Background

CCD

Between launch on 14-Feb-1997 and failure of the "Side-1" electronics on 2001-May-16, CCD temperature was stabilized at -83 C. During the Side-1 era, the CCD median dark rate rose from 0.0013 to 0.0043 e-/sec/pix. CCD temperature cannot be stabilized with the "Side-2" electronics, so after 2001-May-16 thermal fluctuations perturb the dark rate. These fluctuations are discussed in STIS ISR 2001-03. On The Fly Reprocessing (OTRF) currently scales the CCD dark according to temperature, but users who require the best possible dark subtraction may improve upon the OTFR results by scaling their dark files themselves, using a log of CCD housing temperature that is finely sampled in time. This log is available at: ftp://ftp.stsci.edu/pub/instruments/stis/CCD_Housing.tar.gz. Based on an extrapolation of previous trends, the CCD dark current in during cycle 17 is expected to be near 0.009 e-/sec/pixel.

Because bright UV illumination temporarily increases CCD dark rate, acquisition exposures should use the long pass filter, rather than the clear aperture.

See also " Radiation Damage Effects...on the STIS CCD"

MAMA Detectors

When the FUV-MAMA detector HV is initially turned on, the dark rate can be as low as 6.4e-6 counts/pixel/s, but as the detector operates, an extended glow centered in the upper-left hand quadrant of the detector begins to appear. The rate at which this glow increases has itself been increasing with time. The unpredictable nature of this glow has made it impractical to subtract this in the OTFR pipeline, however, mean images of this glow have been made available to assist observers in subtracting this from their data, and can be found at http://www.stsci.edu/hst/stis/design/detectors/fuvDarkGlow.html.

The cause of this glow is not well understood, and this makes it difficult to predict how it will behave after STIS repair. Current predictions suggest that the peak of the glow may increase by about 1e-6 counts/pixel/sec/HST-orbit.

Note that in the lower right hand corner of the detector, the glow is usually absent, and the dark rate in that part of the detector remains near 6.4e-6 counts/pixel/s, even after the detector HV has been on for several orbits.

NUV-MAMA dark rate varies between 0.0017 and 0.00067 counts/sec/pix, depending on temperature, due mainly to phosphorescence by impurities in the detector faceplate. Higher aft-shroud temperatures after SM4 may cause the NUV MAMA dark current to be 5 to 15% higher than previously.

See also " Characteristics of the FUV-MAMA Dark Rate"

Earthshine and Zodiacal background

Sunlight reflected by the Earth or the Moon can scatter onto STIS detectors, so the corresponding background depends on the angle between the target and the bright limb of the Earth or Moon.

Zodiacal background varies as a function of helio-ecliptic coordinates.

Both Zodiacal background and Earthshine depend on wavelength. Tabulated background count rates for imaging filters and intensities vs. wavelength (data) are available.

Low background scheduling can be requested in a Phase 1 proposal, and if approved, specified in the Phase 2 proposal using the LOW-SKY special requirement.

Geocoronal

Tabulated intensities and line widths are available for geocoronal emission lines at 1216, 1304, 1356, and 2471 Å.

Note that FUV-MAMA images with the clear or Ly-alpha filter will be dominated by variable geocoronal Ly-alpha background.

Scheduling when geocoronal lines are weak can be requested in a Phase 1 proposal, and if approved, specified in the Phase 2 proposal using the SHADOW special requirement.