EDP Sciences
Free access
Volume 412, Number 3, December IV 2003
Page(s) 897 - 902
Section Physical and chemical processes
DOI http://dx.doi.org/10.1051/0004-6361:20031432

A&A 412, 897-902 (2003)
DOI: 10.1051/0004-6361:20031432

The nature of the fiber noise with the FOCES spectrograph

Nature, modeling and a way to achieve S/N $> \mathsf{400}$
F. Grupp

Universitäts Sternwarte München, Scheinerstr. 1, 12345 München, Germany
(Received 15 April 2003 / Accepted 8 September 2003 )

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 $\approx$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 $S/N \approx 500$. 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.

Key words: instrumentation: spectrographs -- techniques: spectroscopic -- telescopes

© ESO 2003