The nature of the fiber noise with the FOCES spectrograph - Nature, modeling and a way to achieve S/N (greater than)400

The non-white noise noticed using the fiber-fed FOCES spectrograph is recognized as the result of vignetting to an uneven distribution of monochromatic light in the spectrograph beam behind the fiber exit. This phenomenon - that can be assigned to every fiber-coupled spectrograph with vignetting of the light path behind the fiber - is qualitatively and quantitatively described and modeled. The S/N ratio that seemed to be limited to (almost equal)200 when surrounding the object exposure by two flatfield exposures is shown to reach the theoretical limit (defined by the linear range of the CCD-detector) of S/N (almost equal)500. This is achieved by using multiple flatfield exposures in combination with a fiber vibration device allowing non-harmonic movement of the fiber during exposure time.