Any-weather observing programmes

We summarise some of the observing statistics for Paranal that can be useful when requesting or considering any-weather programmes, also known as "filler programmes".

Idle time

Idle time statistics per month and telescope are publicly available via the La Silla Paranal Observatory factsheets provided to the Users Committee, on a yearly basis.  

In the below table we list the idle time statistics per telescope between August 2021 and August 2023. Idle time is defined as the time when no OBs are available for observing while observing conditions are still acceptable (seeing <~ 1.6", no thick clouds). For this, we typically apply a tolerance of 25% to the requested observing constraints (i.e. idle definition would be met if current seeing is 1.5" and the OB with most relaxed constraint available requires 1.0"). 

We indicate the total amount of idle time, and also the fractional time per type of condition (multiple conditions can apply simultaneously, such that the sum of the condition times may be larger than the total idle time). For example, 1.5% of the time UT1 was idle because in the currently observable RA range, no OBs were available for execution within ~25% tolerance of their requested constraints. 

Idle total [%] 2.3 1.2 0.2 0.3 5.8


0.3 0.1 0.1 0.4
Wind 0.9 0.4 0.1 0.2 1.0
Thin clouds 0.3 0.2 0 0.1 0.3
Moon 0.8 0.3 0 0.1 0.1
RA 1.5 0.6 0.1 0.1 3.6

The fact that the largest individual reason for idle time is RA (and not seeing or thin clouds), shows that there are typically enough programmes that can be observed under bad conditions.

The amount of idle time per telescope generally anti-correlates with the oversubscription rates. Between Periods 108-111, the lowest idle time telescopes UT3 and UT4 have the highest ratio of requested/allocated time, about 3-5. For UT1 and UT2 this ratio is about 2-3, while for VLTI is 1.5-2.

More details on requested RA distributions and observing time pressure factors are available here (note that pressure factors are calculated differently to the above mentioned ratio of requested/allocated time). 

Criteria to be considered as any-weather programme

While there is no hard-coded limit in observing constraints to separate any-weather from a normal programme, usual criteria are that the programme should be OK with THN/THK conditions, less than average seeing (~>1.2"), and grey or bright time. There can be cases of programmes scheduled even for CLR conditions, in case seeing and Moon constraints are very loose. 

An ideal science case is where the science output even for a small sub-set of observed targets is already relevant (i.e. the proposal success does not critically depend on a full completion). Programmes with critical time dependencies for the execution of some or all observations, are generally not scheduled as any weather programmes.

For reference, programmes scheduled in P109 to P112 as any-weather programmes, cover the following constraint combination:

90% of programmes have a seeing constraint of 1.3" or worse
60% of programmes have no seeing constraint, and accept THN or THK clouds
85% of programmes accept THN or THK clouds


Execution probability for any weather programmes

Each semester, about 1000 to 1500 hours of any-weather runs (C rank) are scheduled across all VLT/I telescopes, in ~50 to 100 individual runs. Typically, about 25-50% of these scheduled observations get executed. This rate can of course vary depending on the prevailing conditions per semester and constraints of the scheduled A- and B-rank proposals.