at 1.3mm will introduce a phase error of $\frac{4\pi}{\lambda}\times \frac{180}{\pi}\sim 24^o$ and a loss in amplitude of $\sim 3-5\%$.