CFH12k Queued Observing Programme: Preliminary Description.


4. Factors determining queue execution.

 The sequence in which the queue is executed is a complex function of:

    1. Object observability (Coordinates, LST)
    2. Sky conditions:
    3. sky brightness
    4. seeing
    5. transparency
    6. Degree of programme completion.
    7. TAC ranking
    8. Number of usable observing hours remaining in the run
    9. Current weather forecast.

4.1. Object observability.

 This is a function of the field's location on the sky (celestial coordinates) and current local sidereal time. The investigators will supply information on the airmass constraints for each field.

4.2. Sky conditions

The required S/N for each field constrains the usable sky conditions. The signal and Poisson noise at the required limiting magnitude or target brightness is assumed to be calculable from the filters used, the CCD QE curves and readout noise, and the optical efficiency of CFHT prime focus. Dynamically changeable factors which are relevant during a run are as follows:

4.2.1. Sky Brightness

Sky brightness is commonly defined by any of a number of different methods, including lunar phase, time of night, or angular distance to the moon, but the most straightforward and applicable is to specify the actual, instantaneous brightness of the sky in the target field. This is the measure that the queued observing programme will adopt throughout.

At the telescope, the instantaneous sky brightness will be known, either because:

a) the moon is down and there is currently no twilight - sky brightness will be assumed constant everywhere,

or b) observations have begun and the sky is the same brightness or darker everywhere the current distance or more from the moon,

or c) an estimate of the analytical function of sky brightness with distance from a partial moon will be defined observationally,

or d) we can go and look (if the new programme is otherwise acceptable to begin).

The queued observing programme will therefore adopt a direct sky brightness definition (detected photons per square arcsec per second in the waveband of interest) and will not request lunar phase limits from the investigators. Investigators will be provided with an estimate of the range and distribution of such values as determined from prior experience.

4.2.2. Seeing

The investigator will specify the acceptable seeing for his programme according to a set of seeing bands (see Appendix 1). It is always assumed that a programme can be executed with better seeing than specified by the investigator, unless specifically stated otherwise.

4.2.3. Transparency

Transparency conditions will be set by the investigator, stated as a fraction of normal observing efficiency under photometric conditions.

4.3. Degree of programme completion

Once a programme has been started, it will be given preference over other programmes which have not yet been started.

4.4. TAC ranking

Under most circumstances, the TAC ranking will be among the last parameters considered. This is intuitively obvious since object observability, sky conditions and degree of programme completion directly constrain the group of programmes which it is possible to execute. Such programmes are termed "viable" in this document (see Appendix 1). Only after this group is established will TAC ranking come into play.

4.5. Potential I-time remaining

The queue must always take into account the amount of usable observing time remaining in a run. A new programme will not be started unless there is sufficient useable I-time available in the run to complete it and all other programmes which are already under way and not yet complete. If there are no executing programmes and there is not sufficient I-time remaining in the night or run to complete a new programme, the viable, unstarted programme which may be most nearly completed in the time remaining will be started (this may be overriden, at the observer and support staff's discretion on inspection of other pending programmes).

4.6. Current weather forecast

The observer may decide not to begin a new programme, even if it is viable, if the projected observing conditions are likely to significantly reduce the maximum remaining I-time in a run. If the timescale of weather pattern changes is within the current night, then the observer is best situated to make the necessary decisions. If the changes are expected to occur over more than one night, then decisions should be made in conference with daytime staff in Waimea.



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