Far-UV observations in and near the Hubble Deep Fields demonstrate that the Space Telescope Imaging Spectrograph ( STIS ) can potentially 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 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 \lambda 1493 Å , that is comparable to the dark rate and inseparable from the far-UV background .
Discarding all but the night data from these datasets gives a diffuse far-ultraviolet background measurement of 501 \pm 103 ph cm ^ { -2 } sec ^ { -1 } ster ^ { -1 } Å ^ { -1 } , along a line of sight with very low Galactic neutral hydrogen column ( N _ { HI } = 1.5 \times 10 ^ { 20 } 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 .