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WFC3 STAN - Issue 18, July 2014

WFC3 Space Telescope Analysis Newsletter - Issue 18, July 2014

New information about WFC3 in the "New in the Last 45 Days" and "Late Breaking News" sections.

This and previous issues of the STAN.

Contents:

Cycle 22 Phase II Deadline Reminder and Updates to APT

M. Bourque, K. Peterson, L. Dressel

Observers who have been awarded time on HST for Cycle 22 are reminded that the deadline to submit Phase II proposals is on July 24th, 2014. For more information regarding Phase II development and submission, please visit the Resources for HST Phase II Proposal Development page. Users needing assistance are encouraged to contact the STScI Helpdesk by sending an email to help@stsci.edu.

Phase II proposals must be submitted through the Astronomers Proposal Tool (APT). Users needing to submit Phase II proposals are required to install the latest version of APT (22.2). The latest release of APT contains the following updates/changes:

  1. Removed little-used fields in the Proposal Description - We have removed the Realtime Justification, Calibration Justification, and Additional Comments fields from the Proposal Description form. All of this narrative information about the observation should now go in the remaining Observing Description field.
  2. Updated the version of Aladin - In order to support new features in APT (see below), we have updated to Aladin 8.1. The basic functions of Aladin for visualization are the same, but there are two tasks - zooming and changing the opacity of the aperture overlays - that are different. (See the Aladin training video: Using Aladin with APT in Phase II for details.)
  3. Labels now available in Aladin - It is now possible to overlay labels in Aladin for targets, exposures, and observations. You can also display the current Orientation angle, as well as the POS TARG XY axes. (See the Aladin training videos: Using Aladin with APT in Phase II,Measuring/updating orientations and Creating Pos Targ offsets for details.)
  4. Changes to Hubble Legacy Archive access - Access to HLA data is now through an obvious button on the left hand side of the Aladin Server Selector (it used to be more hidden under the Archives button). More recent HLA data is available, and the data downloads are now JPEG images rather than FITS data (to speed the download times). (See the Aladin training video: Using HLA with Aladin for details.)

    And of particular interest to WFC3 users:

  5. Round-trip Spatial Scans - This new feature of APT is addressed in its own section below.

For more information, please visit the APT website.

Please note: Observers are encouraged to carefully describe the observations that they intend to make in the Observing Description so that Contact Scientists can assess whether the phase II specifications meet the requirements of the observations.

WFC3/IR Backgrounds

G. Brammer

Observers are reminded of a recent study (WFC3 ISR 2014-03) that reported the discovery of a strong terrestrial helium emission line component to the WFC3/IR background at 1.083 microns. The line contribution to the background is seen on the day side of the HST orbit and essentially disappears in the earth shadow. The strength of the line increases with decreasing target-to-limb angle and can vary significantly within a single exposure, reaching count rates as high as 5-6 times that of the nominal zodiacal background.

From a sensitivity standpoint, we report in ISR 2014-03 that the broader F105W and F110W filters are still expected to be more sensitive than the narrower F098M filter for all but the most extreme elevated backgrounds (for background-limited exposures longer than a few hundred seconds). However, such extreme backgrounds are much more likely to occur at low limb angles typical of CVZ observations. Observers planning CVZ observations can consider using the F098M or F125W filters, which are not sensitive to the emission line, in place of F105W or F110W if faint source sensitivity is critical to their observing program. The lowest target-to-limb angles occur at the beginning and end of an orbit. For orbits using multiple filters/grisms, putting F105W/F110W/G102/G141 exposures in the middle of the orbit when possible can help reduce the susceptibility to significantly elevated background levels.

"Round-trip" Spatial Scans

P. McCullough

A new feature in Version 22.2 of APT allows a spatial scan direction to be defined as "round trip", which creates a pair of one "forward" scan and one "reverse" scan. Combined with "number of iterations," the new feature allows users to define time series of spatially-scanned spectra, for example of transiting exoplanets, in a conveniently compact and easily-adjustable format. For example, if "number of iterations" equals 3 and "scan direction" equals "round trip", the phase II file will be equivalent to one in which six exposures are individually defined with scan directions equal to forward, reverse, forward, reverse, forward, and reverse.

