Julien Spronck (Yale) : Fiber Scrambling for Extreme Doppler Precision

The detection of Earth-like exoplanets with the radial velocity method requires extreme Doppler precision and long-term stability in order to measure tiny reflex velocities in the host star. Recent planet searches have led to the detection of so called “super-Earths” (up to a few Earth masses) that induce radial velocity changes of about 1 m/s. However, the detection of true Earth analogs requires a precision of 10 cm/s. One of the factors limiting Doppler precision is variation in the instrumental profile from observation to observation due to changes in the illumination of the slit and spectrograph optics. Thus, this stability has become a focus of current instrumentation work. Fiber optics have been used since the 1980’s to couple telescopes to high-precision spectrographs, initially for simpler mechanical design and control. However, fiber optics are also naturally efficient scramblers. Scrambling refers to a fiber’s ability to produce an output beam independent of input. Our research is focused on understanding the scrambling properties of fibers with different geometries (circular, octagonal), different lengths and fiber sizes. We also report all results obtained with fiber scramblers on the Hamilton spectrograph at Lick Observatory and on the HIgh-Resolution Echelle Spectrometer (HIRES) at Keck Observatory. We demonstrate an improvement in the stability of the instrumental profile by a factor 10 or more using these fiber scramblers. Additionally, we present data obtained with a double scrambler at Lick Observatory that further improves the stability of the instrument by a factor 3. These results show that errors related to the coupling between the telescope and the spectrograph are the dominant source of instrumental noise at Lick and Keck observatories.