Observations are arranged in groups. A group is a set of observations of a target which share the same scheduling mode and observing setup. For example, an observer wishing to observe HD123456 every night for 4 weeks months might create a block of 28 identical observations using “monitoring mode�. The modes available in 2026B are as follows.

Single observations

A group of just one observation, where the sole time constraint is that it must be carried out between two given times (technically the “group enable� and “group disable� times), which are set in Phase 2. If the times are the beginning and end of the semester the observation can be carried out at any time, but much tighter constraints (for example to force overlap with observations from another facility) can be given. If the observations are highly time constrained (to less than a one-month period) please state this in the technical section of the proposal.

Phased observations

This scheduling mode is primarily intended for programmes where observations of a transit or eclipse are required, and the event in question lasts significantly less than a night. However, it can be used for any periodic phenomenon. The timing of the observations is defined by three parameters set in Phase 2. The first two are a phase zero time and a period with which the event recurs at the Solar System barycenter, in Barycentric Dynamical Time (TDB). The final parameter is the half width of a window within in which the observation must occur. The entire observation must lie within the window.

Monitoring observations

This scheduling mode is intended for programmes which want to observe a source regularly, for example every hour or once a week. In Phase 2 a monitoring period is set, and an allowed range around the period defined by a window half-width. The differences between this and phased observations are as follows.

1. The timing is in UT as opposed to TDB.
2. An observation can be carried out after the window has closed if it has not occurred within the window.

For example, consider a target with a four-day monitoring period but a window which is only a night long. If a given night is missed, the scheduler carries on attempting to make an observation, rather than waiting three more nights for the next slot. Once the observation is made (maybe after a five-night gap) the scheduler will next try to observe on the fourth night after the successful observation. Note that the mid-point for the observation window is always moved to the nearest occurrence of 01:12UT (roughly local midnight) to allow the user to set sensible window widths which cover entire nights.

This scheduling mode behaves rather differently if the period is set to less than a day. If, for example, the period is set to be 2 hours then at the beginning of the night windows will be set centered on 17:12, 19:12 etc, and the scheduler will attempt to place observations close to each of those times.

Targets of opportunity

Entry of targets after the start of the semester will not be possible in 2026B.

Additional Constraints

In Phase 2, in addition to the above scheduling modes observers should be able to request additional constraints. Below we list those ones that we hope to implement for 2026B.

1. That a target is not observed when it is lower in the sky than a given altitude.
2. That a target is not observed if
   1. the Moon is in the sky and
   2. brighter than a given fractional illumination.
3. That a target is not observed if
   1. the Moon-target distance is smaller than a given value and
   2. the Moon is brighter than a given fractional illumination.
4. That a target is not observed if
   1. the difference between the radial velocity of the target and that of scattered moonlight is smaller than a given value and
   2. the Moon is brighter than a given fractional illumination and
   3. the Moon is higher in the sky than a given altitude.
5. That a target is not observed unless the Sun is a given distance below the horizon.
6. That a target is not observed if the seeing is worse than a given value.

Observation length

Observations requiring more than 3 consecutive hours on the same night are not possible given the scheduling constraints imposed by the THE survey.