CTE Correction for UVIS Subarrays Without Pre-Scan Bias Pixels

M. Bourque, J. Anderson

The pixel-based Charge Transfer Efficiency (CTE) correction software performs corrections on "raw" and "flt" images to produce "rac" and "flc" images, respectively. The software is a stand-alone FORTRAN program that is available in three different flavors: (1) wfc3uv_ctereverse.F, which operates on full-frame UVIS images, (2) wfc3uv_ctereverse_wSUB.F, which operates on UVIS subarrays, and (3) wfc3uv_ctereverse_parallel.F, which operates on full-frame UVIS images and uses parallel processing. These routines are available to download from the CTE Tools webpage.

The wfc3uv_ctereverse_wSUB.F version of the software operates on UVIS subarrays that contain pre-scan bias pixels. As a result, there are some subarrays that are not supported by the software, as shown in the following table:

SupportedUnsupported
UVIS1-2K4-SUBUVIS2-M1K1C-SUB
UVIS1-2K2A-SUBUVIS2-M512C-SUB
UVIS1-2K2B-SUB
UVIS1-2K2C-SUB
UVIS1-2K2D-SUB
UVIS2-C1K1C-SUB
UVIS2-C512C-SUB


However, CTE corrections for the UVIS2-M1K1C-SUB and UVIS2-M512C-SUB subarrays can be achieved by placing the subarray into a full-frame raw bias image, performing the CTE correction on this "hybrid" file, and extracting the subarray from the CTE-corrected full frame. This procedure is outlined below:

(1) Retrieve a full-frame UVIS bias frame that was acquired closest in time previous to the sub-array observation. This can be done by using the following parameters in the HST search form on MAST: Target Name = "BIAS*, DARK*", Resolver = "Don't Resolve", Exp Time = "0, 0.5", Apertures = "UVIS", select "WFC3" under Imagers, select "Calibration" under Observations, unselect "Science" under Observations, and finally, enter a Start Time which complements the observation date of the subarray observation. Please note that, due to Flight Software constraints, post-flash biases are acquired as 0.5 second DARK images, and can be regarded as BIAS frames.

(2) Copy the subarray onto the full-frame raw bias image at the appropriate area where the subarray exists on the detector. The below table shows which rows and columns of the full-frame bias file that correspond to the subarray placement.

SubarraySubarray DimensionsFull-Frame RAW RowsFull-Frame RAW ColumnsFull-Frame FLT RowsFull-Frame FLT Columns
UVIS2-M1K1C-SUB1024 X 10241049 - 20721028 - 20511024 - 20471028 - 2051
UVIS2-M512C-SUB512 X 5121561 - 20721540 - 20511536 - 20471540 - 2051


Below is an example of how to perform this placement using IRAF/PyRAF (Note that IRAF/PyRAF uses 1-based indexing):

For UVIS2-M1K1C-SUB:
--> imcopy <subarray>_raw.fits[1] <bias>_raw.fits[1,overwrite+][1049:2072,1028:2051]
--> imcopy <subarray>_flt.fits[1] <bias>_flt.fits[1,overwrite+][1024:2047,1028:2051]

For UVIS2-M512C-SUB:
--> imcopy <subarray>_raw.fits[1] <bias>_raw.fits[1,overwrite+][1561:2072,1540:2051]
--> imcopy <subarray>_flt.fits[1] <bias>_flt.fits[1,overwrite+][1536:2047,1540:2051]


(3) Run the full-frame CTE correction software over the "hybrid" file.

./wfc3uv_ctereverse.e <bias.fits> <FLC+>
or
./wfc3uv_ctereverse_parallel.e <bias.fits> <FLC+>


(4) Extract the subarray from the CTE-corrected "hybrid" file and place back onto subarray file.

