6 Conclusion and possible extensions

We have confirmed that the incomplete scrambling action of a fiber feeding a spectrograph leaves RV errors at a level of few m/s. We have described a fiber-locked autoguider specifically built to reduce the stellar-beam geometrical fluctuations within the spectrograph; these were reduced by a factor of $\sim\! 3$ even while instantaneous image blurring was not improved. While fluctuations do not vanish, the gain is appreciable, and the system is fully compatible with a future double scrambler. On the other hand, a mere attempt to make FLAG faster does not seem worthwhile: our 0.05 arcsec RMS residual photocenter-motion under 2.2-arcsec seeing, seems to imply that the corresponding RV errors from any spectrograph will be negligible compared with those arising from changes in image blurring; these might be corrected only by AO.

Presently FLAG needs 4 plane-parallel plates and is adapted to a f/27 Coudé beam, but we can imagine other configurations with better efficiency. If many 152-cm telescope users at OHP were interested by a fiber link, FLAG could be adapted for the primary focus, which would eliminate 4 mirrors and reduce the telescope central obstruction. Also, the plane-parallel plates might be replaced by a single ellipsoïdal off-axis mirror; this would be supported by a PZT-driven tip-tilt platform and used both for scanning and guiding in X and Y. The field of acquisition/correction would be small ($\sim\! 2$ arcsec), but one might keep a pierced mirror as finder; very low-frequency guiding errors might be sent to the fine guiding telescope motors. The fiber input itself would oscillate in Z. The electronic control would be almost unchanged. A similar FLAG could also be built for the Cassegrain focus of larger telescopes.

Altogether, we make no claim to have solved the scrambling problem; we merely hope that our plain FLAG will help, as long as adaptive optics are unavailable.

Acknowledgements

We are grateful to the members of the Observatoire de Haute-Provence J.P. Sivan, D. Kohler, G. Adrianzyk, A. Moulet and G. Rau for their help. We thank J. Schmitt, M. Martic, J.C. Lebrun and F. Moulin for their help during ELODIE campaigns. The FLAG project was made possible thanks to the support received from the Institut National des Sciences de l'Univers and from the Programme National de Planétologie.