Measurements of the Diffuse Ultraviolet Background and the Terrestrial Airglow with the Space Telescope Imaging Spectrograph

Far-UV observations in and near the Hubble Deep Fields demonstrate that the Space Telescope Imaging Spectrograph (STIS) may obtain unique and precise measurements of the diffuse far-ultraviolet background. Although STIS is not the ideal instrument for such measurements, high-resolution images allow Galactic and extragalactic objects to be masked to very faint magnitudes, thus ensuring a measurement of the truly diffuse UV signal. The programs we have analyzed were not designed for this scientific purpose, but they would be sufficient to obtain a very sensitive measurement if it were not for a weak but larger than expected signal from airglow in the STIS 1450–1900 Å bandpass. Our analysis shows that STIS far-UV crystal quartz observations taken near the limb during orbital day can detect a faint airglow signal, most likely from N I λ1493, that is comparable to the dark rate and inseparable from the far-UV background. Discarding all but the night data from these data sets yields the diffuse far-ultraviolet background measurement 501 ± 103 photons cm-2 s-1 sr-1 Å-1, along a line of sight with very low Galactic neutral hydrogen column (NH I = 1.5 × 1020 cm-2) and extinction [E(B-V) = 0.01 mag]. This result is in good agreement with earlier measurements of the far-UV background and should not include any significant contribution from airglow. We present our findings as a warning to other groups who may use the STIS far-UV camera to observe faint extended targets and to demonstrate how this measurement may be properly obtained with STIS.