Keywords: astro-noema-sensitivity-estimator

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# Observing mode and elapsed telescope time

The goal of a time estimator is to find the elapsed telescope time ( ) needed to obtain a given rms noise, while a sensitivity estimator aims at finding the rms noise obtained when observing during . The total integration time spent on-source is shorter than the elapsed telescope time due to several factors. As of Gildas Jul17 release, the input time of the sensitivity estimator is telescope time. The actual on source time is then computed taking into account the following two points:

1. Instrumental setup time: At the beginning of an observing track a significant time ( minutes according to history of observations) is spent in receiver tuning and calibration observations before observing the actual astronomical target. This means that even for a very short ON source time, a project cannot be shorter than . Also, for long projects observed in several ( ) tracks the time spent for tuning and calibration is . We thus define the time spent for observations (i.e. without instrumental setup) as:

   | (19)

   The number of tracks is computed as where is the typical duration of a track, which depends on the source declination:
   - Sources above are observed 8 hours at most,
   - 8.2 hours for a declination of (truncated to 8 hours),
   - 6.5 hours for a declination of ,
   - 3.9 hours for a declination of ,
   - 0.0 hours for a declination of .
   - Sources below can not be observed.
   A linear interpolation with the declination is performed in the appropriate range between and .
   For short projects ( ), the number of tracks is set to 1. Otherwise, the floating value of is used in the computation of . Since is constant whatever the length of a track the use of a floating value for is somehow unnatural but it ensures that the conversion from to is a monotonic function.
2. Observing efficiency: After the initial phase of instrumental setup, the observing mode does not dedicate 100% of the time to the astronomical target. Part of the time is spent for calibration (pointing, focus, atmospheric calibration,...) and to slew the telescopes between useful integrations. The time actually spent on source is defined as

   | (20)

   where is the observing efficiency.

The exact computation depends on the observing mode. There are three main observation kinds that are exclusive from each other.

Single-source, single-field observations
  where the telescope tracks a single source during the full integration time. This mode is used when the signal-to-noise ratio is the limiting factor.
Track-sharing, single-field observations
  where the telescope regularly cycles between a few close-by sources. This mode is used when the sources are so bright that the limiting factor is the Earth synthesis, not the signal-to-noise ratio.
Single-source mosaicking
  where the telescope regularly cycles between close-by pointings that usually follows a hexagonal compact pattern whose side is , where is the diameter of the interferometer antennas. This modes is used to image sources wider than the primary beam field of view.

These kinds can be combined with two others possibilities.

Dual band observations
  where the use of an additional dichroic allows the interferometer to observe simultaneously at 3 and 1mm.
Frequency cycling observations
  where the tuning frequency is regularly cycled between several predefined values in order to, e.g., allow to observe a full atmospheric window in a single track.

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

In the following, we will work out the equations needed by the sensitivity estimator for each of these observing modes.

Subsections

- Single-source, single-field observations
- Track-sharing, single-field observations
- Mosaicking
- Dual band observations
  - Python implementation
- Frequency cycling observations
  - Python implementation

Next: Single-source, single-field observations Up: IRAM Memo 2015-2 NOEMA Previous: Actual computations Contents