Anomalies

Charge transfer efficiency (CTE) describes how effectively a CCD transfers charge from one pixel to another during readout. While CTE was measured to be >99.9999% during TV2 on the backup UVIS detector, it has been gradually declining on WFC3/UVIS as a consequence of ongoing radiation damage to the silicon lattice of the detector.

CTE losses, also known as charge transfer inefficiency (CTI), manifests as vertical trails attached to sources, pointing away from the output amplifier. In full-frame images, CTI can make sources appear to be "melting" towards the chip gap. 

You can increase the background to mitigate CTI (effectively filling potential traps) by adjusting filter choice, increasing post-flash, or using a longer exposure time. For WFC3/UVIS observations, the calwf3 pipeline produces CTI-corrected files (FLCs) for WFC3/UVIS observations.

The WFC3 team monitors CTI through yearly calibration programs; current information is posted on the CTE webpage. 

 

Right: an example of CTI in a UVIS/F467M image (HST observation rootname idaz05ieq). Note that sources at the top of UVIS 1 and bottom of UVIS 2 appear much more point-like than sources near the center, which are accompanied by trails pointing towards the chip gap.

 

 

Crosstalk in the WFC3/UVIS CCD detectors occurs when a source in one quadrant generates an electronic mirror-image negative ghost in the neighboring quadrant on the same chip. The crosstalk is at levels of about -2 e-04 in quadrants B and D due to sources in A and C and at levels of about -0.7 e-04 for crosstalk in quadrants A and C due to sources in B and D.

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 ISR 2012-02.

 

Right: an example of crosstalk in a UVIS/F200LP image (HST observation rootname idgbb8pgq). The bright point source on UVIS 2 in quadrant C causes the dark vertical artifact to the right in quadrant D. Careful examination reveals two additional, minor instances of crosstalk appearing in quadrant D. 

 

Many stars lie immediately outside the image captured by the WFC3/UVIS detector. These stars can create a notable pattern of artificial light on the detector that is called ‘Dragon’s Breath’ when it originates from the edge of the detector, and ‘Scattered Light’ when it’s disconnected from the detector edge.

In 2016, the WFC3 team created two tools to help users plan around this effect.

• A searchable table containing observations in which the dragon's breath anomaly appears
• An interactive plot showcasing the locations of off-frame sources that caused dragon's breath to occur

 

Right: An example of dragon's breath in a full-frame UVIS/F606W image (HST observation rootname ie5h62irq). A bright source off-frame is causing the anomaly to appear on UVIS 1 in quadrant A (top left).

 

 

 

Figure-8 ghosts are an anomaly caused by refections off the CCD detector window. They occur on a diagonal from a bright source observed in UVIS quad D, and their confined appearances across the quad A-to-D diagonal is due to the detector's tilt.

 

Right: schematic diagram of figure-8 ghosts. Starting from the top left quadrant and ordering clockwise, we have the A, B, D, and C UVIS quadrants. This figure is a co-added image of 37 individual frames obtained during ground testing in TV3. Each star in the lower right quadrant produces four elliptically-shaped ghosts to the upper left. To aid in identification of the four ghosts associated with each star, the six stars and their associated ghosts have been color-coded.

 

 

Detector-filter ghosts are caused by a bright source reflecting off both the filter wheel and the IR and UVIS detector windows. It appears as a large, diffuse single or double donut, quartered by the shadows of the four struts. While they are less commonly seen, they can appear on both IR and UVIS images.

Filter ghosts are small, donut-shaped ghosts that appear near or within the source's PSF. They are caused by a reflection off layers in the UVIS filters. These are more frequently observed in UVIS images than detector-filter ghosts.

 

Right: an example of both filter and detector-filter ghosts in a full-frame UVIS/F606W image (HST observation rootname ic3t84jcq). The largest source, positioned near the chip gap between CCDs, is accompanied by a prominent detector-filter ghost. The smaller source, midway across UVIS 1, has a filter ghost appearing above the PSF. 

Fringing is a pattern of constructive and destructive interference created by multiple reflections betwen the CCD detector surfaces. It is seen at wavelengths longer than 650 nm because only then does the silicon of the CCD become transparent enough - that is, the absorption efficiency decreases enough - to allow the reflections to occur. 

Fringing is highly wavelength-dependent. Narrow-band red filters (especially F953N, FQ889N, FQ906N, FQ924N, and FQ937N) generally show the strongest fringing. 

