VLT Laser Guide Star Facility Policies
In the following we describe the Paranal Science Operations policies for the operation of the 4 Lasers Guide Star Facilities (4LGSF).
Operational impact of the LGSFs
The operation of the 4LGSF creates two sources of potential interference with the observations of neighboring telescopes on Paranal (UT1,2,3, ATs, and VISTA and VST): first the 4 laser guide stars themselves, and second the Rayleigh and Mie scattered laser cone (all at 5892 Å). In case any of those sources contaminate the observation of another telescope, we refer to a beam collision. To avoid beam collisions, a minimum separation of several arcminutes between the Rayleigh cone or the laser star and a non-laser observation needs to be maintained. Note that the telescope active optics is equipped with a notch filter rejecting the light from the laser.
For the laser avoidance zone it has to be considered that the laser guide stars and the laser cones appear at different positions on the sky when seen from different telescopes. These effects are taken into account by the laser visualization and beam collision tools available from each Telescope ('LTCS'). Scientific observations in a wavelength range that include the laser wavelength (at 5892 Å) will be affected by a collision. In case of such a beam collision, either UT4 or the other telescope needs to yield, i.e., interrupt its observation and point to a different target avoiding collision.
The rules that define the priorities in case of such beam collisions are defined hereafter.
Science Operations Policies for LGSF Observations
The following priorities are applied for science operations with the 4 Lasers Guide Star Facilities currently available in Paranal.
4 Laser Guide Star shall be operated only when the Aircraft Avoidance System (AAS) is operational and online. Operations using any of the lasers shall immediately be suspended should this system be temporarily unavailable. Operation can only be resumed if the AAS is operational again.
The LTCS will provide real time information on ongoing or predicted beam collisions with any of the other Telescopes on the mountain while the laser is propagating. Information is available from both the real time tool accessible from each telescope console and from the Visitor Observation Tool (vOT) used to execute the observations.
In case of issues with the LTCS, the UT4 Night Support should discuss with the operators of the other telescopes, to check before each preset for possible beam collision with on-going scientific observations on other telescopes.
If a collision between UT4 LGSF and any of the non-LGSF telescopes is ongoing or predicted, the following priorities apply:
(o) If the science observations carried out with the non-LGS 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.
Exceptions to this rule are only considered if the yielding program were to consist of a single (or very few) targets that are scheduled to be observed in the single specific slot, while the program that pointed first has a large suitable target list and/or a large time allocation, thus allowing flexibility. Paranal staff will communicate to and coordinate such cases with the affected visitors, to minimize any potential related time losses.
(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 their 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-LGSF 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.
(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-mode observations should be aborted. Exception is made when the LGS-mode observations are nearly completed (>75%), then the visitor is asked permission to wait that the LGS-mode observations are completed prior to stop propagation of the laser. If the visitor refuses and needs immediate access to the field, the LGS-mode observations should be aborted.
Additionally, the following rules and guidelines govern the operations using the 4LGSF:
- 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 in this context are understood as those that were defined as such in the Phase 1 proposal and that have to be executed within the same night to remain without their defined time window (e.g. transits), the Rapid Response Mode observations, and some Target of Opportunity observations that have to be executed within the same night to remain within their validity period.
- Weather conditions:
- If the weather conditions are such that the transparency is "Thin", observations with the 4LGSF can proceed. In case of conditions such that the transparency is "Thick" observations with the 4LGSF become unfeasible, and therefore lose any priority.
- N.B.: With "Thin" cloud coverage the possibility exists that the laser light is reflected by clouds and thus visible over a larger area than nominally checked for collision. In this case, special attention to the data for laser sensitive instruments with coordinates close (~10 degrees) to the laser position needs to be paid. If contamination by the laser light is detected at a level that affects the data quality, the same rules (i) to (iv) as above apply.
- If during "Thin" conditions, visitor data on a non-LGS telescope are severely affected by such reflected light at a position where no nominal beam collision was detected, compensation for the lost time can be considered on a case by case basis.