For UVIS2-M1K1C-SUB:
--> imcopy <bias>_rac.fits[1][1049:2072,1028:2051] <subarray>_raw.fits[1,overwrite+]
--> imcopy <bias>_flc.fits[1][1024:2047,1028:2051] <subarray>_flt.fits[1,overwrite+]

For UVIS2-M512C-SUB:
--> imcopy <bias>_rac.fits[1][1561:2072,1540:2051] <subarray>_raw.fits[1,overwrite+]
--> imcopy <bias>_flc.fits[1][1536:2047,1540:2051] <subarray>_flt.fits[1,overwrite+]

You are now left with a CTE-corrected subarray image. Users may want to consider renaming the end products to "<subarray>_rac.fits" and "<subarray>_flc.fits" to be consistent with that of nominal CTE correction software products.

WFC3 Presentations and Posters at 2014 Summer Meetings

M. Bourque

A SPIE Astronomical Telescopes & Instrumentation conference was held on June 22 - June 27th, 2014 in Montreal, Quebec, Canada and The 224th American Astronomical Society Meeting was held on June 1 - June 5, 2014 in Boston, Massachusetts. One WFC3 talk and one WFC3 poster were presented at the SPIE meeting, and four WFC3 posters were displayed at the AAS meeting. The abstracts are shown below. Electronic copies of the posters, talk slides, and accompanying materials are available at http://www.stsci.edu/hst/wfc3/documents/meeting_posters/. Please feel free to send any questions that you have to help@stsci.edu.

HST/WFC3: New Capabilities, Improved IR Detector Calibrations, and Long-term Performance Stability - SPIE - Talk 9143-72
John MacKenty, Sylvia Baggett, Gabriel Brammer, Bryan Hilbert, Knox Long, Peter McCullough, Adam Riess (STScI)

Wide Field Camera 3 (WFC3) is the most used instrument on board the Hubble Space Telescope. Providing a broad range of high quality imaging capabilities from 200 to 1700nm using Silicon CCD and HgCdTe IR detectors, WFC3 is fulfilling both our expectations and its formal requirements. With the re-establishment of the observatory level “spatial scan” capability, we have extended the scientific potential of WFC3 in multiple directions. These controlled scans, often in combination with low resolution slit-less spectroscopy, enable extremely high precision differential photometric measurements of transiting exo-planets and direct measurement of sources considerably brighter than originally anticipated. In addition, long scans permit the measurement of the separation of star images to accuracies approaching 25 micro-arc seconds (a factor of 10 better than prior FGS or imaging measurements) enables direct parallax observations out to 4 kilo-parsecs. In addition, we have employed this spatial scan capability to both assess and improve the mid- spatial frequency flat field calibrations.

WFC3 uses a Teledyne HgCdTe 1014x1014 pixel Hawaii-1R infrared detector array developed for this mission. One aspect of this detector with implications for many types of science observations is the localized trapping of charge. This manifests itself as both image persistence lasting several hours and as an apparent response variation with photon arrival rate over a large dynamic range. Beyond a generally adopted observing strategy of obtaining multiple observations with small spatial offsets, we have developed a multi-parameter model that accounts for source flux, accumulated signal level, and decay time to predict image persistence at the pixel level. Using a running window through the entirety of the acquired data, we now provide observers with predictions for each individual exposure within several days of its acquisition.

Ongoing characterization of the sources on infrared background and the causes of its temporal and spatial variation has led to the appreciation of the impact of He I 1.083 micron emission from the earth’s atmosphere. This adds a significant and variable background to the two filters and two grisms which include this spectral feature when the HST spacecraft is outside of the earth’s shadow.

After nearly five years in orbit, long term trending of the scientific and engineering behavior of WFC3 demonstrates excellent stability other than the expected decline in CCD charge transfer efficiency. Addition of post-flash signal to images is shown to markedly improve the transfer efficiency for low level signals. Combined with a pixel based correction algorithm developed at STScI, CCD performance is stabilized at levels only slightly degraded from its initial values.


