WFC3 Data Handbook V. 4.0

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5.5	Image Anomalies

5.5.1 Ghosts

5.5.2 Stray Light

5.5.3 Cross Talk

Some UVIS images may contain features that are not direct images of astronomical sources. The causes of these features include multiple reflections between optical surfaces (detector, filters, and windows) of light from the astronomical scene, scattered light from bright sources outside the detector FOV, light from the bright Earth that is scattered in the OTA, and electronic cross talk between readout amplifiers. In general, these artifacts are not calibrated and cannot be removed by the WFC3 pipeline.

Examples of the different kinds of image anomalies can be found on the WFC3 website.

5.5.1 Ghosts

Ghosts appear as images of the pupil formed from the light of a bright target in, or near the UVIS detector FOV. The target light is scattered twice (or more) by optical surfaces forming one (or more) out of focus images. The separation of the ghost from the source depends upon separation of the scattering surfaces and the angle of scattering. Filter ghosts are formed by scattering of the near-normal source light at the surfaces of a filter and, as such, are found close to, or overlapping, the source image. Further details concerning these ghosts may be found in WFC3 ISR 2007-09.

Scattering between the UVIS CCD and either the detector, or dewar window forms so-called ‘optical’ ghosts. These ghosts are separated by ~80 arcsec from their source generally in pairs (a "figure eight"). Further details may be found in WFC3 ISR 2001-17, WFC3 ISR 2004-04 and WFC3 ISR 2007-21.

5.5.2 Stray Light

Diffuse, structured linear features may be occasionally found in UVIS images. The features are approximately aligned either along rows, or along columns. The stray light may be scattered from astronomical sources outside and close to the detector field of view. In a dithered set of exposures, the stray light feature follows the dithers. When the line of sight passes within ~30 degrees of the bright limb of the Earth, stray light may be scattered by the OTA onto the UVIS detector.

5.5.3 Cross Talk

Whenever two or more quadrants are read out simultaneously, there is a chance of generating electronic crosstalk (Janesick 2001). In fact, both channels in WFC3 do exhibit some crosstalk (CT) though the level is very low. In the UVIS detectors, point sources and extended targets generate low-level mirror images in the quadrant adjoining the target quadrant, i.e., amps A+B and amps C+D are coupled. In the IR channel, the CT is also a low-level mirror image although in this case, the coupled amps are 1+2 (upper left and lower left, when image is displayed with x=1,y=1 at lower left) and 3+4 (upper right and lower right). In both channels, the CT appears as a negative image; thus, these electronically induced features are unlikely to be confused with e.g. optical ghosts.

Figure 5.7 shows a UVIS image with CT (from WFC3 ISR 2009-03). The UVIS crosstalk is linear, negative, and appears at the level of ~10-4 to 10-5 of the source. Specifically, in full-frame, unbinned UVIS readouts, the CT level is ~2x10-4 of the source when the target is in quadrants A or C and about 8x10-5 when the target is in quadrants B or D (WFC3 ISR 2009-03).

Crosstalk in the UVIS channel only occurs in the chip containing the target, it does not cross between chips. To within the errors, the CT due to hot pixels and cosmic rays is the same for point or extended sources. Dithering of observations can help mitigate the low-level effects of CT: the mirror image nature of the CT moves the features in a direction opposite to the target motion, i.e., they will appear to be transients and thus be removed during the drizzling procedure.

A standalone IDL procedure is available for correcting UVIS data for crosstalk, effectively restoring pixels to a mean, which is well within 1 sigma of the mean of surrounding pixels. The code as well as a description of its use is available as part of WFC3 ISR 2012-02.

Figure 5.7: UVIS Crosstalk.

UVIS chip image taken from the PSF Wing calibration proposal 11919, shown with a hard stretch to emphasize the CT. The slightly saturated star is in the B quadrant at right; the resulting mirror image CT (circled) is in the A quadrant at left. The partial dark column below and to the left of the PSF is a bad column.