FOCES - A FIBER OPTICS CASSEGRAIN ECHELLE SPECTROGRAPH

We have designed and built the echelle spectrograph FOCES fed by 100 m u 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 spectr ograph itself follows a white-pupil design collimated with two off-axi s parabolic mirrors. The 15 cm beam leaving the 31.6 lines/mm R2 echel le is refocussed in the vicinity of a small folding mirror which allow s efficient removal of scattered light. The cross-dispersion is achiev ed with a tandem prism mounting, and the beam imaged with. an f/3 tran smission camera onto a field centered on a 1024(2) thinned Tektronix C CD with 24 mu m pixel diameter. The echelle image covers the visible s pectral 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 R = lambda/Delta lambda = 40600 with a 2 pixel resolution element; unvignetted resolution as defined by the fibre alone would be obtaine d at R = 18000. Replacing the CCD by a 2048(2) chip With 15 mu m pixel diameter and taking into account light losses from a reduced entrance slit width a full 2 pixel resolution of R = 65000 is obtained. The ab ove concept has made FOCES an extremely well-defined instrument. A num ber of successful test installations at the Cassegrain foci of the Wen delstein 80 cm telescope, the Calar Alto 2.2 m and 3.5 m telescopes ha s produced spectra of high quality for up to 60 min exposures. The lim iting magnitude for al hr exposure with an S/N ratio of 100 scales to V = 12 for a 3.5 m telescope which is only slightly less than expected from laboratory tests. In an alternative mode FOCES offers a second f ibre carrying the sky background signal to correct extremely faint obj ect spectra. This mode obtains the required higher cross-dispersion fr om an additional grism resulting in a correspondingly reduced spectral coverage.