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The high-resolution FIbre-fed Echelle Spectrograph (FIES)

FIES is a cross-dispersed high-resolution echelle spectrograph with a maximum spectral resolution of R = 67000. The entire spectral range 370-830 nm is covered without gaps in a single, fixed setting. Extended coverage up to 910 nm is available with minor inter-order wavelength gaps. To isolate it from sources of thermal and mechanical instability, FIES is mounted in a heavily insulated building separate from and adjacent to the NOT dome.

The optical fibres that connect the telescope with the spectrograph are permanently mounted with a movable 45-degree mirror near the focal plane of the telescope. The fibres run from the instrument adapter through the telescope and azimuth axis to the spectrograph enclosure. Being a permanently mounted instrument, FIES can be used at any time, also for short periods of time while other instruments are mounted.

The currently installed fiber bundle contains a high-res 1.3 arcsec fibre offering a spectral resolution of R=67000. This bundle also provides two 1.3 arcsec fibres giving R=46000 (med-res), and a fibre with a larger entrance aperture of 2.5 arcsec but a lower spectral resolution (R=25000, low-res).

FIES offers the option of simultaneous observations of wavelength reference (Thorium-Argon) spectra, for the medium- and high-resolution modes. Simultaneous sky spectra can be obtained in medium-resolution mode, restricted to wavelengths shorter than 650nm. An exposure meter is available to predict the S/N of an ongoing exposure, allowing to optimise the exposure time. A later upgrade of FIES to full polarimetric capability is being investigated.

A recent (2014) paper describing FIES can be found here or here.

Questions about FIES should be directed to the instrument specialist, John Telting.

Advantages and drawbacks

The particular advantages of FIES compared to other high-resolution spectrographs are:

  • A high degree of mechanical and thermal stability, which allows good precision in radial-velocity determinations and accurate flat-fielding in demanding spectrophotometric work.
  • The freedom to schedule observations with FIES independently of the instrumentation mounted at the main focus, and thus to follow variable objects with periods of the order of weeks without blocking the telescope for other programmes in the meantime.
The main drawback of FIES is the low UV sensitivity, due to unavoidable Rayleigh scattering losses in the long fibre. At 400nm the sensitivity is about one third of the peak in the VR range.
Also, long-slit spectroscopy is, of course, not possible.

Data reduction

FIES comes with dedicated reduction software (FIEStool), which provides a fully reduced spectrum for quick-look analysis after the end of each exposure, using library calibrations. The same software is also available to observers for final analysis of the data using the nightly calibration frames.

General documentation
Instrument specifications
Standard stars and Data reduction
Observing guides and tools

Back to top Last modified: 03-Feb-2017