Evidence for a far-ultraviolet spacecraft glow in the ROSAT wide-field camera

During a 6 month all-sky survey and subsequent operations, the ROSAT Wide-Field Camera (WFC) was subject to a dominant and unexpected source of background. The contributions of the expected components are well understood, and once these are removed the residual background shows a strong correlation with the ram angle, suggesting that it may be due to a spacecraft glow phenomenon similar to that seen in other missions, most notably the Atmospheric Explorer ( AE- C) and the Space Shuttle. The microchannel plate detector used in the WFC is very insensitive to photons of energy E ⩽ 6 eV (λ ⩾ 2000 A ̊ ) , which implies that the spacecraft glow, if it is the cause of the excess background, may lie in the ultraviolet or far-ultraviolet bands, outside the wavelength range of the majority of the previous observations. We present the results of a computer model which show that in some orientations the sensitive surfaces are “shadowed” from the ramming gases by the bulk of the spacecraft, and that in these orientations the observed background is much reduced.