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The FLAIR multi-object spectroscopy system

 

The Schmidt Telescope is unique among the world's large Schmidts in providing a facility for multiple-object CCD spectroscopy using an optical-fibre feed system. Known as FLAIR (Fibre-Linked Array-Image Reformatter), the multi-object feed couples the telescope's focal surface to an intermediate dispersion spectrograph mounted on an optical table in the dome. This arrangement gives very high wavelength stability.      

At their input ends, the fibres are located in the focal surface by an optical alignment method adopted because of the high positioning accuracy (10um) required by the Schmidt's plate scale. The end of each fibre is terminated with a 2mm-square right-angled prism and, during positioning, the fibre/prism assembly is cemented to the surface of a 1mm-thick copy plate of the target field so that the image of the fibre in its microprism is aligned with the target image on the plate. When all the fibres have been attached to the copy plate, it is deformed to the focal curvature in a special plateholder and loaded into the telescope for the observations to be made. After observing, the fibres can be removed from the copy plate and reused with the next field.

At their output ends, the fibres are aligned in the collimator focus of the spectrograph. An earlier Pentax-lens instrument has been replaced with a purpose-built spectrograph which uses Schmidt-type optics for both the collimator and camera. Known as FISCH (for FIbre SCHmidt), the new spectrograph uses gratings from the AAT's RGO spectrograph to give a range of spectral resolutions from 12Å/pixel to less than 1Å/pixel. The detector is a cryogenic CCD camera with a dye-coated chip for enhanced blue sensitivity, and on-chip binning allows the effective pixel size to be matched to the fibre diameter. The CCD system was built for FLAIR by the Physics Department of the University of Durham.

As currently configured, FLAIR is optimised for measuring galaxy redshifts. Using absorption lines in the blue, an accuracy of approximately 130 km/s can be obtained to a limiting B magnitude of 16.8 in 9000 seconds with an 80% success rate for galaxies of all Hubble types. Red spectra are used for narrow emission-line galaxies, and here an accuracy of approximately 50 km/s is attained at B=17.5 in 3000 seconds.

Work is now well underway on a second generation system (FLAIR II) to replace the prototype. The main improvements will be in the number of available fibres (which will increase to about 100) and the turnaround time from one field to another, along with the improved blue performance already yielded by the new spectrograph.

A new manual for FLAIR II has recently become available. A link to it should be placed here.



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