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WFC3 STAN - Issue 12, January 2013

WFC3 Space Telescope Analysis Newsletter - Issue 12, January 2013

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 20 Calibration Plan

E. Sabbi

The Cycle 20 calibration program started November 1, 2012 and will continue until the end of October 2013. The program was formulated with the actual usage of WFC3 in mind. During Cycle 20, the UVIS and IR channels are both heavily utilized: 64% of the WFC3 exposures are to be acquired with the IR channel, of which 30% use the IR grisms. GOs are using 14 out of the 15 available IR filters. On the UVIS side, there has been a request for 43 of the 63 available filters, with 43% of the exposures acquired in the near UV.

In Cycle 20, for the first time WFC3 has enabled post-flash to mitigate the increasing degradation of the charge transfer efficiency (CTE) of the UVIS detectors. As a result, 44% of the GO UVIS exposures are taken using post-flash.

The WFC3 Cycle 20 Calibration Program has been designed to measure and monitor the behavior of both the UVIS and IR channels and to provide the best calibration data for the approved scientific programs. The calibration activities consist of 30 different programs and can be divided into 5 categories: UVIS and IR Detector Monitor, Photometry Performances, Spectroscopy, Astrometry, and Flatfields Quality:

For Detector Programs on the UVIS side, particular attention is devoted to the characterization and correction of the Charge Transfer Efficiency (CTE) degradation in the CCDs, while for the IR channel the primary effort is to improve the model of the persistence that is caused by previous observations of bright sources.

A total of 83 external and 1833 internal orbits have been allocated for the Cycle 20 calibration program. Further information about the Calibration Plan can be found at the WFC3 web page http://www.stsci.edu/hst/wfc3/calibration/CY20

Calibration of Post-Flashed Science Data

J. Biretta, S. Baggett

The WFC3 post-flash capability has become available to Cycle 20 observers, and is being widely used to mitigate charge transfer errors (CTE degradation) in images with low backgrounds.  This capability uses an LED lamp to briefly illuminate the rear of the shutter blades at the end of an exposure, and hence imposes a well-controlled and near-uniform background light on the image.  We have recently released two new reference files to calibrate post-flashed data and remove the post-flash light pattern from science images.  These reference files, wc52031oi_fls.fits and wc52031pi_fls.fits, replace previous dummy reference files, and are automatically used in the archive calibration pipeline to remove the light pattern.  There is one reference file for each of the "A" and "B" shutter blades, since they have slightly different reflectivity. The appropriate reference file is automatically selected during pipeline processing using the SHUTRPOS header keyword.  These reference files were generated from stacks of very long flashes (typically ~28000 e-/pixel total taken on-orbit in November 2012), and then scaled to the count rate per second of lamp "on" time.   During calibration, the reference file is scaled by the actual lamp on time (given by the FLASHDUR header keyword) before being subtracted from the science image.

The two new reference files are for images taken with the "low" lamp current and un-binned format, which make up the vast majority of the science data.  Work is still underway to deliver additional reference files for other observing modes.  Specifically, the 15 and 22 electron flash levels use the "medium" lamp current, and separate reference files will be needed for these.  Separate reference files will also be needed for data taken with 2x2 and 3x3 binned format.  Work on these additional reference files is underway and they should soon be available.  We are also monitoring the post-flash lamps and shutters for any changes, and new reference files will be delivered as needed in the future.

Observers who obtained their data prior to release of the new reference files can simply re-request the data from the HST archive, and will receive updated images with the latest calibrations applied. The new post-flash reference files were installed in the archive pipeline on Dec. 5, 2012; images taken after that date should automatically have the new calibration applied.  Observers can also check the “FLSHFILE” header keyword to verify that one of the new reference files mentioned above were applied to the data.

Post-flash calibration can also be performed outside the archive pipeline with the usual CALWF3 program.   CALWF3 can be run separately on each input xxxxxxxxx_RAW.FITS file to apply the post-flash calibration. However, we caution against using CALWF3 with an association table (xxxxxxxxx_ASN.FITS) to process multiple post-flashed files together.  Current and past versions of CALWF3 will read the reference file list from the first image in the association table, and apply those files to all the data. This may give incorrect results since different images in a sequence will typically need different post-flash reference files, as the shutter shuttles back and forth between the "A" and "B" blades.

CALWF3 software now available outside of IRAF

M. Sosey, W. Hack

The calwf3 processing software and its associated routines, such as wf3ccd and wf3ir, are now available independent of the IRAF environment as part of the HSTCAL package. The previous IRAF/PyRAF version, while still available within the current STSDAS/IRAF release, will no longer be maintained or updated. The STScI archive pipeline started running the updated HSTCAL-version of the software as of Sept 26, 2012. Users wishing to run the most recent version of the calibration pipeline should not use the version of the software found in the IRAF stsdas.hst_calib.wfc3 package.

