The objective transmission grating (OGS) onboard the Einstein Observatory, launched in 1978, consisted of two transmission gratings (500 and 100 lmm-1), either of which could be placed in the X-ray optical path at the exit from the mirror . The mirror had a focal length of 3.4m. The gratings consisted of gold bars nominally 0.2 |m thick and with a bar width equal to half of the grating period. The OGS was supplied by SRON in Utrecht/Netherlands. The high resolution imager (HRI) was used to detect the diffracted spectra. The grating resolution was limited by the telescope/detector angular resolution at short wavelengths and by optical aberrations at long wavelengths because the OGS did not follow the Rowland curvature. Therefore, the resolving power was limited to less than 50. The maximum effective area including telescope and detector efficiency was 3 cm2 at 44 A .
The instrument was used between 1978 and 1983 to investigate about 40 targets for a total of 500 ks observing time.
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