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.
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
So-called 'optical' ghosts are formed by scattering between the UVIS CCD and
either the detector, or dewar window. 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
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.
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.
shows a UVIS image with CT (from WFC3 ISR 2009-03
). The UVIS crosstalk (CT) is linear, negative, and appears at the level of ~10-4
that of the source. Specifically, in full-frame, unbinned UVIS readouts, the CT level is ~2x10-4
that 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, i.e., the CT does not cross between chips. To within the errors, the CT due to hot pixels and cosmic rays is the same as that due to point or extended sources.
Dithering of observations should 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. Tests using ground-based single images of an isolated source showed that the UVIS CT could be effectively removed by scaling the target image quadrant by the amp-dependent factor noted above, flipping the image about the y-axis, and subtracting it from the CT image quadrant (WFC3 ISR 2009-03
). On-orbit data, of course, typically contain more complicated fields with sources in all four amps, so an iterative solution will likely be required in order to achieve acceptable corrections. An option to correct for UVIS CT is a planned enhancement for the calwf3
pipeline but has not been implemented yet.