Anomalies

This page will serve to document anomalies or odd behavior observed on WFC3. A brief description and links to relevant documents will be provided for each behavior.

UVIS

Charge Transfer Inefficiency (CTI)

CTE data taken in TV2 on the backup UVIS detector indicate a current charge transfer efficiency of >99.9999%. For a full description, see the ISR below.

ISR 2007-13: UVIS CCD EPER CTE measurements performed during the April 2007 Ambient Calibration campaign (SMS UV02S01)
M. Robberto 30 Apr 2007 (Pdf 210 Kb)

Crosstalk

Crosstalk on a CCD detector occurs when a mirror image of a source from one amplifier output appears on neighboring amplifier outputs. In effect, sources detected in one quadrant cause electronic mirror-imaged ghosts in other quadrants. In the WFC3 UVIS channel, the only measurable crosstalk occurs when pixels on the detector saturate. Detailed findings of the latest crosstalk investigation will be presented in an upcoming ISR.

Filter Ghosts

Ghosts due to scattering from the window and filters of the UVIS channel have been observed in WFC3 ground testing. In all filters except for two (FQ232N and FQ243N), these ghosts contain less than 0.6% of the source flux. Typical ghosts are in the 0.3 - 0.4% range. For the two filters mentioned above, the ghosts can contain up to 7% and 5% of the incident source flux, respectively.

For full details on the UVIS ghosts, see:
ISR 2007-09: UVIS Channel Filter Ghosts after Filter Replacement
T. Brown 25 Apr 2007 (Pdf 1900 Kb)

IR

Dark Current Tail

The dark current behavior of the pixels in the IR detector cannot be completely described by a single number. The distribution of dark current values across the detector is a skewed Gaussian, with a tail of high dark current pixels. The dark current distribution of the IR flight detector will be characterized in the upcoming TV3 testing.

Inter-Pixel Capacitance

The IR channel is affected by inter-pixel capacitance (IPC), in which the signal measured by one pixel is felt by its neighbors, resulting in an over-production of electrons. This can be thought of as an artificial increase in QE of the detector. From ground testing, we have determined that the effects of IPC can be removed by scaling the measured signal downwar by a factor of 0.88. This correction will be implemented in CALWF3.

Crosstalk

Crosstalk effects have been observed in the IR channel. Unlike the UVIS channel, crosstalk appears at very low levels (1e-6) for all point sources observed in the IR channel, regardless of whether the source contains saturated pixels. Details of the latest crosstalk investigation will be presented in an upcoming ISR.

Filter Ghosts

Unlike the UVIS channel, no significant ghosts were observed in the IR channel during TV testing. See the report below for details.

ISR 2007-16: WFC3 TV2 Testing: IR Channel Ghosts and Baffle Scatter
T. Brown 15 Aug 2007 (Pdf 2234 Kb)