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 might create a group of 28 identical observations (blocks) using "monitoring mode". The currently available modes 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.

An example of how to schedule the observation of a exoplanetary transit with some constraints. Suppose a transit taking 3.3h and you wish to take individual exposures of 1020s. The whole transit will require 11 exposures, including a two-min detector readout. Now you want to add 4 more exposures before ingress time to serve as a baseline. Using HTOTE, 15 exposures will take 4.42h, and adding the last detector readout, 4.45h. 4 exposures take 1.23h. The phase zero time, or the middle time of your observations will be:

t0=(tingress-1.23h)+4.45h/2

Then convert t₀ into TBD format. 'Period (frequency)' is the period of the transiting exoplanet. 'Window half size' is 4.45h/2=2.225h, or 0.0927 days.

If when validating this group of observations you get the error your window size is too short, then increase the window half size slightly, usually around 10%. This error happens when the number of exposures and individual exposure time is longer that the window size.

Click on 'Display Observation Windows' to show all the slots. If you require observing one specifically, adjust the 'Group enable and disable times' to select that one. Note that if the group enable/disable times are not compatible with the t₀ time and half window size, then your observation can't be scheduled. If you require observing several slots, then set 'Number of observations required' to the desired number.

The chance of scheduling and observing time-critical observations like the one described above depends very much on the scientific ranking of the proposal and the observing constraints. The higher ranking and the more relaxed observing constraints, the higher probability of getting it scheduled and eventually observed.

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 is not currently possible.

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 currently implemented.

A target is not observed if...

1. it is lower in the sky than a given altitude.
2. the Moon is in the sky and brighter than a given fractional illumination.
3. the Moon-target distance is smaller than a given value and the Moon is brighter than a given fractional illumination.
4. the difference between the radial velocity of the target and that of scattered moonlight is smaller than a given value, the Moon is brighter than a given fractional illumination and higher in the sky than a given altitude.
5. the Sun is a given distance below the horizon.
6. the seeing is worse than a given value.
7. the extinction is worse than a given value.

There is no default minimum Moon separation. If required, for instance, when using the second fibre for observing the sky and the sky-brightness gradient can be large as a consequence of the distance and brightness of the Moon, then set the corresponding constraints.

The seeing the observer-supplied value will be compared with is the estimated seeing at zenith, hence the actual seeing may be a little worse than this and still satisfy the constraint.

Similarly the extinction value supplied by the observer will be compared with the estimated excess extinction at the zenith compared with a "clear" night. So, an observation taken at airmass 1.5 with the constraint that the extinction has to be zero will have to be taken on a "clear" night, but will also suffer more extinction than an observation at at airmass 1. To have a reasonable chance of being scheduled we suggest using an extinction limit of 0.15, but we will have more information after commissioning.

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.