HST/WFC3: New capabilities, improved IR
detector calibrations, and long-term
performance stability
John W. MacKenty, Sylvia M. Baggett,
Gabriel Brammer, Bryan Hilbert, Knox S.
Long, Peter McCullough, and Adam G. Riess
Space Telescope Science Institute
26 June 2014
SPIE 9143-72
WFC3 Summary
• Installed in May 2009 during Servicing Mission 4 by the crew of STS-125
• Provides HST with powerful imaging capabilities
– Visible light capabilities to complement ACS
• More filters, smaller pixels, new CCDs
– Greatly improved near UV and near IR performance
– >50% of all HST observations in every year since SM4 (including Cy22!)
• WFC3 is operating nominally
– Photometric zero points (including UV) stable to ~1% since 2009
– Astrometric calibration is stable (dominated by HST OTA “breathing”)
– UVIS CCD Detectors CTE declining as expected with radiation damage
and mitigation/correction methods have improved considerably
– IR Detector shows essentially zero evolution of its performance in flight
– 5 year life requirement surpassed last month!!
• Kudos to GSFC, Ball Aerospace, e2Vv Teledyne, etc…..
WFC3 Optical Layout
UVIS Channel
• FOV 160 X 160 arc sec
• F/31
• OTA + WFC3 focal length = 78,000 mm
• 4k X 4k CCD, 15 um pixels
• 200 - 1000 nm
IR Channel
• FOV 123 X 139 arc sec
• F/11
• OTA + WFC3 focal length = 29,000 mm
• 1k X 1k HgCdTe array, 18 um pixels
• 850 - 1700 nm
WFC3 in HST
Outline
– Spatial Scans
– IR Persistence
– IR Background Variations
– CCD CTE Decline Mitigation
Spatial Scans
• Spatial Scans allow the light from astronomical objects to be trailed
across the WFC3 detectors in a predictable and repeatable manner
during exposures
– Rates up to 4.8 arc seconds per second (FGS control) and 7.8 “/s
(GYRO control) are supported
– Restoration of an Observatory capability not used since 1996
• Enables:
• Significantly increased S/N observations – especially for infrared
spectroscopy of bright sources (e.g. exo-planet transits)
• Higher precision astrometric measurements orthogonal to the scan
direction (measurements to <30 micro-arc seconds)
• Opportunity to observe brighter sources (especially with the IR
detector– 0th mag achieved with Grism in -1 order)
• Flat field verification and improvement of mid-spatial frequency
residuals in near UV CCD flats
IR Spectra
Direct (left) and Scanned (right)
Pair of Stars Scanned in Raster
(multiple scans co-added on right)
WFC3 Spatial-Scan L Flats
(Validation of F606W)
High Precision Astrometry
• Nominal precision of centroiding of bright stars is 0.01 pix (400 µas)
• Scans over several 1000s of pixels
•  factor of >10 better (reaching ~25 µas)
• Exceeds FGS precision and compares favorably with GAIA
• Successful measurement of Cepheid parallax to ~2 kpc (5 kpc possible)
• Many details must be carefully considered (ongoing efforts to improve)
• See Riess, Casertano, Anderson, MacKenty, Filippenko 2014 Ap.J.
785,161 (arXiv:1401.0484) for a full description
MW Cepheid SY AUR @ 2.3 Kpc
Two Advantages of Spatial Scans,
Jitter Removal and Repetition
Jitter between lines
is coherent,
subtracted in line
separations (vs time),
PASS is doubly
differential
Target scanned
over ~4000 pix,
improves S/N by
up to ~40 (or 10 for
correlated errors
on scales of 40 pix)
Astrometric Precision Per Exposure
Need high S/N
per pixel so bright
targets give best
results
Cepheid photometry w/ same zeropoints as SN hosts by scanning!
