The ORFEUS II Echelle Spectrometer: Instrument description, performance and data reduction

During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spectrometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane instruments of the ORFEUS telescope. We present the in-flight performance and the principles of the data reduction for this instrument. The wavelength range is 90 nm to 140 nm, the spectral resolution is significantly better than <~mbda/Delta <~mbda = 10 000, where Delta <~mbda is measured as FWHM of the instrumental profile. The effective area peaks at 1.3 cm(2) near 110 nm. The background is dominated by straylight from the Echelle grating and is about 15% in an extracted spectrum for spectra with a rather flat continuum. The internal accuracy of the wavelength calibration is better than +/- 0.005 nm.