The Status of HST/WFC3

John W. MacKentySpace Telescope Science Institute

12 August 2014Calibration Workshop

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 this summer!!• Kudos to GSFC, Ball Aerospace, e2Vv Teledyne, etc…..

WFC3 Optical Layout

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

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

WFC3 in HST

WFC3 Posters by STScI Team

• Jay Anderson: WFC3/UVIS Charge-Transfer-Efficiency Losses: Mitigation and Correction

• Matthew Bourque: The WFC3/UVIS Dark Calibration and CCD Monitor• Susana Deustua: The WFC3 Two Chip Solution: New Zeropoints and

Flatfields for WFC3/UVIS• Michael Dulude: A New Generation of WFC3/IR Dark Calibration Files• Catherine Gosmeyer: Longterm photometric trends in WFC3/UVIS• Heather Gunning: The WFC3/UVIS Gain Monitor and Low Sensitivity

Pixel Population• Jennifer Mack: WFC3/UVIS Flat Field Accuracy & Improved Solutions

for UV Filters• Kai Noeske: Charge Transfer Efficiency in WFC3/UVIS: Monitoring and

Corrections from Star Cluster• Nor Pirzkal: The IR background as seen by WFC3

WFC3 Talks by STScI Team

• Knox Long: Persistence in Near-IR Detector Arrays• Peter McCullough: WFC3 Spatial Scanning (Exoplanets)• Stefano Casertano & Adam Riess: WFC3 Spatial Scanning

(Astrometry)• Kailash Sahu: WFC3 UVIS Shutter Blade• Gabe Brammer: Elevated Sky Backgrounds in WFC3 from He 10830• Bryan Hilbert et al.: WFC3 Time Variable Background Correction

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 SpectraDirect (left) and Scanned (right)

Pair of Stars Scanned in Raster(multiple scans co-added on right)

WFC3 Spatial-Scan L Flats(Validation of F606W)

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

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 non-

uniform 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

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)

Radiation Effects of CCDs

• CCD Charge Transfer Efficiency Declines due to radiation damage• 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 4.6e-

3) 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

360 Second Dark ImagesCentral 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)

Future Calibration Initiatives (1)

• PSF Library– ~10^7 stars “reasonably isolated” with “reasonable S/N” in F606W– Will expand to entire set of WFC3 observations– First application: improve focus monitoring from ~2 mm to <1mm– Exploring methods for making this usefully available

• Astrometric improvements– Initial requirement: 4 mas (0.1 pixels) for AstroDrizzle– WFC3 very stable internally due to thermal control of optical bench– Inclusion of photolithographic mask offsets (2013) 2 mas– Inclusion of filter induced mid-spatial frequencies 1 mas

• Done for ~10 UVIS filters with sufficient Omega Cen data• Considering expanding to most filters

– How to best exploit GAIA?• All Guide Stars and many (most?) frames to <1mas absolute!

Future Calibration Initiatives (2)

• Better instrument calibration/understanding– Time correction for UVIS zero points (<0.3 percent per year)

• This affects visible (not UV or IR); WHY??– Continued exploration of infrared detector persistence

• Improved model for predicted images (but will hit limits)• Repeated observations of bright objects noise floor?

– Very high precision astrometric calibrations for spatial scans

• Advanced GRISM data reduction algorithms/software– Tools to handle observations at multiple roll angles– Modeling approach to extract fainter sources and understand errors– Highly synergistic with JWST and WFIRST-AFTA needs

Where to get more information

• http://www.stsci.edu/hst/wfc3– WFC3 Instrument Handbook– Instrument Science Reports (“ISRs” also on ADS)

• help@stsci.edu