The more restrictions that you put on an AOR, the harder it is to schedule. HSpot allows you to define many restrictions on observations such as chopper avoidance angle(s), detector orientation, sequencing observations, requesting very long AORs that are close to the maximum permitted time, concatenation, etc. These should be used warily in the knowledge that putting to many or too strict limitations on an observation may make it difficult, or even impossible to schedule. Long scan maps with a strict orientation constraint can be exceptionally difficult to schedule.
In general, splitting a long AOR into several shorter ones will generally give the scheduler more flexibility to programme them and thus compensate for any slight loss of observing efficiency.
Any chopper angle constraint, or detector orientation requirement, translates into a physical constraint on the (already limited) dates when the observation can be made. A strong constraint on chopper avoidance angle may mean that a particular observation can never be scheduled. Similarly, requesting a specific detector orientation obliges the scheduler to programme the observation for a particular day, something that may or may not be possible given the limitations on scheduling instruments whereby each instrumnent is scheduled in a regular block of days within a 28 day planning cycle.
So, the best advice is to be as flexible as possible with your requests. Check carefully to see if a constraint is really necessary before requesting it (even if you see a bright star close to your source in a 2MASS image, that star is unlikely to have significant emission at Herschel wavelengths). Avoid, where possible, both very short (i.e. inefficient because the overheads are longer than the integration time) and very long observations (that are close to or may even overrun the maximum permissible time).
Time constraints on observations: if it is important to carry out an observation at a particular epoch or range of dates, the appropriate time constraint should be added and the extra overhead included in your time request to HOTAC. Time constraints correctly entered and approved by HOTAC are of obligatory acceptance for HSC when scheduling Priority 1 observations unless there is a major scheduling efficiency problem, in which case the user will be contacted in search of an alternative. Priority 2 observations are incompatible with time constraints as they will be scheduled only as fillers.
The more complex and restrictive the time constraints, the harder it will be to schedule the observations. Some overconstrained observations may be impossible to schedule.