The NEVEC Group are focussing on the following sources of error in PRIMA astrometric measurements
The definition of the baseline relevant for narrow-angle
astrometry observations, and the error in the definition of this
baseline (e.g. from misalignment of optical components). The baseline
is defined by an image of the
metrology system retroreflector, situated in the Star Separator module of each Auxiliary Telescope (AT). The location,
tilt and curvature of this image and of the co-located image plane
of the stellar beams are crucial to the baseline determination. A point in the
AT pupil plane is conjugate to this image plane for each AT, and
this defines the baseline. For further information read
this and
this.
Numerical simulations indicate that wavefront corrugations
across the telescopes may make fringe tracking very difficult with
the PRIMA FSUs (click
here for
further information). The NEVEC group are studying this in more
detail in order to determine whether the narrow-angle astrometric performance
can be improved by e.g. increasing the number of spectral channels in the FSU.
The dependence of the phase output on atmospheric seeing and refraction effects in the two beams from the Star Separator module
when it is in calibration mode is of
some concern as this is on of the fundamental calibrations for PRIMA
astrometry. This will have to be investigated in detail to determine
whether it can be calibrated out with an appropriate observing strategy.
Brief discussions can be found here and
here.
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. Preliminary work with a
simplified model for the spatial filter can be found here.
The deformation of the delay line tunnel, and the resulting impact
on the stability of the light beams and the position of re-imaged pupils.