Spectroscopy

Setting up a spectrograph into a large telescope is not a trivial problem. It is necessary to cope with telescope movements and the stability needed for the spectrograph.

The use of optical fiber is a great help to solve such problem. The idea is to build various communication links between the telescope focus point and the spectrograph.

Examples of such links:

  • Single fiber for single object observation (MOS, MEDUSE)
  • Bundle of 20 to 40 fibers for larger area observation (IFU, UVES)
  • Bundles of several hundred fibers for large field observation keeping a high resolution (ARGUS, LCB, LIFU)

The fibers are arranged in a so called pseudo-slit to be connected and match in the best possible way the slit of the spectrograph.


Fibers

Fiber selection is an essential consideration for such systems. Only a few fiber manufacturers are able to propose the quality level required: product line offering best transmission for the wavelengths needed, process control to guaranty consistency of the characteristics over the full fiber spool and between different spools coming from one or from several preforms.

Fiber protection

The link length between the telescope focus point and the spectrograph platform can vary from 10 to 40 meters or more. Consequently a strong and reliable protection of the fibers is needed able to survive the telescope movements


Design optimization

Telescope side

The mechanical design must allow for a very precise positioning of the fibers at the different focus points within a minimum space to let free other observation points.

It has also to match the lens or lens arrays necessary to adapt the fibers numerical aperture to the telescope as well as the possibility to move and collect light from different points.

Last but not least it has to be adapted to the mechanical and optical characteristics of the fibers.

Spectrograph side

On this side it is necessary to create a line of fibers, often very long and with best possible alignment (linearity) to match maximum surface of the spectrograph slit (density).

Besides it is necessary in some configuration to compensate for the field curvature of the telescope.

The mechanical parts have also to guarantee an excellent parallelism of the fibers which is a crucial factor on spectrograph side as well as on telescope side.


Zero fiber breakage and FRD

The assembly of the link is a very difficult and delicate operation in order to garantee best possible FRD (Focal Ratio Degradation) with a fiber breakage proportion < 1%.


Projects / Years

Telescope fiber links are generally unique. Each instrument, each telescope and each spectrograph has its own configuration. However it is mandatory to be able to trace the complete system, procedures, part list and drawings and to maintain the documentation for a minimum of ten years or more according to contract.

References

SEDI-ATI a déjà réalisé de nombreux projets de systèmes fibrés pour télescope, tels que :

FLAMES for VLT spectrograph GIRAFFE

MEGARA for GTC in Canaries Islands,

ARES Spectrograph, integrated to Joan Oró telescope,

ESTranGIS spectrograph for the sun observatory,

– Prototypes for WEAVE,  William Herschel telescope in Canaries Islands

– Prototypes MOONS Spectrograph multi-objects for the VLT

– Prototypes OPTIMOS EVE  future fiber system(MOSAIC.


Achievements / Products

Single fiber link, packaging from a few fibers to several hundred, bundles, slits, the most sophisticated assemblies are used in spectroscopy applications.