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
370-730 nm is covered without gaps in a single, fixed setting. 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
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
Questions about FIES should be directed to the instrument specialist,
Advantages and drawbacks
The particular advantages of FIES compared to SOFIN and other
high-resolution spectrographs on La Palma are:
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.
- 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.
long-slit spectroscopy is, of course, not possible.
In a transition period, programmes requiring capabilities not
covered by FIES (UV coverage; NIR coverage 0.7-1.0 micron; Zeeman
measurements) may still request to use SOFIN, but with limited freedom
as regards scheduling.
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.