  We demonstrate that artificial bipolar structure can be detected using
spectro-astrometry when the point spread function (PSF) of a point source
suffers distortion in a relatively wide slit. Spectro-astrometry is a technique
which allows us to probe the spatial structure of astronomical sources on
milliarcsecond scales making it possible to detect close binaries and to study
the geometry and kinematics of outflowing gas on scales much smaller than the
seeing or the diffraction limit of the telescope. It is demonstrated that
distortion of the PSF, caused by tracking errors of the telescope or unstable
active optics during an exposure can induce artificial signals which may be
misinterpreted as a real-astrometric signal. Using simulations we show that
these may be minimised by using a narrow slit relative to the seeing. Spectra
should be obtained at anti-parallel slit position angles (eg. 0 and 180
degrees) for comparison in order to allow artificial signatures to be
identified.

Astrophysics