Requirements for AO-ELT operation and ELT site monitor

Abstract

Performance of AO systems on ELTs depends not just on seeing; other parameters such as strength of high-altitude turbulence are more relevant for the laser tomography. Nights good for some instruments are not so good for others, so there is a clear advantage to go from the simple queue scheduling on seeing to a more complex strategy of selecting in a multi-parameter space. The need to know those atmospheric parameters in real time drives the requirements to the ELT site monitors and their data. It is suggested to measure atmospheric parameters and internal seeing along the line of sight by an internal seeing monitor, complementing the external site monitor. 1 Standard and advanced queue scheduling Extremely large telescopes (ELTs) will work with adaptive-optics (AO) most of the time. The per- formance of AO and, consequently, the science output of ELTs, strongly depend on the atmospheric conditions. Operation of ELTs in queue-scheduled (QS) mode, when observations are done under op- timum, rather than random, conditions, is an obvious choice. Modern large telescope already work in the QS mode driven by the seeing measured by site monitors. The QS on seeing will not be the best choice for ELTs, however. Their AO systems will use laser guide stars (LGSs) and tomography and their performance will mostly depend on the turbulent conditions in the high atmosphere. Therefore, a better choice will be to schedule observations on relevant atmospheric parameters related to the quality of AO science. Those parameters will differ, depending on the type of AO. Let us call this strategy advanced queue scheduling (AQS) to distinguish it from the standard queue scheduling (SQS) on seeing and the classical scheduling (CS) when nights are assigned in advance without regard to atmospheric conditions. Considering that ELTs and their instruments represent large investment, even a modest gain in productivity achieved by changing the strategy from SQS to AQS is worth the effort. In this contribu- tion, SQS and AQS are compared on two examples using real atmospheric data. Then we detail the requirements to the ELT site monitors and their data products as needed for the AQS.