Extreme Ultraviolet Explorer spectrometer

The design and calculated performance is described for a spectrometer included on the Extreme Ultraviolet Explorer (EUVE) astronomical satellite. The instrument is novel in design, consisting of three plane reflection gratings mounted in the converging beam behind a grazing incidence telescope. This configuration is based on new varied line-space (VLS) gratings which have recently been proposed. A sample EUVE grating has been mechanically ruled and experimentally characterized. It recovered over 80% of the theoretical efficiency of perfectly formed grooves, reaching 38% absolute at a wavelength of 114 Å. The grating was used to construct a laboratory spectrographic version of the EUVE spectrometer and recorded the spectrum of helium from 228 to 320 Å. The spectral resolution was λ/Δλ ∼ 2000 and the image heights were ∼10 sec of arc. Individual spots were ∼25 × 50 μm, which is a significant improvement over existing grazing incidence spectrographs. A line profile measurement at 1 Å away from first order 304 Å showed <1.5%/ Å of grating stray light and a rapid decline toward the wings. In visible light, no grating stray or ghost light could be seen. Three flight spectrometer channels in combination span the 70–760-Å band with an effective collecting area of 0.3–1 cm2 over the 80–600-Å region. The spectrometer has an inherent resolution of λ/Δλ ∼ 300, but if combined with a worst-case satellite performance will yield a spectral resolution of λ/Δλ = 110–240 and a spatial resolution of 1–2 min of arc. For a 40,000-sec observation, the average 3σ sensitivity to continuum flux is ∼2 × 10−27 erg/cm2/sec/Hz. This is a factor of 100 dimmer than a bright known EUV source and is comparable to the sensitivity of the all-sky survey which will be carried out on the EUVE. At a 5σ detection threshold, the spectrometer sensitivity to individual spectral lines is 1–4 × 10−3 photons/cm2/sec, which is a factor of 50 better than available with the EUVE wide bandpass telescopes. Simulated observations of two known classes of extrasolar EUV sources reveal rich spectra. During a six-month spectroscopic phase, target selection will be conducted by guest investigators chosen by NASA.