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Quicklook

Background

The term "Quicklook" initially referred to a sub-team of the WFC3 instrument team tasked with quality checking new images and tracking unexpected behavior. Over time, Quicklook's scope and tools have grown significantly, encompassing a complex system of software, database, filesystem, and an internal website. Today, members of the team use the internal website to visually inspect the newest observations and flag images with specific anomalies; these flags are then stored as entries in a database table. 

Just because an anomaly is identified in an image, it does not necessarily mean that the feature has compromised the science quality of the image, that the feature is unexpected, or that the feature's cause is unknown. We use "anomaly" to describe certain features appearing unwanted anywhere in the frame, regardless of whether they impact the science target.

Various anomalies for the UVIS and IR detectors are described in the last two sections of this page. For many anomalies, mitigation strategies and/or corrective techniques have been developed; this information is included in the anomaly entry where applicable. We also include some known detector defects which may impact image quality.

Database

We provide a public version of the Quicklook database, comprising of all non-proprietary observations taken from installation through June 6th, 2022. Each row contains observation information (from the file header) and anomaly flags (identified by Quicklookers).

Download wfc3_quicklook_database_2022-06-06.csv 

A previous version of this database was released in 2020. 

Documentation

For more information regarding the WFC3 instrument, detector specifics, file formats, and relevant terminology, please see:

For more information about the Quicklook project, please see:

UVIS Anomalies

Charge Transfer Inefficiency

Charge Transfer Inefficiency

Known Defect

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.

 

 

A full-frame UVIS image with a source in the top right corner and trails pointing towards the center chip gap.
WFC3/UVIS observation idaz05ieq features prominent trails caused by charge transfer inefficiency (CTI).

 

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

More information:

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

 

Crosstalk

Crosstalk

Detector Anomaly

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. 

A full-frame UVIS observation taken in the F200LP filter. In the bottom half of the image, there is a bright point source and a dark vertical artifact to it's right.
WFC3/UVIS observation idgbb8pgq contains examples of crosstalk.

 

More information:

WFC3 Crosstalk webpage

ISR 2009-03: WFC3 TV3 Testing: UVIS-1 Crosstalk (S. Baggett)

ISR 2012-02: WFC3/UVIS Crosstalk and Crosstalk Correction (A. Suchkov & S. Baggett)

Dragon's Breath/Scattered Light

Dragon's Breath/Scattered Light

Detector Anomaly

 

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).

 

 

A full-frame WFC3/UVIS image with a bright anomaly originating in the top left corner.
WFC3/UVIS observation ie5h62irq contains a prominent example of the "dragon's breath" anomaly.

 

 

Figure-8 Ghost

Figure-8 Ghost

Detector Anomaly

 

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.

A plot showing the locations of bright sources in quad D (lower right) of UVIS detector, and how their resultant figure-8 ghosts are projected across the quad A (upper left) to quad D diagonal.
Sources in quadrant D cause figure-8 ghosts to appear across the A-to-D diagonal. Credit: P. McCullough (WFC3 ISR 2011-16)

 

More information:

ISR 2011-16: Geometric model of UVIS window ghosts in WFC3

ISR 2022-03: WFC3/UVIS Figure-8 Ghost Classification using Convolutional Neural Networks (F. Dauphin, M. Montes, N. Easmin, V. Bajaj, P. McCullough)

UVIS Filter/Detector Ghosts

UVIS Filter/Detector Ghosts

Detector Anomaly

 

 

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. 

A full-frame WFC3/UVIS image featuring a large bright star as well as a bright donut-shaped optical ghost,

HST/UVIS observation ic3t84jcq has prominent optical ghosts.

 

 

 

Fringing

Fringing

Detector Anomaly

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). 

A full-frame UVIS image of a star field with a wood-grain-like pattern apparent.
WFC3/UVIS observation idhm02jvq contains prominent fringing.  

 

 

Satellite Trail

Satellite Trail

Detector Anomaly

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). 

 

A full-frame WFC3/UVIS image of a spiral galaxy, with a diagonal bright line crossing the image.
WFC/UVIS observation id8m11g4q contains a very bright satellite trail. 

 

 

IR Anomalies

Blobs

Blobs

Known Defect

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). 

 

A WFC3/IR image of a star field with dark blobs interspersed.
WFC3/IR observation idq293c3q features many dark blobs. 

