Detailed contributions

sect:ctb-1-1-1-seeing

The largest contribution from atmospheric seeing on the measured astrometric parameters will be a zero mean process. Isoplanaticity will be the principle limitation on the separation of target star and reference star. Systematic errors in the astrometry can be generated in the StS calibration mode through a combination of seeing fluctuations and atmospheric refraction away from the zenith. This would encourage the use of an observing strategy where the PS and SeS were switched regularly between alternate FSUs.

Atmospheric seeing is introduced through the combination of:

1. Atmospheric turbulence, typically between shearing wind layers
2. Fluctuations in the density of air or in the density of water vapour, due to changes in temperature, humidity and to a much lesser extent pressure