SIMBAD references

The precision premium, a concept in astrometry that was first presented by Pascu in 1994, initially means that the relative positional measurement of the Galilean satellites of Jupiter will be more accurate when their separations are small. Correspondingly, many observations have been obtained of these Galilean satellites since it was introduced. However, the exact range of separations at which the precision premium takes effect is not clear yet, nor the variation of the precision with separation. In this article, observations of open cluster M35 are used to study the precision premium and the newest star catalogue Gaia DR2 is used in data reduction. Our results show that the precision premium applies at less than 100 arcsec for two specific objects and the relative positional precision can be well fitted by a sigmoidal function. Observations of Uranian satellites are also reduced as an example of the precision premium.