Radial velocity stability

When doing precise radial-velocity measurements it was found to be important to position the rotator at a fixed angle such that the fibers are stretched the least. The telescope parameters to achieve this are now included in the setup for FIES, while the instructions in the cookbook were updated to advice people to point the telescope such that the rotator always goes to the same position.

A new fiber-shaker was constructed from an old foot-and-leg massage machine. This ``2nd generation'' shaker (the successor to the initial LEGO device) was used for several tests. The results seem to indicate that if the seeing is stable the shaker does not have any significant effect on the stability of the measured radial-velocities, with the RMS values being below 10 m/s for both the high- and medium-resolution fiber with or without the use of the shaker. When the seeing is variable the RMS value of the radial-velocities measured with the medium-resolution fiber is much worse ($\sim$25 m/s) but when the shaker is used the RMS become $\sim$10 m/s, i.e., nearly as good as under stable seeing conditions. No tests were done yet with the high-resolution fiber under variable seeing conditions, but we do not expect this to be very different from the result for the medium-resolution fiber.

The above points to variable seeing being the cause of low stability in radial-velocity measurements and the need of having a shaker available for precise measurements on a permanent basis. Two main issues that we hope to clarify in the coming months is if there is any danger to the fibers in shaking them for extended periods during various nights, and if a reliable system can be made that is capable of doing the shaking over such extended periods.

Thomas Augusteijn 2012-02-21