 

Right: an example of fringing in a full-frame UVIS/F953N image (HST observation rootname idhm02jvq). 

Because Hubble is in low-Earth orbit, satellites can intersect with the area of sky being observed. Although they most often appear as strong single lines, tumbling satellites in UVIS may cause dashed-line trails.

 

 

Right: an example of a satellite trail across a full-frame UVIS/F350LP image (HST observation rootname id8m11g4q). 

 

 

Blobs are defocused images of particulates on the channel select mechanism (CSM), corresponding to regions of slightly lower than nominal sensitivity. New blobs appear occasionally throughout the mission as the CSM mirror accumulates particulates. In images, they appear as dark blobs. 

To mitigate blobs in observations, users may want to consider utilizing the "WFC3-IR-DITHER-BLOB" dithering pattern. 

 

Right: an example of prominent blobs in an IR/F140W image (HST observation rootname idq293c3q). 

 

 

Crosstalk effects have been observed in the IR channel. Positioned symmetrically opposite the source about the dividing line between each of the coupled readout amplifier quadrants, IR crosstalk appears at a lower level than the surrounding background, about ~1e -06 that of the source signal. The level is low enough that it should not be an issue for most programs; dithering can help mitigate the effect.

 

Right: An example of crosstalk in an IR/F164N observation (HST observation rootname idn105heq). Because of bright point source in quadrant 2, a dark spot appears above the PSF in quadrant 1. 

 

Earth limb (also called Earth shine or scattered Earth light) is stray light that appears as a higher background on the left of the IR detector. It is dependent on HST's pointing direction.

 

Right: an example of Earth limb in an IR/F110W image (HST observation rootname idgb26dtq). 

 

 

While no significant ghosts were observed in the IR channel during TV testing, two types of detector ghosts have since been observed for WFC3/IR.

Detector-filter ghosts are caused by a bright source reflecting off both the filter wheel and the IR and UVIS detector windows, appearing as a large, diffuse single or double donut, quartered by the shadow of the four struts. While they are less commonly seen, they can appear on both IR and UVIS images.

An additional diamond feature is rare, but can be caused by reflection from the refractive IR corrector element, appearing as a diamond-shaped outline balanced on the top of a star's PSF. 

Right: a cropped section of a WFC3/IR observation (HST rootname idq2hlcuq) that features both a detector-filter ghost and a diamond feature. 

 

Image persistence in the IR array occurs whenever a pixel is exposed to light that exceeds more than about half of the full well of a pixel in the array. Persistence can occur within a single visit, as the different exposures in a visit are dithered. Persistence also occurs from observations in a previous visit of completely different fields.

Image persistence is seen in a small, but non-negligible fraction of WFC3/IR exposures. Its properties are discussed in the WFC3 Instrument Handbook in Section 7.9.4. Persistence is primarily a function of the degree to which a pixel is filled (in electrons) and the time since this occurred.

Right: an example of persistence occuring in an IR/F110W image (HST observation rootname id1k72qhq). A previous grism observation taken before this exposure is the cause of the bright horizontal lines appearing to the left of the bright sources. 

Snowballs are transient events observed in some HgCdTe detectors that occur instantaneously and deposit at least 200,000 electrons in a small area. They often do not appear in the final calibrated FLT file because snowballs are treated like cosmic rays in the calibration pipeline.  In the FLT example shown here, the snowball (near the center of the lower right quadrant) was not completely removed by the automated pipeline and as a result, appears as a fuzzy ring. Over an 11-year baseline from 2009 to 2020, ~13,000 snowballs were detected. 

A table containing WFC3 IR snowball data from 2009 to 2019 is available to the public. It contains HST exposure ID (rootname), observation date (MJD), peak flux (electrons), number of saturated pixels, and spatial position. For snowballs detected after 2014, the snowball classification assigned is also included.

Download (CSV)

More information:

ISR 2020-03: IR ‘Snowball’ Occurrences in WFC3/IR: 2009-2019 (J. Green, H. Olszewski)

ISR 2015-01: IR “Snowballs”: Long-Term Characterization (M. J. Durbin, M. Bourque, S. Baggett)

ISR 2009-44: Radioactivity in HgCdTe devices: potential source of “snowballs” (P. McCullough)

ISR 2009-43: "Snowballs" in the WFC3-IR Channel: Characterization (B. Hilbert)