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Restrictions due to the LGS
Use of the Laser Guide Star imposes some restrictions
Targets must be submitted to the Laser Clearing House at least one week in advance. Usually, each target will have 1 to 5 shutter windows. Typical shutter window are of the amount of few seconds only. Nevertheless, this creates some overheads, as we need to open all the laser and laser related loops, and re-close them once window is cleared. We estimate that the overheads due to LCH window is about 1 to 3 minutes / observation hour.
We are using a combination of laser spotters and radar information coming from the DGAC (Chilean Department of Aviation) to monitor the sky for planes. Since the beginning of GeMS commissioning in 2010, we have been gathering air traffic statistics. The data shows that most of the air traffic is concentrated during the first three and last two hours of the night. Low elevation targets are more susceptible to being in an airplane route. Overheads due to airplanes are slightly longer than LCH shutter windows, on the order of 2-5 minutes, as we must wait for the plane to be sufficiently clear of our target before we can propagate the laser.
The laser cannot be propagated in presence of clouds because spotters cannot detect the presence of airplanes. In case of thin cirrus, the laser can be propagated, but the absorption of the cirrus may reduce consequently the laser brightness, and affect the AO performance.
For the same reasons as with clouds, the laser cannot be propagated during twilights.
Sodium Season, Sodium Return
The brightness of the Laser Guide Star changes with the Sodium abundance during the year. We observe a yearly variation by a factor of three, with a low season during the Chilean summer, and high season during winter. On top of this yearly variation, strong variations (also by a factor 2 to 3) are observed from night to night, and within a night. As a matter of fact, during low season, low return night, we have to run the AO loop at a lower rate to cope with the low return. This can affect the performance, or sometimes even prevent us from observing. We are still gathering data in order to characterize properly this effect, and provide an efficient queue planning.