projects:micado:atmosphere [OmegaCEN]

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= OmegaCAM technical time proposal: Atmospheric effects on astrometry

The turbulent behavior of the atmosphere causes jitter in the apparent position of sources on short (subsecond) timescales. This thus limits achievable astrometric accuracy.  The adaptive optics system counters the turbulent behavior by determining and applying a real-time a correction to mirror shapes that optimizes the spatial resolution  optimally at a single location in the FoV. Away from this point the correction also has an effect, but corrects less optimal. 

Current image simulations for MICADO at the E-ELT take as input simualated atmospheres based on extensive measurements of atmsopheric turbulence properties (e.g., [[http://ao4elt3.sciencesconf.org/13383/document | Sarazin et al., 2013]]). 

Adaptive optics system optimize for image spatial resolution, not for astrometric accuracy. The combined effect of atmospheric turbulence and deformation of mirrors has a complex impact on astrometry. Studies on the direct impact of atmospheric turbulence on astrometry (i.e., without AO correction) are scarce on time scales ranging from minutes up to a full night in combination with simultaneous coverage of a wide range of baselines, spanning from the arcsecond-scale to the tens of arcminute-scale. 

We therefore propose a small pilot with OmegaCAM on the VST in bright technical time to measure the power spectrum of astrometric variability on small and large baselines in space and time which has an atmosphere expected very similar to the E-ELT atmosphere only 20 miles away. The combination of a seeing limited and stable PSF over a wide FoV (1sq.deg.) makes OmegaCAM a very appropriate instrument to determine  We propose  a night-long sequence of short (39 sec) i-band observations of a dense, but not crowded, stellar field (e.g., a southern standard field) that crosses zenith around midnight during technical time. The dithered observations will be unguided to optimize observing efficiency. This strategy follows the successful short-exposure observing strategy of the OmegaWHITE program now running for several years on the VST. 

Details of the observations will be finalized with OmegaCAM instrument staff.