APT Support for Spatial Scans
IR Detector Persistence
• The WFC3/IR detector has excess dark currents for several hours
following exposures to >50% full well
• Produces a typical signal ~0.3 e-/s after 1000 seconds from bright
sources
• This is a complex charge trapping behavior
• Mitigation Approaches:
1) Users should dither IR images
2) STScI manually identifies the majority of the “worst events” in
advance and attempts to preclude other IR observations during several
subsequent orbits
3) STScI now provides a predicted residual image for every WFC3/IR
observation within 7 days of your observations
• See: http://archive.stsci.edu/predps/persist/search.php
IR Detector Persistence
• Continuing to refine our understanding and models
Persistence is a function of
• Exposure Time and Arrive rate of photons
• Total number of photons collected in a pixel
• Time since last exposure
– Effects accumulate (e.g. multiple bright exposures build up persistence)
–
• Recent emphasis on short exposures of bright objects to improve exoplanet
transit observations
– Systematic effects (not fully repeatable) impacting series of short exposures of
bright sources (a limiting factor for exoplanet observations)
• Combination of deep survey programs (e.g. Frontier Fields) with 300+
orbits of spatial scan exoplanet observations in Cycle 21
– Team has expanded “bad actors” manual classification and scheduled longer
gaps
• Resurrection of persistence has been observed and now confirmed
– Residual imaged faded completely after 24+ hours
– Internal flat (not saturated) made persistence image re-appear 48 hour after
original observation
– Not understood
Persistence Model
• At each fluence level, fit
the persistence to a
power law of the form:
Variation with exposure time
Infrared Background Variation
•
A major strength of the WFC3/IR channel is that Broad Filters and
GRISMS are intended to be Zodiacal background limited
•
Nominal Backgrounds: 0.5 to 1.0 e-/s/pixel
• HOWEVER: sometimes brighter (up to 3 to 5e-/s) and nonuniform backgrounds are observed
• Particularly problematic for deep Grism surveys
• Causes now well understood
• Pointing traversing central part of zodiacal cloud
• i<80°sun angle near ecliptic plane
• Long dwells near bright earth limb (i.e. CVZ or near CVZ
situations).
• Inclusion of He I 10830Å line within the passbands of G102,
G141, F105W, F110W
• Careful scheduling required for faint observations – consult STScI
Sections From Two GRISM Observations
(background level & structure varies)
He I 10830A Line
Background function of Day/Night
Location with He Line
Minimal Effect without He Line
Spectroscopic Confirmation
• Dust particles on in-focus IR channel
select mirror provide “negative”
spectrum
• He I 10830 line present when
background is high in both Grisms.
Radiation Effects of CCDs
• CCD Charge Transfer Efficiency Declines due to radiation damge
• Results in Loss of S/N for faint sources; charge trailing; photometric &
astrometric errors
• Mitigation Options for Observers:
1) Place sources of interest near the readout amplifiers in corners!
2) Post-Flash (i.e. Background Enhancement)
• Recommended option for Cycle 20+
• 10-12 e- per pixel restores most CTE
• Benefits: recovers faint sources, better darks
• Cost: increased effective noise from 3.1e- to 4.6e3) Post-Observation Corrections
• Pixel Based Charge Transfer Model (Anderson&Bedin)
• Benefits: Restores trailed charge to correct locations
• Limitation: Cannot recover lost S/N
• Available for post-processing at
http://www.stsci.edu/hst/wfc3/ins_performance/CTE/
• To be included in OPUS pipeline in late 2014
Post-Flash Recovers Faint Sources
CCD Trending and Dark Current
•
CCD Detectors performing nominally (radiation damage growth on trend line)
– ~50% usage of PostFlash for CTE mitigation (appropriate fraction; GO
understand option)
– Unexplained glitches in hot pixel recovery (revert to trend line)
360 Second Dark Images
Central 1000x100 pixel region
Zero e- Post-Flash
15 e- Post-Flash
Improved CCD Dark Calibrations
• Correcting for CTE and using anneal-cycle average values for “good” pixels
improves the background of deep programs
• Developed by Matthew Bourque and Marc Rafelski (GO)
CTE & UVIS L-flats
F336W
4th
order
5th order
•
UVIS L-flats are derived from Omega Cen
stellar photometry and may therefore be
subjected to CTE losses
•
Figure shows the ratio images between L-flats
derived from CTE corrected and non-CTE
corrected images.
•
Here we have used the full set of 28 images at
multiple rotations from both Cy 17 and Cy 18.
•
Result: CTE losses are visible
- Peak-to-peak ~0.3%
Ratio images flc/flt, peak2peak: 0.3%
Mean ratio as a function of y-position
Where to get more information
• http://www.stsci.edu/hst/wfc3
– WFC3 Instrument Handbook
– Instrument Science Reports (“ISRs” also on ADS)
• help@stsci.edu