shows that the STIS
zero point (still from ground-based calibration) is fairly well determined.
Most images are taken with multiple 150s exposures interleaved with longer
spectral exposures. A star with a V magnitude about 19 will saturate (>33000
ADU/pixel) in 150s with this mode in GAIN=1. The background count rate is
about 0.3 ADU/s/0.1x0.1'' pixel.
The figure shows a lowish Galactic latitude field centred at 00h 38m 44.2s +48 23' 33'' (J2000) (Galactic coordinates l,b 120.8,-14.4). The total exposure times is 1200s (8 150s exposures). The images have been bias subtracted, combined and cosmic ray cleaned (using the STIS pipeline). The faintest star visible has a STIS magnitude of about 28.5.
two or more stars along a row or adjacent rows, it becomes impossible to extract
useful spectra. The figure shows the slitless spectrum
(total exposure time 7200s) obtained on the crowded star field.
Here the crowding is not very severe but enough that it is impossible to choose
a region truely free of stellar spectra for background subtraction. The background
results from the sky light from the dispersed spectra of the whole CCD array.
It has a peak count of about 0.06 ADU/s/0.1'' long pixel.
A simple visualization and extraction tool is under development at the
Space Telescope European Co-ordinating
Facility Clicking on the
two brightest stars in the image (STIS magnitude about 20.5)
shows the extracted spectrum
and the estimated mean background produced by this tool. No attempt has been
made to flux calibrate these spectra
nor to attempt to flat field them (the fringing is strong above 7500A). The
background was taken as the mean of the
lowest areas between the spectra, although these are contaminated by stellar
spectra. The lower star is a late type (K-M) star,
whilst the upper star is probably of type F-G.
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01 Jul 97