VLT Laser Guide Star Facility Policies
Revision History: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). LGSF policies have been in place since the beginning of the ESO Observing Period 79. This policy has been reviewed and updated after one year of operations. The present version is valid as of P81.
The complete LGSF Operations Plan is described in VLT-PLA-ESO-11860-2290.
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 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 the laser cone appear at different positions on the sky when seen from different telescopes. These effects are taken into account by the laser visualization tools running on each telescope. On some of the foci, the active optics is equipped with a notch filter rejecting the light from the laser. On these foci, scientific observations in a wavelength range that does not include the laser are not affected, so the beam collision can be ignored.
Science Operations Policies for LGSF Observations
The following priorities are applied wrt the Paranal LGSF science operations.
Safety policy
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.
Operations
For every new preset of any telescope, the Telescope and Instrument Operator (TIO) checks for bean 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 non-LGSF telescope is equipped with notch filter, and the corresponding science observations are not affected by the laser, the collision can be ignored.
(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.
- Weather conditions: 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. As a result, in order to secure maximum efficiency of UT4 time, general AO-LGS observations are currently restricted to Service Mode only; the only exception is made for LGS GTO observations which are executed in Visitor Mode.