VLT Laser Guide Star Facility Policies

(Prepared/Revised: O.Hainaut 2007-03-31 - Approved: A.Kaufer 2007-04-01 - Released: C.Cesarsky 2007-04-01)

Purpose of this Document

In the following we describe the Paranal Science Operations policies for the operation of the Laser Guide Star Facility (LGSF). These policies are in place since the beginning of the ESO Observing Period 79.

The complete LGSF Operations Plan is described in VLT-PLA-ESO-11860-2290.

Beam Collision

The operation of the LGSF creates two sources of potential interference with the observations of neighbouring telescopes on Paranal (UT1,2,3, ATs, ASM, and in the future VISTA and VST): first the laser guide star itself, and the second the Rayleigh and Mie scattered laser cone. In case any of the two sources contaminates and/or interferes with the observation of another telescope, we refer to a beam collision

To avoid beam collisions, the minimum separation between the Rayleigh cone or the laser star and a non-laser observation is 3 arcmin, equivalent to a flux decrease of 3 orders of magnitude. For this avoidance zone it has to be considered that the laser guide star, and in particular, the laser cone appear at different positions on the sky when seen from different telescopes.

Science Operations Policies for LGSF Observations

Safety policy: the LGSF shall be operated only when the Aircraft Avoidance System is operational and online. LGSF operations shall immediately suspended should this system be unavailable.

As a general policy, also followed by other astronomical observatories,
observations performed with the LGSF have lower priority than observations on any other telescope.
This general policy is implemented on Paranal as follows:

  • For every new preset of UT4 and LGSF, the Telescope and Instrument Operator (TIO) checks for conflicts with on-going observations on the other telescopes before launching the laser beam. In case of conflict, the incipient LGSF observation is aborted.
  • For every new preset on another telescope, the corresponding TIO checks for collisions with the laser beam during the coming observations. If a conflict is identified, the UT4 TIO will be notified accordingly. Then,
    • if more than 3/4 of the total execution time of the currently executing UT4 Observing Block (OB) have been completed, this observation will continue until its normal ending. The OB execution time is displayed on the P2PP/OT tools and includes the time for the execution of the acquisition template and the science observing templates,
    • otherwise, the LGSF is shut off within one minute of receipt of the conflict warning. This rule applies equally to Service Mode (SM) and Visitor Mode (VM) observations.

The general policy is amended as follows:

  • In case of a beam collision between LGSF observations in Visitor Mode and another telescope in Service Mode:
    • For every new preset of UT1, 2, 3, VST or VISTA, operated in Service Mode, the TIO checks for conflicts with on-going visitor mode LGSF observations on the UT4. In case of conflict, the incipient Service Mode observation is aborted.
    • For every new preset on UT4 operated in Visitor Mode, the UT4 TIO checks for collisions between the laser and that of another telescope operated in Service Mode during the coming observations. If a conflict is identified, the other telescope's TIO will be notified accordingly. Then,
      • if more than 3/4 of the total execution time of the other telescope's current OB have been completed, this observation will continue until its normal ending. The OB execution time includes the acquisition template and science templates, as displayed on the P2PP/OT tools.
      • otherwise, the other telescope's OB is terminated within one minute of receipt of the conflict warning.
  • The 3 other possible combinations (LGSF and other in VM, LGSF in SM and other in VM or SM) are handled by the general policy above.
  • <>Scientific observations have priority over technical or commissioning activities, notwithstanding on which telescope these observations and activities are taking place.
  • Time critical scientific observations have priority over any other type of observations, notwithstanding on which telescope they take place (laser or no laser). Time critical observations include those that were defined as such in the Phase 1 proposal and that have to be executed within a short time slot, the Rapid Response Mode observations, and some Target of Opportunity observations that have to be executed within a short time slot.
  • If the weather conditions are such than the fractional lunar illumination is >50% and the transparency is "Thin" or "Thick", the LGSF observations become unfeasible, and therefore lose any priority they could have gained through previous amendment of the general policy.

Observational Mode

Meteorological and atmospheric conditions have a much greater impact on AO-LGS observations than on classical observations. As these conditions can change rapidly during the night, even short term planning is difficult.

In order to achieve the necessary flexibility with minimum negative impact on the scientific output of UT4, and to gain the necessary on-site experience, AO-LGS observations are currently restricted to Service Mode only (with the exception of LGS GTO observations which are executed in Visitor Mode only).

 
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