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One of the main methods used for finding extrasolar planets is the radialvelocity technique, in which the Doppler shift of a star due to an orbitingplanet is measured. These measurements are typically performed usingcross-dispersed echelle spectrographs. Unfortunately such spectrographs arelarge and expensive and their accurate calibration continues to be challenging.The aim is to develop a different way to provide a calibration signal. Acommonly used way to introduce a calibration signal is to insert an iodine cellin the beam. Disadvantages of this include that the lines are narrow, do notcover the entire spectrum and that light is absorbed. Here I show thatinserting a birefringent element or an imaging Michelson, combined withWollaston prisms eliminates these three shortcomings, while maintaining most ofthe benefits of the iodine approach. The proposed designs can be made verycompact, thereby providing a convenient way of calibrating a spectrograph.Similar to the iodine cell approach, the calibration signal travels with thestellar signal, thereby reducing the sensitivity to spectrograph stability. Theimposed signal covers the entire visible range and any temperature drifts willbe consistent and describable by a single number. Based on experience withsimilar devices used, in a different configuration, by the Helioseismic andMagnetic Imager, it is shown that the calibration device can be made stable atthe 0.1 m/s level, over a significant wavelength range, on short to medium timescales. While promising, many details still need to be worked out. Inparticular a number of laboratory measurements are required in order tofinalize a design and estimate actual performance and it would be desirable tomake a proof of concept.

Using birefringent elements and imaging Michelsons for calibration of high precision planet finding spectrographs