The HST/WFC3 Flux Calibration Ladder - SPIE - Poster 9143-123

Susana E. Deustua, Ralph C. Bohlin, John W. MacKenty, Norbert Pirzkal (STScI)

Astronomical flux calibration’s primary goal is to convert measurements recorded in some particular instrumental units into physical quantities, e.g. Watts/m2, removing as much as possible all instrumental signatures. Absolute flux calibration in astronomy relies on transferring the calibration from SI traceable laboratory standards to astronomical objects of interest. However, this is far from trivial - the transfer must reach outside the atmosphere, extend over 4π steradian of sky, cover a wide range of wavelengths, and span an enormous dynamic range in intensity.


Vega is one of only a few stars calibrated against an SI-traceable blackbody, and is the historical flux standard. Photometric zeropoints of the Hubble Space Telescope’s instruments rely on Vega, through the transfer of its calibration via stellar atmosphere models to the suite of standard stars. HST’s recently implemented scan mode has enabled us to develop a path to an absolute SI traceable calibration for HST IR observations. To fill in the crucial gap between 0.9 and 1.7 micron in the absolute calibration, we acquired -1st order spectra of Vega with the two WFC3 infrared grisms. At the same time, we have improved the calibration of the -1st orders of both WFC3 IR grisms, as well as extended the dynamic range of WFC3 science observations by at least 10 magnitudes (down to 0 mag). We describe our progress to date on the WFC3 ‘flux calibration ladder’ project to provide currently needed accurate zeropoint measurements in the IR.


HST Wide Field Camera 3: Improvements to WFC3/UVIS Photometric Calibration - AAS - Poster 122.03
Catherine Gosmeyer, Sylvia Baggett, Ariel Bowers, Tomas Dahlen, Susana Deustua, Derek Hammer, Jennifer Mack (STScI)

The Wide Field Camera 3 (WFC3) is a fourth-generation imaging instrument installed on the Hubble Space Telescope (HST) in May 2009. It contains both an IR and a UVIS channel. The latter, which covers the 200-1000nm spectral range, consists of two 2K x 4K CCD chips along with 62 spectral elements and one grism. The two CCD chips were manufactured on different foundry runs. As a result, there are differences in the two chips’ properties and behaviors, such as their lithography imprint patterns, their sensitivity responses, and their measured quantum efficiency (QE), particularly in the UV. Therefore, the WFC3 team developed a chip-dependent approach to the photometric calibration, where each chip now has its own separate calibration. We discuss the impacts of this new approach and its implementation in the calibration pipeline, presenting the new zero points and header keywords, as well as the new flat fields. We also present the latest trends in the contamination monitoring, which obtains regular imaging and grism spectroscopy of the white dwarf, GRW+70, in key filters F218W, F225W, F336W, F814W, and F606W. No contamination effects have been detected, though there is evidence for a small photometric drift (<1% over 3 years). We anticipate that these efforts will improve UV imaging with WFC3.

HST WFC3: UVIS Dark Calibration - AAS - Poster 122.04
Matthew Bourque, John Biretta, Jay Anderson, Sylvia Baggett, Heather Gunning, John MacKenty, and the WFC3 Team (STScI)

Wide Field Camera 3 (WFC3), a fourth-generation imaging instrument on board the Hubble Space Telescope (HST), has exhibited excellent performance since its installation during Servicing Mission 4 in May 2009. The UVIS detector, comprised of two e2v CCDs, is one of two channels available on WFC3 and is named for its ultraviolet and visible light sensitivity. We present the various procedures and results of the WFC3/UVIS dark calibration, which monitors the health and stability of the UVIS detector, provides characterization of hot pixels and dark current, and produces calibration files to be used as a correction for dark current in science images. We describe the long-term growth of hot pixels and the impacts that UVIS Charge Transfer Efficiency (CTE) losses, post-flashing, and proximity to the readout amplifiers have on the population. We also discuss the evolution of the median dark current, which has been slowly increasing since the start of the mission and is currently ~6 e-/hr/pix, averaged across each chip. We outline the current algorithm for creating UVIS dark calibration files, which includes aggressive cosmic ray masking, image combination, and hot pixel flagging. Calibration products are available to the user community, typically 3-5 days after initial processing, through the Calibration Database System (CDBS). Finally, we discuss various improvements to the calibration and monitoring procedures. UVIS dark monitoring will continue throughout and beyond HST’s current proposal cycle.

