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Unpredictable displacements in the photocentre of an optical feed at the entrance slit of a spectrograph produce
corresponding barycentre offsets that impose limits to very high resolution schemes. These limitations not only apply to
direct light from a science object, but also light relayed via an optical fibre or image slicer. Several mitigation strategies
are in development or are currently in use, however these all have potentially restrictive idiosyncrasies.
An alternative approach is proposed to remove displacement effects from the spectra by nulling barycentre offsets.
Correction is achieved by time-integrating at the detector a sequence of multiple normal and 180-degree inverted images
of the input aperture, thus eliminating optical asymmetries about the axis of inversion, which is aligned orthogonal to the
spectral direction. The flip is generated with a path-length compensated, non-dispersive ‘reversion prism’, driven on a
high precision translation stage. The prism is periodically chopped in and out of the beam, and the resulting time-averaged
image thus has an imposed central axis regardless of barycentre shifts.
The method works regardless of the specifics of the spectrograph feed (fibre, multiple fibres, slit, slicer etc.) With a
relatively simple and inexpensive scheme it should be possible to stabilise an image to better than one part in 104 potentially permitting detection down to cms-1 regimes.
The concept is currently at a very early stage of development, so this paper outlines the basic principles and details a
practical reversion component that is currently under development at Durham CfAI. There then follows a description of
how the component will be implemented in a laboratory prototype scheme. The paper concludes with a proposed test
plan and suggests the focus for future work.
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