|WFC3 Data Handbook v.4|
5.5 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.5.5.1 GhostsGhosts 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 the 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" from their source generally in pairs (a "figure eight"). Further details may be found in WFC3 ISR 2001-17, WFC3 ISR 2004-04, WFC3 ISR 2007-21 and WFC3 ISR 2011-16.Bright sources immediately outside the image frame on WFC3/UVIS can create (mostly minor) light anomalies in the form of spurious signal redirected into the field of view and affecting the image (see Figure 5.9). Dragon’s Breath (when the light originates from the edge) or Scattered Light (when the light is somewhat displaced from the edge) affect about 12% of a subset of images recently surveyed. It occurs when a bright source lies within the 20’’ area outside of the image frame, and peaks when the neighboring source has a V band magnitude around 11.9 or 14.6 ( WFC3 ISR 2017-02). The source of scattering along the optical path is not completely understood. Most cases of Dragon’s Breath and Scattered Light are minor and will not adversely affect a science image.Figure 5.9: Dragon’s Breath and Scattered Light on WFC3/UVISExamples of Dragon’s Breath and Scattered Light on WFC3/UVIS. Left: Dragon’s Breath spills into the image frame from the bottom of the image. Right: Scattered light projects into the center of the image from a source directly underneath. (These examples are shown because they obviously visually demonstrate the effect, but most cases of Dragon’s Breath are much less dramatic both in number of affected pixels and flux from the effect.) For an interactive catalog of Dragon’s Breath affected images see: http://www.stsci.edu/hst/wfc3/ins_performance/anomalies/bokeh_dragon.html5.5.3 Cross TalkWhenever two or more quadrants are read out simultaneously, there is a chance of generating electronic crosstalk (Janesick 2001). 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 5.10 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 ~2 × 10-4 of the source when the target is in quadrants A or C and about 8 × 10-5 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 as 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 is described in WFC3 ISR 2012-02 and can be downloaded at:A standalone python procedure is also now available for correcting UVIS data for crosstalk. The code as well as a description of its use is available at:Figure 5.10: UVIS Crosstalk.UVIS crosstalk (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.