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 no t the ideal instrument for such measurements, high-resolution images allow Galactic and extragalactic objects to be masked to very faint magnitudes, t hus 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 Ang strom bandpass. Our analysis shows that STIS far-UV crystal quartz observat ions taken near the limb during orbital day can detect a faint airglow sign al, most likely from N I lambda 1493, that is comparable to the dark rate a nd inseparable from the far-UV background. Discarding all but the night dat a from these data sets yields the diffuse far-ultraviolet background measur ement 501 +/- 103 photons cm(-2) s(-1) sr(-1) Angstrom(-1), along a line of sight with very low Galactic neutral hydrogen column (N-HI = 1.5 x 10(20) cm(-2)) and extinction [E(B- V) = 0.01 mag]. This result is in good agreeme nt with earlier measurements of the far-UV background and should not includ e any significant contribution from airglow. We present our findings as a w arning 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 ob tained with STIS.