FOCES - a fibre optics Cassegrain échelle spectrograph

We have designed and built the éch elle spec tro graph FOCES fed by m optical fibres to be mounted at the Cassegrain focus of either the 2.2 m or the 3.5 m telescope at the Calar Alto Observatory. The spec tro graph itself follows a white-pupil design collimated with two off-axis parabolic mirrors. The 15 cm beam leaving the 31.6 lines/mm R2 éch elleis refocussed in the vicinity of a small folding mirror which allows efficient removal of scattered light. The cross-dispersion is achieved with a tandem prism mounting, and the beam imaged with an transmission camera onto a field centered on a 10242 thinned Tektronix CCD with m pixel diameter. The éch elleimage covers the visible spectral region from 380 to 750 nm displayed in 70 spectral orders with full spectral coverage. Spectral orders are separated by 20 pixels in the blue and by 10 pixels in the red. The maximum spectral resolution is with a 2 pixel resolution element; unvignetted resolution as defined by the fibre alone would be obtained at . Replacing the CCD by a 20482 chip with m pixel diameter and taking into account light losses from a reduced entrance slit width a full 2 pixel resolution of is obtained. The above concept has made FOCES an extremely well-defined instrument. A number of successful test installations at the Cassegrain foci of the Wendelstein 80 cm telescope, the Calar Alto 2.2 m and 3.5 m telescopes has produced spectra of high quality for up to 60 min exposures. The limiting magnitude for a 1 hr exposure with an ratio of 100 scales to for a 3.5 m telescope which is only slightly less than expected from laboratory tests. In an alternative mode FOCES offers a second fibre carrying the sky background signal to correct extremely faint object spectra. This mode obtains the required higher cross-dispersion from an additional grism resulting in a correspondingly reduced spectral coverage.

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