The number of polarizations

Heterodyne mixers are coupled to a single linear polarization of the signal. Hence, heterodyne receivers have at least two mixers, each one sensitive to one of the two linear polarization of the incoming signal. Both mixers are looking at the same sky position. This implies that we have to distinguish between the time spent on a given position of sky and the human elapsed time. Indeed, we will use the time spent on a given position of the sky when estimating the sensitivity, while we will give human elapsed time for the telescope and the on and off times.

If the mixers are tuned at the same frequency, the times spent on and off in the same direction of the sky will be twice the human elapsed time. We thus have to introduce the number of polarization simultaneously tuned at the same frequency, , which can be set to 1 or 2. It happens that for EMIR, the two polarizations are always tuned at the same frequency, i.e. $\ensuremath{n_\ensuremath{\mathrm{pol}}}=2$. The simplest way to take into account the distinction between human time and sky time is to slightly modify the radiometer equation to take into account the number of polarization

$$\ensuremath{\sigma_\ensuremath{\mathrm{}}}= \frac{\ensurema... ...ath{\mathrm{on}}}+\ensuremath{t_\ensuremath{\mathrm{off}}}}.$$ (5)

This equation implies that , , and will be human times.