Users may still access the routines while running PyRAF by importing the new wfc3tools package (released as part of the latest stsci_python), which can be imported into any python session. The calibration routines may also be called directly from the command line prompt. The wfc3tools package and the calwf3 executable are included with the HSTCAL package released alongside the latest STSDAS public release and can be downloaded from http://www.stsci.edu/institute/software_hardware/stsdas/download-stsdas.  

All versions of calwf3 past 3.0 represent the new HSTCAL software. 

Examples of how to run the code:

In Python, without Teal:

>>> from wfc3tools import calwf3
>>> calwf3.calwf3(filename)
In Python, with Teal:
>>> from stsci.tools import teal
>>> from wfc3tools import calwf3
>>> teal.teal('calwf3')

In PyRAF:

>>> import wfc3tools
>>> epar calwf3

From the command line:

> calwf3.e filename

WFC3/UVIS CTE and Mitigation

S. Baggett, J. Anderson, J. MacKenty, J. Biretta, K. Noeske

The harsh low-earth orbit environment of HST is known to damage CCDs, generating hot pixels, increasing dark current, and decreasing charge transfer efficiency (CTE). Studies of CTE in previous HST instruments as well as WFC3 have shown that CTE losses due to radiation damage depend on the
1) distance from the amplifier: sources further from the amplifier experience more transfers during readout and thus encounter more charge traps
2) intrinsic source signal level: fainter sources experience a higher fractional loss than brighter sources
3) image background: a higher background fills some of the charge traps and thus mitigates the CTE losses and
4) observing scene: sources preceding the target source can fill traps and improve CTE.

In addition, the CTE losses are continuously increasing with time as new charge traps are formed on-orbit. Currently, bright sources furthest (2051 pixels) from the amplifiers suffer ~2-4% losses of their total flux within a 3-pixel radius aperture while moderately faint sources (total fluxes of several 100 electrons) show ~10% and ~50% losses in high (~20 electrons/pixel) and low (close to zero) image backgrounds, respectively. Even fainter sources will experience even larger fractional losses, or become entirely undetectable. The mitigation options for WFC3 CTE losses fall into two general categories: those that can be implemented in the proposal planning stage (during data acquisition) and those that can be applied after the images are in-hand.

During proposal planning:

-- Consider target placement. This is an option for observers with small targets. If the target is placed close to a readout amplifier, it will minimize the number of transfers during readout and thus minimize the amount of source signal lost to charge traps.

-- Increase the image background. This can be accomplished by lengthening exposure times, using a broader filter, and/or applying a low-level post-flash. The optimal level of total background to achieve in WFC3/UVIS science exposures is 12e-/pix, as determined from a series of exposures of the globular cluster Omega Cen (http://www.stsci.edu/hst/wfc3/ins_performance/CTE/).  When sky+dark alone will not provide 12e-/pix of background, observers should consider adding a post-flash to make up the difference.
A post-flash can be easily added to an exposure by specifying the desired flash level in the phase II proposal via the optional parameter 'flash' along with the desired flash level in e-/pix. Post-flash of a typical exposure will add 2-3 seconds of overhead (up to ~5 sec for a full 12e-/pix). Achieving the recommended background will improve the detection of faint sources in low background observations where CTE losses would otherwise remove much or all of the flux from those sources. The modest amount of background required improves the CTE and increases the source signal faster than it increases noise. For higher levels of background, the Poisson noise continues to increase but CTE does not continue to improve significantly. Sky background levels can be estimated using the Exposure Time Calculator. In addition, ISR-2012-12, using all UVIS archival data, summarizes the typical backgrounds achieved in each filter.

** Ensuring that images with faint sources contain a minimum of 12e-/pix background is expected to be a key CTE mitigation strategy for many WFC3/UVIS science proposals. Observers should be aware that the ETC currently does not include the effect of CTE losses, or the effect of any post-flash, in its signal-to-noise calculations. **

After data acquisition:

-- Apply formula-based corrections to photometry. The measured CTE losses of point sources as a function of MJD (Modified Julian Date), source flux and detector Y position have been fit with 2-parameter polynomials. The derived fit coefficients are available to observers who wish to either estimate CTE losses in their data or apply them as a correction to their photometric results. Further details, including the fit coefficients, are available in ISR 2012-09.

