Astrometric reductions of ESO-Schmidt platesfn1fn2

We have performed a series of astrometric reductions of ESO-Schmidt plates. In this paper we compare the results of reductions with linear transformation formulae with the results of reductions with the full third degree transformation formulae. We find that there are only small differences between both reductions, provided that an equidistant projection is used between the sky and the tangential plane rather than the usual gnomonic projection, and that enough reference stars are used. These small differences are almost entirely due to the larger errors in the third-degree reduction, showing the superiority of linear transformation formulae over third degree formulae for ESO-Schmidt plates. Reductions using only stars from a certain magnitude class show that there is no measurable magnitude effect. And finally, comparing reductions using only one single plate with reductions using the mean measurements of seven plates show that taking the mean of seven plates does improve the positions.