Limitations due to PRIMA hardware

The current design of the PRIMA FSU hardware should be compatible with both phase-stabilised operation (Section 2.2.5) and non phase-stabilised operation (Section 2.2.3). The low spectral resolution of the FSUs is slightly better optimised for phase-stabilised operation, however. If the data transfer architecture at the VLTI precludes the use of $C_{PS}'C_{SeS}'^{*}$ as the primary observable for PRIMA, non phase-stabilised operation will be impossible and the limiting magnitude for phase-stabilised operation will be slightly poorer. For the remainder of this document I will assume that astrometric measurements are based on $C_{PS}'C_{SeS}'^{*}$ provided from both phase-stabilised operations and non phase-stabilised operations of PRIMA.