Tm. Brown et al., Measurements of the diffuse ultraviolet background and the terrestrial airglow with the space telescope imaging spectrograph, ASTRONOM J, 120(2), 2000, pp. 1153-1159
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.