Critical Issues

1. The definition of the baseline relevant for narrow-angle astrometry observations. This is defined by an image of the metrology system retroreflector, situated in the StS. The location, tilt and curvature of this image and the corresponding image plane of the stellar beams are crucial to the baseline determination. See Sections 21 and 15 for further details.
2. Numerical simulations indicate that wavefront corrugations across the telescopes may make fringe tracking very difficult with the PRIMA FSUs (see Section 28). It will be important to do a more detailed study of this.
3. The dependence of the phase output in the two beams in StS calibration mode on atmospheric seeing and refraction effects is of some concern as this is the fundamental calibration for PRIMA astrometry. This will have to be investigated in detail to determine whether it can be solved with an appropriate observing strategy. A brief discussion is included in Sections 3 and 5.
4. The dependence of the spectral sensitivity of PRIMA on the seeing conditions due to the effect of spatial filtering, will have to be studied in detail. An accurate model of the FSU spatial filters will have to be developed in order to determine the seeing dependence of the sensitivity of PRIMA.
5. Other potential show-stoppers from items being investigated by Richard Mathar, Murakawa Koji and Rudolf Le Poole.