1. Preliminary Remarks and Definitions

Target of Opportunity Observations (ToO) are needed to follow-up transient phenomena of great  scientific interest.  There are basically two types of events that trigger ToO:

• Unpredictable sudden astronomical events which require urgent or immediate observations. Those should be submitted as Director Discretionary Time (DDT) proposals, following the procedure described in the DDT Proposal web page. In this document, we refer to these ad DDT ToOs.
• Events predictable in a generic sense only. This is typically (but not limited to) transient phenomena, and follow-up or co-ordinated observations of targets of special interest. Telescope time to observe these events is obtained using the normal proposals for observing time that go through the OPC review. In this document, we refer to these as OPC ToOs.

Because the support scientist on La Silla are already fully occupied with normal observing programmes, ToO impose an additional, often heavy, work load on them. ToO can also consume a significant fraction of a scheduled observing programme.  Therefore, it is necessary to restrict ToO's in order to limit their effect on staff and scheduled programmes, while preserving their scientific return. This document describes the restrictions to be applied at La Silla to achieve this goal.

Also, to ensure the success of the ToO observations, it is critical that La Silla receives all the information needed in the shortest possible time. This document describes the information needed and the procedure to make it available to La Silla.

2. Delegation of Authority

2.1- DDT ToOs

DDT ToO's can only be authorized by the following persons:

• The Director General or his delegate (the DDTC)
• The Director of La Silla 

The normal procedure is that the proponent sends a request to Director's Discretionary Time Committee (DDTC) following the procedure described in the DDT Proposal Page. The DDTC transmits its decision to La Silla. During holiday periods, week-ends, or after working hours in Garching, the proposals are transmitted automatically to La Silla. For these cases, upon receipt of a proposal, the La Silla Shift Leader must contact the La Silla Director to request approval of the observations. When the Director cannot be reached, his authority is automatically delegated to the Head of SciOp, then to the Head of Astro, then to the Shift Leader.

2.2- OPC ToOs 

Only the PI of a OPC ToO program can trigger an observation for that program. For large consortia, the PI can send in advance a list of authorized triggerer. An observation triggered by a Co-I not included in that list will be rejected. Once a trigger is received from an authorized person, the SciOp Shift Leader will check that the requested observations fit within the available time quota for that program.

3.ToO Rules

3.0 Dead line

ToO trigger must reach La Silla before 16:00 local time.

3.1 ToOs on National Telescopes

ToO observations on national telescopes during national time must be requested through the chairpersons of the corresponding committees, to be executed either by the scheduled observers, or by La Silla staff.  In the latter case, the La Silla Shift Leader must be kept fully informed, and s/he will follow the rules described below.

3.2 Trigger of ToO observations

An OpC ToO trigger must be submitted using the ToO Web Form available on the SciOp Web Page, and it must include the name of the Observation Block(s)  (OBs) to be executed. In case these OBs cannot be updated for coordinates of the objects, the coordinates must be included in the form.
A DDT ToO trigger must by submitted following the procedure listed on the DDT page. Once the program is accepted, you will receive further instructions. In case of extreme emergency, a request can be sent to lasilla@eso.org. Ideally, this should include the name of the OB to be executed. However, considering the urgency of the observations, this can be relaxed to a detailed description of the OBs.

3.3 Time spent on ToO

• Telescope Time (TT) is the total time spent for the observations. TT includes time to change configuration, point the telescope, acquire guide stars, expose, read-out, calibrate, etc.  It is measured from the time the telescope is taken over by the ToO observer, until the time the scheduled observer begins the first exposure after the ToO.
• Shutter time (ST) is the time during which the telescope is collecting photons on the ToO target. ST does not include pointing, acquisition, or calibrations.

The OPC recommends programmes for telescope time (TT) allocation for OPC ToO observations. The Director (or DDTC) allocate shutter time (ST) for emergency DDT ToO observations. 

3.4 Restrictions

• ToO observations cannot exceed the following limits of telescope time:
  • OPC ToOs: 3 hours of TT per run (i.e. scheduled programme) or per 3 nights in the case of longer runs (within the limit of  the total OT approved by the OPC).
    
