The ORFEUSII Echelle Spectrometer: Instrument description, performance anddata reduction

During the second flight of the ORFEUS-SPAS mission in November/December 19 96, the Echelle spectrometer was used extensively by the Principal and Gues t Investigator teams as one of the two focal plane instruments of the ORFEU S telescope. We present the inflight 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 lambda/Delta lambda = 10 000, where Delta lambda is measured as FWHM of the instrumental profil e. The effective area peaks at 1.3 cm(2) near 110 nm. The background is dom inated by straylight from the Echelle grating and is about 15% in an extrac ted spectrum for spectra with a rather flat continuum. The internal accurac y of the wavelength calibration is better than +/-0.005 nm.