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The abundance of interstellar molecular nitrogen (N2) is of considerable importance: models of steady-state gas-phase interstellar chemistry, together with millimetre-wavelength observations of interstellar N2H+ in dense molecular clouds predict that N2 should be the most abundant nitrogen-bearing molecule in the interstellar medium. Previous attempts to detect N2 absorption in the far-ultraviolet or infrared (ice features) have hitherto been unsuccessful. Here we report the detection of interstellar N2 at far-ultraviolet wavelengths towards the moderately reddened star HD 124314 in the constellation of Centaurus. The N2 column density is larger than expected from models of diffuse clouds and significantly smaller than expected for dense molecular clouds. Moreover, the N2 abundance does not explain the observed variations in the abundance of atomic nitrogen (N I) towards high-column-density sightlines, implying that the models of nitrogen chemistry in the interstellar medium are incomplete.

The ultraviolet groove efficiency for a holographically ruled diffraction grating with a trapezoidal profile has been measured. The efficiencies for the +/-1 and the zero orders are in good agreement with those derived from scalar theory. The +/-1 orders have equal efficiency as a function of wavelength. The peak of the sum of fitted groove efficiency functions is 76%, a level that is competitive with the groove efficiency of a mechanically blazed grating. We suggest that a normal-incidence grating mount with detectors at both orders will offer a system with twice the efficiency and provide a built-in redundancy. We discuss design considerations for reducing astigmatism equally in both orders in such dual-order mountings.

Narcissistic ghost lines in a Rowland-grating spectrograph have been identified as arising from the reimaging of bright spectral features, which are incident on the face of a detector photocathode [Cesium iodide (CsI) on a microchannel plate], back onto the detector by the grating in zero order. The mean of the wavelength of the diffracted light and the apparent wavelength of the ghost allows the angle of the grating normal with respect to the input beam (alpha) to be determined. Measurements of ghost intensity as a function of wavelength are presented and are found to range between 7 x 10(-4) and 7 x 10(-3) of the parent line. We find that the sum of the CsI photocathode reflectivity and quantum efficiency <1/2, showing the bulk of the light incident upon the detector, is neither reflected nor detected. We caution that any Rowland circle spectrograph with a detector normal nearly aligned with the grating normal and with a sufficiently reflective detector face (or surrounding mounting structure) will suffer from these narcissistic ghosts.

The UV quantum efficiency of a Scientific Imaging Technologies (SITe, Beaverton, Ore., formerly Tektronix) 1024 pixel by 1024 pixel thinned and backside-illuminated charge-coupled device has been measured in the wavelength band from 1164 to 5200 Å. The 24-µm-pixel device, which does not have an antireflective coating, has an average quantum efficiency of more than 40% in the band from 1164 to 4000 Å and a read noise of 15 e(-)/pixel.

The absolute quantum efficiency (QE) of two 1024 × 1024 pixel phosphor-coated (lumigen) chargecoupled devices has been measured in the band from 1216 to 5200 Å. It is found that the average QE is 17.7 ± 1.8% for the lot 1 detector and 18.1 ± 1.8% for the lot 2 detector in the band from 1600 to 3800 Å. The sensitivity rises on the long-wavelength side of the measurement band to 32% at 5200 Å and falls with the wavelength below 1600 Å. The two detectors have different spatial uniformity characteristics; both have a very uniform response (±2%) to 5200-Å flat-field illumination, although only one of the devices has a flat ultraviolet response. The read noise of these devices is 8 electrons (e(-)).