Radial velocity stability

Prompted by the radial velocity stability degradation seen for the medium-resolution fiber with the current bundle in use, we have looked into the possibility of detecting modal noise in FIES. Modal noise may appear as a consequence of the fiber-output speckle pattern being different at different wavelength, and different as well when the fibers are moved. When a variable or moving speckle pattern is inserted into the spectrograph, the amount of light vignetted by the exit slit (for the high-resolution fiber) or the grating (all fibers) or other items that may vignette the beam will be variable as a function of time due to the inclusion/exclusion of speckle points close to vignetting edges, and hence as a function of wavelength and time, 'noise' will be recorded in the spectra.

Such modal noise will not flat-field out, as due to the fiber movements invoked by the telescope, the flat field will change as a function of time as well. Modal noise is a symptom of non-perfect scrambling of the light in the fibers, and it may be possible that the radial velocity stability problem is another symptom of this.

From the experiment it is clear that modal noise induced by fiber movements (by telescope movement) is severely limiting the maximum S/N that can be reached in FIES spectra to about S/N=450 (high-resolution) and S/N=650 (medium-resolution).

In consultation with Gerardo Avila from ESO it was decided to do some test moving the fiber around during an exposure in an attempt to scramble the average signal as much as possible and obtain a better radial velocity stability. A fiber ``shaker'' was constructed using Lego and placed in the basement of the telescope relatively close to the exit of the fiber and well removed from the part of the fiber that moves with the telescope. The linear-slide shaker operated at 1.5 Hz, with a stroke of 6.5cm. See,

http://www.not.iac.es/instruments/fies/images+drawings/fibershaker/prototype1/DSC00694.JPG
http://www.not.iac.es/instruments/fies/images+drawings/fibershaker/prototype1/DSC00696.JPG

Preliminary tests were very promising with an intra-night stability of 5m/s been achieved with the shaker switched on compared with about twice that value when the shaker was not used.

A more permanent design is being made for the shaker to be able to do more extensive tests (also because the owner of the Lego wanted his property back). To solve the issue of controlling the unit remotely a simple motor controller has been designed that can be operated via Ethernet. For the mechanical part of the shaker we are looking at modifying a leg/massage unit that has two modes of operation, one similar to the Lego shaker with a slow ($<$3.5Hz) large amplitude ($\sim$3cm) moving arm and a fast low amplitude pair of vibrating plates. Once modified we will have to test if the slow large amplitude motion or the faster lower amplitude is better. From the literature it has been found that the typical way of doing 'optical fiber mode scrambling', as it is formally referred to, is to use piezos and squeeze the bare fiber. We do not readily have access to the bare fiber but it is noted that these system use very low amplitude and high frequencies to do the scrambling.

Information was added to the web pages about the simultaneous sky mode observing a target and the sky with the two medium-resolution fibers. Instructions were provided on how to calibrate the relative throughput of the two fibers.