5 Summary

The Echelle spectrometer was one of the two focal instruments of the ORFEUS telescope, which was flown on its second mission in November/December 1996. The in-flight performance and the principles of data reduction for this instrument are presented. The wavelength range is 90nm to 140nm and we showed, that the spectral resolution is significantly better than $\lambda$/$\Delta$$\lambda$=10000, where $\Delta$$\lambda$ is measured as the FWHM of the instrumental profile. The effective area peaks at 1.3cm² near 110nm. 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 $\pm$0.005nm. No corrections could be applied which would correct for a possible systematic shift of the object within the 20$^{\prime\prime}$ aperture of the telescope.

For the future we plan to improved the data quality by further reduction steps and investigations:

• Integration of single photons with correction of actual orbital radial velocity and possibly correction for the ASTRO-SPAS pointing jitter.
• Correlation of count rate variations with ASTRO-SPAS pointing jitter to estimate the object's position within the aperture and the size and the shape of the telescope's image of the object.
• Investigation of small scale flat field variations.
• Investigation of the cause for the "tilted'' absorption lines within the spectral orders.
• Stray light model for improved background subtraction.

Acknowledgements

This project could not have been successfully carried out without the very efficient cooperation of all the many individuals and institutions involved. In particular we acknowledge the excellent performance of the teams of DASA with the construction and mission operations of the ASTRO-SPAS satellite, of Kayser-Threde with the construction of the ORFEUS telescope, the ground support teams at the Kennedy Space Center, the crew of STS-80 Columbia, the mission representatives of DARA and NASA and last not least our colleagues at the Space Sciences Laboratory of the University of California at Berkeley. We wish to thank Dr. Ralf Napiwotzki for calculating the atmosphere models for BD+28$^\circ$4211 and making them available to us. The ORFEUS program was supported by DARA grants WE3OS8501 and WE2QV9304 and NASA grant NAG5-696.