HST WFC3: UVIS Charge-Transfer Efficiency Losses: Mitigation and Correction - AAS - Poster 122.05
Jay Anderson, Sylvia Baggett, John MacKenty, and the WFC3 Team (STScI)

WFC3/UVIS was installed on HST in 2009 and has been subjected to the harsh radiation environment of space for the past five years. One consequence of radiation damage is charge-transfer-efficiency (CTE) losses, and these losses became apparent much more quickly than expected for WFC3/UVIS because of its low dark current and the low backgrounds in the ultraviolet. Several years ago, we developed a pixel-based model to describe the WFC3/UVIS charge-transfer process, similar to the model that was developed for ACS/WFC. This model showed us that a low-level of post-flash (12 electrons) significantly improves CTE, and many observations now take advantage of this preventative strategy. We have recently taken some calibration data to allow us to improve the pixel-based CTE-correction model in anticipation for the model's inclusion in the WFC3 pipeline. This poster will report on the latest model and CTE-losses.

WFC3 News Regarding IR Backgrounds, Spatial Scans, and Cycle 22 Phase 2 Advice - AAS - Poster 122.06
John MacKenty and the WFC3 Team (STScI)

We report on recent developments in the characterization and calibration of the Hubble Space Telescope's Wide Field Camera 3. Installed in 2009 during HST Servicing Mission 4, WFC3 continues as the most used instrument on the observatory with stable performance and steady improvements in its photometric, flat field, dark current, and astrometric calibrations. An important recent development is the recognition of the impact of the Helium 1.083 micron emission line from the upper atmosphere. This sometimes results in significant background variations of factors of 3 to 5 higher than the nominal zodiacal light background in the F105W, F110W, G102, and G141 spectral elements. We also report on progress in using observatory level spatial scans to achieve higher dynamic range observations, to obtain higher precision and more efficient photometric measurements of bright sources, to improve our knowledge of the flat fields, and to increase the precision of astrometric measurements.

New Documentation

ISR 2014-02: The Impact of x-CTE in the WFC3/UVIS detector on Astrometry - J. Anderson
ISR 2014-03: Time-varying Excess Earth-glow Backgrounds in the WFC3/IR Channel - G. Brammer, N. Pirzkal, P. McCullough, J. MacKenty
ISR 2014-04: WFC3 Cycle 19 & 20 Dark Calibration: Part I - J. Biretta, M. Bourque
ISR 2014-05: WFC3 Cycle 20 Proposal 13168: UVIS Gain - H. Gunning, S. Baggett
ISR 2014-06: New WFC3/IR Dark Calibration Files - M. Dulude, S. Baggett, B. Hilbert
ISR 2014-07: WFC3 Cycle 21 Calibration Program - E. Sabbi & the WFC3 Team
ISR 2014-08: WFC3 Side-Switch Observatory Verification Programs - H. Gunning, E. Sabbi, J. MacKenty, S. Baggett & the WFC3 Team
ISR 2014-09: Use of the Shutter Blade Side "A" for UVIS Short Exposures - K. Sahu, S. Baggett, J. MacKenty
ISR 2014-10: Improved TinyTIM Models for WFC3/IR - J. Biretta
ISR 2014-11: The Near Infrared Sky Background - N. Pirzkal
ISR 2014-12: Astrometric Correction for WFC3/UVIS Filter-Dependent Component of Distortion - V. Kozhurina-Platais
ISR 2014-13: No Evidence Found for WFC3/UVIS QE 'Overshoot' - M. Bourque, S. Baggett, L. Dressel
ISR 2014-14: Attempts to Mitigate Trapping Effects in Scanned Grism Observations of Exoplanet Transits with WFC3/IR - K. Long, S. Baggett, J. MacKenty, P. McCullough
ISR 2014-15: Enabling Observations of Bright Stars with WFC3 IR Grisms - S. Deustua, R. Bohlin, J. MacKenty


The complete WFC3 ISR archive is at: /hst/wfc3/documents/ISRs/
Need help? /hst/wfc3/help.html


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