# NOEMA instrumental modes

The most usual instrumental mode for a radio-interferometer is to observe with a single band of receiver a single LO frequency. This mode will be called “standard” mode hereafter. NOEMA introduces two other possibilities:

In dual band mode,
  two frequencies (one at 3 and one at 1 mm) are observed simultaneously. The beam has thus to be split into two beams with the help of a dichroic. This adds some instrumental noise that we will encode as a higher value of the receiver noise.

  As the dichroic can be removed from the optical path when doing single band observations, this receiver noise increases will only happen when the sensitivity estimation is done in dual band mode.

In frequency cycling mode,
  the tuning frequency is regularly cycled between predefined values inside the same receiver RF band. This implies that the on-source observing time must be split between the different tuning of the frequency cycling. To do this, the user will have to give the percentage of the time required per tuning. The sum of the percentage will have to be equal to 100%. By default, PMS will divide equally the on-source time between the tunings, but the user will have the possibility to modify this time repartition.

  After the setup phase, each cycle observed at a given frequency must be surrounded by gain calibration observations at the same frequency. This means that the observing efficiency decreases with respect to the standard instrumental mode: in practice this is like doubling the number of calibrators, since each calibrator will have to be observed at the 2 frequencies (the frequencies of the previous and of the next cycle, whatever the number of cycles).

These two additional possibilities can in principle be combined during the same observation.

This memo describes how the sensitivity estimator tool used in the Proposal Management System (`https://oms.iram.fr/oms/?pms`) encodes the sensitivity estimation for these three instrumental modes. Sections , , and first reminds the basic equations used to compute the sensitivity of a radio-interferometer, namely, the interferometric point source and extended sensitivity, as well as the system temperature that characterizes the noise added by the receiver, the telescope optics, and the atmosphere. Section details the actual computation as a function of the observing mode.