Anomalies

Here we document known anomalous behavior and underlying causes. For detail on any of the following items, see the Instrument Handbook.

CCD

• CCD Fringing occurs at wavelengths longer than 7000 Å with both the G750L and G750M gratings. Peak-to-peak fringe amplitude can be up to 30% of the mean in raw data, dropping to less than 3% after calibration with contemporaneous flats.
• Charge transfer efficiency (CTE) may be recast as charge transfer inefficiency (CTI = 1 - CTE), which is the fractional loss of charge per transfer to an adjacent pixel. Measured CTI is worse for weak sources, low background, and time since launch. CTI increases by ~15% per year. CTI mitigation is achieved with pseudo-apertures, which place targets closer to the readout amplifier.
• Amplifier Ringing is visible in serial overscan pixels and leads to a small bias depression proportional to the signal in each row.
• Extreme UV over-illumination elevates CCD dark current for times up to 40 minutes.
• Interference noise produces a ripple pattern with a maximum amplitude of 1 ADU in the CCDGAIN=4 setting.

MAMA Detectors

• Phosphorescence of impurities in the NUV-MAMA faceplace creates a temperature dependent background.

• TIMETAG errors in raw STIS data used to cause spurious gaps, negative steps, or large positive jumps in time. On 2001-Sep-13, HST archive software was updated to fix all known TIMETAG problems on-the-fly, even for old datasets.

Optical

• Echelle scattering is severe in the FUV, making interorder light a poor estimate of background under adjacent orders. A 2-D background subtraction algorithm reduces fractional errors to at most 1%.
• Order sorting filters used with first order gratings also give rise to fringing visible in monochromatic images and spatial profiles.
• Image ghosts are caused by reflection off the CCD surface and then the CCD window. Out of focus annular ghosts are offset from the main image and contain about 1% of the signal.
• Parallel ghost spectra (railroad tracks) are extremely rare, occuring for unknown reasons when observing bright red targets.

Electrical

• Short circuit in "Side 1" electronics on 2001-May-16 forced a switch to the backup "Side 2" electronics. Closed loop control of CCD temperature is not possible with Side 2.
• Opto-isolaters in MAMA electronics are susceptible to spurious control signals generated by cosmic rays. To protect the MAMA detectors, they are only used for 5 consecutive SAA-free orbits each day.