  • DDT ToOs: 2 hours of TT per run or per 3 nights for longer runs.

• No ToO observations can be scheduled during 1-night runs.
• ToO observations requiring more time than specified above must be spread over consecutive observing runs, if justified.
• ToO cannot be imposed over time-critical scheduled observations as established in the OPC-approved proposal (e.g. occultations).
• Follow-up and monitoring ToOs must be scheduled in Service, Reserved, Technical or Idle nights. If not at all possible, the timing of the follow-up or monitoring observations must be adjusted in order to minimize the impact on regular programs.
  

The above restrictions can only be over-ridden by the Director General or the Observatory Director, and this approval will be granted only in very exceptional cases.

To summarize, the table below lists the maximum number of hours that can be allocated to ToOs.

4.Compensation time

Observers of Visitor Mode programs interrupted by a ToO for more than 1 hour TT (per run or 3-night period) may request compensatory observations in "Queue Scheduling" Service Mode by the La Silla Science Operation Department., for a total telescope time less than or equal to the time they lost minus 1h (e.g. a program that has lost 2.5h can request 1.5h compensation). Whenever possible, SciOp will schedule and execute these observations during special nights reserved for this purpose.  The affected visitor will have to submit a reduced "Phase II" package using the ToO Web Form, including

• list of OBs to be executed, totalling to the TT lost.
• finding charts, mandatory for spectroscopy
• Strategy instructions
  

SciOp will attempt to schedule the lost OT on a best effort basis, subject to the usual astronomical constraints (weather, coordinates, moon, etc.).  Since in general it may not be feasible to compensate all the lost time, priority will be given to the observers who lost the largest fractions of their scheduled time. The pending compensations will be declared obsolete and removed from the queue at the end of the visibility period of the OBs.

5- Requirements to ToO PI's

5.1- OPC ToO

In order to be scheduled, an OPC-approved ToO PI must provide the following information:

• Phase II package, as described in the Phase II instructions, including
  • a README 
  • a set of OBs with dummy coordinates

This Phase II package has to be submitted before the Phase II deadline defined by SciOp, and has to be approved by the corresponding support astronomer.

• With ToO Web Form triggering the alert, the PI must submit
  • a list of the OBs to be executed. Ideally, the OBs should be updated in the Repository, but if that is not possible, the coordinates can be included in the email
  • a list of calibrations to be acquired (either generic "standard calibrations", or a list of pre-defined OBs).
  • Upon reception of the confirmation email from the ToO Web Form, a finding chart of the target if it is identified must be sent following the instructions in the confirmation email.

5.2- DDT ToOs

For DDT ToO proposals, an effort should be made to provide as much as possible of this information, and the PI must be available for questions before and during the observations. The following information must be included in the ToO Web Form

• A description of the observing strategy
  
• A set of OB descriptions:
  
  • Precise coordinates precessed to J2000 and integrated to the epoch of observations
  • Exposure times
  • Filters with ESO numbers for non-standard filters
  • Wavelength range and resolution. Grism or grating ESO number and central wavelength when applicable.
  • Slit position angle if relevant.
  • Detector setup (readout speed, binning, windowing)
  • Any constraints such as time, seeing, photometric conditions, moon distance, airmass,  etc., must  be clearly specified. For example, for transient sources such as GRB it is critical to observe  the sources as soon as possible after the alert.  This and any other science constraints  must be clearly specified in order to preserve the scientific value of the observations
• Calibrations in addition to those provided by the calibration plans must be specified  completely.  Photometric and spectro-photometric standards will be  selected by the observer unless  indicated otherwise by the proponent.
• A finding chart must always be provided for spectroscopy.  For imaging, it should be clearly indicated whether the field is to be checked, and a finding chart provided accordingly.  The scale and field of the finding chart should be appropriate to the crowding and magnitude of  the targets and correspond approximately to the wavelength of the ToO observations. The finding chart must be submitted as a PostScript file (no other format accepted), emails following the instructions given in the confirmation email sent by the ToO Web Form.