A High Resolution Spectrometer For EUV/FUV Wavelengths

We have considered various designs for a high resolution spaceborne spectrometer for point-source astronomy in the 400 - 1200Å wavelength region. Our designs utilize as the pri-mary collecting optic a 1-meter normal incidence primary mirror of 1"quality and are constrained to fit within an envelope defined by the size of the advanced Spartan space platform now under consideration for development by West Germany. We find the most efficient design to be a multiple Rowland circle spectrograph, with four toroidal gratings each intercepting a fraction of the beam from the primary mirror. The advantage of this design is that each spectrometer can accept a relatively slowly diverging beam (thus reducing grating aberra-tions) without the loss of efficiency caused by an additional reflection or the magnification of the primary mirror blur and pointing jitter that would be introduced if a secondary mirror were used to slow the beam. We examine the detector requirements for the multiple Rowland circle spectrometer and find that no appreciable loss of resolution occurs if the circular tangential focal surface is approximated by two flat detectors. Furthermore, each pair of flat two-dimensional detectors can receive the spectra from two of the toroidal gratings simultaneously, thus reducing the number of detectors and associated electronics required. The specific parameters of the design (line densities, detector size and pixel size , etc.) are dependent on the drift rate of the space platform. We present a design optimized for an uncorrected drift of This design delivers a resolution of 3000 - 4000 and an effective area (including detector quantum efficiency) of 1 to 6.5 square centimeters across the bandpass.