The nature of thefibernoise with the FOCES spectrograph

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 ratio that seemed to be limited to ≈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 . 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.