VLT Laser Guide Star Facility PoliciesRevision History:
2013-02-19 Revision after LGSF upgrade to PaRLa, cdumas
2009-03-06 Revision wrt use of LGS in VM starting P84, cdumas
2008-02-25 1yr revision: priorities and cleanup, cdumas, ohainaut, akaufer
2007-08-28 akaufer/ohainaut, updated for Aircraft Avoidance System
2007-03-31 ohainaut, created from LGSF operation manual (Ageorges et al) and Science Recovery policies (Lidman et al)
2007-04-01 original version, prepared Hainaut, approved Kaufer, Released Cesarsky
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 have been in place since the beginning of the ESO Observing Period 79. The present document has been reviewed in the light of the recent (Feb. 2013) upgrade of the LGSF and further updates will be done when the new system performances have been fully characterized, as well as we have better determined the the impact on the operations of all instrument modes and of the survey telescopes (e.g. beam colision and minimum acceptable separation).
The complete LGSF Operations Plan is described in VLT-PLA-ESO-11860-2290 (also to be updated).
Operational impact of the LGSF
The operation of the LGSF creates two sources of potential interference with the observations of neighboring telescopes on Paranal (UT1,2,3, ATs, ASM, and 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 (TBC).
For this avoidance zone it has to be considered that the laser guide star and the laser cone appear at different positions on the sky when seen from different telescopes. Some of these effects are taken into account by the laser visualization tools running (or soon to be installed) on each telescope, whose active optics is equipped with a notch filter (on most PO telescopes) rejecting the light from the laser. As a result, scientific observations in a wavelength range that does not include the laser are likely to be not affected, so the beam collision can be ignored (if the telescope is equipped with a notch filter).
Science Operations Policies for LGSF Observations
The following priorities are applied wrt the Paranal LGSF science operations.
The LGSF shall be operated only when the Aircraft Avoidance System is operational and online. LGSF operations shall immediately be suspended should this system be temporarily unavailable.
The UT4 operations team should inform the other telescope teams as early as possible about the coordinates of the targets that will likely be observed with the LGS during the upcoming night.
The UT4 TIO uses the LGS monitoring tools, and/or discusses with the operators of the other telescopes, to check before each preset for beam collision with on-going scientific observations on other telescopes. If a collision between LGSF and non-LGSF telescopes is suspected, the following priorities apply:
(o) If the science observations carried out with the non-LGSF telescope are done at a wavelength range not affected by the laser, the collision can be ignored (assuming the telescope is equipped with a notch filter).
(i) Conflict between two telescopes in Service Mode:
Observations are done on a
"first-come, first-served" basis. In other words, if
non-LGS service mode scientific observations have been started and
there is a risk of impact with the LGS observations to be initiated,
then the LGSF propagation cannot be activated.
Similarly, if the LGS observations have already been initiated and the non-LGS observations to be started run a risk of collision with the laser, then the non-LGS preset should not be initiated.
(ii) Conflict between two telescopes in Visitor Mode
Here again, the rule of "first-come, first-served" is applied. If there is a risk of collision between the observations of the two visitors, the first telescope presetting to a particular field has the priority to carry out the observations.
(iii) Conflict between UT4-LGS in Visitor Mode and any other telescope in Service Mode
The visitor using UT4-LGS has priority to preset to his/her field
of interest and any observations carried out with the non-LGS
telescope must be aborted immediately.
Exception is made when the non-LGS observations are nearly completed (>75%), then the visitor is asked permission to wait that the non-LGS observations are completed prior to propagate the laser. If the visitor refuses and need immediate access to the field in LGS-mode, the non-LGS observations should be aborted (time loss accounted as execution).
(iv) Conflict between UT4-LGS in Service Mode and any other telescope in Visitor Mode
Similarly to the case described above, the visitor has priority to carry-out the non-LGS observations and the LGS observations should be aborted. Exception is made when the LGS observations are nearly completed (>75%), then the visitor is asked permission to wait that the LGS observations are completed prior to stop propagation of the laser. If the visitor refuses and need immediate access to the field, the LGS observations should be aborted (time loss accounted as execution).
- Scheduled Scientific observations have absolute priority over technical or commissioning activities, regardless of the telescopes involved.
Time critical scientific observations have absolute priority over any other type of observations, regardless 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.
- Weather conditions: If the weather conditions are such that the transparency is "Thin" or "Thick" the LGSF observations become unfeasible, and therefore lose any priority.