 

 

More information: 

Blob Flats webpage 

ISR 2010-06: The WFC3 IR Blobs

ISR 2012-15: The WFC3 IR "Blobs" Monitoring

ISR 2014-21: Infrared Blobs: Time-dependent Flags

ISR 2018-06: WFC3/IR Blob Monitoring

ISR 2021-10: WFC3/IR Blob Flats 

Crosstalk

Crosstalk

Detector Anomaly

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. 

 

An IR image of a star field with a bright star and a dark spot above the PSF.
WFC3/IR observation idn105heq features a prominent example of crosstalk. 

 

More information:

WFC3 Crosstalk webpage

ISR 2010-02: WFC3 TV3 Testing: IR Crosstalk (A. Viana, S. Baggett)

Earth Limb/Shine

Earth Limb/Shine

Detector Anomaly

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). 

 

A WFC3/IR image of a field. The left side of the image is much brighter
HST observation idgb26dtq contains prominent Earth limb.  

 

 

More information:

ISR 2009-21: WFC3 SMOV Results: IR Channel Dark Current, Readnoise, and Background Signal

ISR 2012-12: WFC3/UVIS Sky Backgrounds

ISR 2014-03: Time-varying Excess Earth-glow Backgrounds in the WFC3/IR Channel

IR Filter/Detector Ghosts

IR Filter/Detector Ghosts

Detector Anomaly

  

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. 
An image of a bright star with two detector anomalies - a donut-shaped ghost to the right of the PSF and a diamond-shaped ghost above the PSF.
A section of the FLT for a WFC3/IR observation (HST rootname idq2hlcuq)

 

 

More information:

ISR 2007-16: WFC3 TV2 Testing: IR Channel Ghosts and Baffle Scatter (T. Brown)

ISR 2017-22: WFC3 Anomalies Flagged by the Quicklook Team (C.M. Gosmeyer & The Quicklook Team)

 

Persistence

Persistence

Detector Anomaly

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. 

An IR image in the F110W filter depicting a star field. Bright horizontal lines appear to the right of the brightest stars' PSFs, indicating persistence from previous grism exposures.

 

Below are two relatively extreme examples of persistence artifacts in WFC3/IR imaging.

Two examples of persistence in WFC3/IR imaging.
Left: an example of persistence in imaging a field. The bright diffuse source in the center is a persistence after-image from observations of a galaxy 2 hours prior. Right: several observations of bright fields taken before this observation resulted in persistence artifacts that show the current observation's dithering pattern. 

More information:

MAST Search Portal for WFC3 Persistence Data Products

WFC3 IR Persistence webpage

ISR 2011-09: IR Detector Timing and Persistence (K. Long, T. Wheeler, and H. Bushouse)

ISR 2018-03: Persistence in the WFC3/IR Detector: Intrinsic Variability (K. Long, S. Baggett)

ISR 2018-04: Persistence in the WFC3/IR Detector: An Area Dependent Model (K. Long, S. Baggett)

Satellite Trail

Satellite Trail

Detector Anomaly

Because Hubble is in low-Earth orbit, satellites can intersect with the area of sky being observed. In IR images, calwf3 can sometimes cause the trail to look blacked-out because it tries to fit up-the-ramp a source that appears suddenly in a read.

Snowballs

Snowballs

Known Defect

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)

WFC3 image of some stars and galaxies, with a snowball anomaly present in the lower left quadrant of the image.
FLT  image for a WFC3/IR observation (HST rootname ib8d95lpq). 

 

Anomaly Statistics Plots

Satellite Trail Statistics

WFC3 images are occasionally contaminated by satellites passing through the field of view, which leaves a bright trail in the final image (WFC3 ISR 2017-22).  The WFC3 team tracks these as part of routine instrument monitoring. Figure 1 shows the percentage of satellite trails in external WFC3 images over time. Note that no normalization to exposure time and/or field of view have been done.

Figure 1: Percentage of satellite trails detected in WFC3 images over the years. Satellite trails have maintained relatively steady counts overall; however, there is an apparent decreasing trend since 2009 in the IR channel. Normalizing by the total number of IR images per year, we see that the rate of satellite trails per year in the IR channel has periodically increased and decreased every ~three years between 2009 and 2018, and has decreased from 2018 to 2022, where only half as many IR observations saw satellite trails in 2022 compared to the average between 2009 and 2018.  Conversely, there has been a slow increase in the presence of satellite trails in the UVIS channel, from 0.46% in 2009 to 0.94% in 2022. In 2023, we see an increase in satellite trails across both UVIS and IR channels compared to rates from 2020-2022.  Generated on October 25th, 2023. 

Last Updated: 06/13/2024

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