-- Apply empirical pixel-based corrections to the image. Currently under development for WFC3 (Anderson et al.), the correction algorithm is similar to the one currently in use for ACS. The WFC3 team has acquired the necessary data for constraining the pixel-based model and work is underway to optimize the algorithm for UVIS. While the correction is expected to work well, the nature of the algorithm is such that it may not be able to completely recover what was lost, particularly at the faintest levels. In addition, in order to avoid amplification of read noise, the algorithm must be conservative in its reconstruction at the low background levels where losses are non-linear.

Release of a standalone version of the software is planned for spring 2013 and will be linked to the WFC3 CTE page. Observers who would like to participate in alpha-testing the correction algorithm are asked to send an expression of interest to help@stsci.edu.

Further details on the WFC3/UVIS CTE are available in the WFC3 Instrument Handbook (Section 6.9) and on the WFC3/UVIS CTE page.

TDFTRANS Data Keyword now populated differently, affecting CALWF3 IR processing 

M. Sosey

Last fall, a FSW update was made to the telescope that changed the way the UVIS detector exposed images when the Take Data Flag (TDF) was down.  Now, if the exposure time is less than 200 seconds, the FSW ignores the TDF state and opens the shutter for the full exposure unless the TDF goes down again, in which case the shutter is closed. If the exposure time is greater than 200 seconds and if the TDF is down at the start of the exposure, the FSW never opens the shutter and sets the exptime = 0. The TDF response is currently disabled for short exposures, earth calibration exposures, and spatial scans. The FSW update has no effect on how the IR detector takes exposures; it has always ignored this flag. 

However, at the same time, the TDFTRANS keyword (the number of TDF transitions) was found to be populated incorrectly in the IR data headers and the software was changed to reflect the value of the TDFNUM for the exposure. In addition we added more details to the headers and trailer file to help describe associations. For association processing in CALWF3 (ASN tables), there can be a mix of exposures that are good and exposures which experienced TDF transitions. Currently, CALWF3 uses the header values from one of the members in the association, if that dataset happens to have no TDF transition, then the high level data product headers reflect only its keyword values, which can be misleading for the observer about the dataset as a whole. An update to CALWF3 processing was made so that more precise information is used to populate the headers. If ANY of the members of the association have an EXPFLAG (exposure interruption indicator) which is other than NORMAL, then the EXPFLAG will be set to MIXED in the high level data product and the EXPFLAG information for each of the member datasets will be added to the history information in the high level products header (and TRA trailer file). Since the UVIS shutter pays attention to the TDF and it consists of only one continuous exposure, the proper processing of the data and update of the actual exposure time is done.

The processing for the IR data is a little more complicated. Since the IR data is a multiaccum, non-destructive read sequence, there are flags for each of the reads reporting the state of the TDF. CALWF3 pays attention to the TDFTRANS keyword for each image set and if the TDF was down, it marks ALL pixels in that read as bad. When the fit to the ramp is performed, any bad reads are ignored. If ALL the reads have been marked as bad, then the software still uses ALL of the reads to get a best estimate ramp for the FLT science image.

The TDFNUM is now being used to populate the TDFTRANS keyword in the IR imset headers. This keyword keeps track of the cumulative number of times the TDF went down during the exposure. If it is ever >0 during an IR exposure, the EXPFLAG will be set to "INDETERMINATE". The catalog will log this in the wfc3_times table TDFTRANS values. If the TDF was down at the beginning of an exposure, TDFNUM will always be >=1 and TDFTRANS will be marked as 1 in all reads. If TDFNUM is zero until a later read, all reads AFTER the TDF went down will also be marked as "bad".

These changes went into effect in the STSCI Archive pipeline fully with the delivery of opus2012.4c in December 2012.

Since the keyword update is a change in generic conversion and how the keywords are populated in the headers and archive tables, this means that data previously downloaded from the archive could come back with quite a different reduction and with different DQ values depending on the state of the TDF. Users should look at the new data carefully and decide what, if any, are useful. Any imsets which a user feels should be considered as valid data can be edited to remove the bad data flag (DQ=4) from the affected imset(s) RAW DQ extensions and be reprocessed through calwf3. In the future a tool may be provided to assist users with this option.

aXe Compatibility with Astrodrizzle

B. Hilbert, J. Lee, N. Pirzkal, M. Sosey

aXe, a software package designed to extract and calibrate slitless spectroscopic data, was originally written to support Multidrizzle formatted astrometric information. Current (aXe v2.3) and earlier versions of the software rely on output files from the Multidrizzle process to transform object pixel coordinates between the user supplied "drizzled" direct image and corresponding spectral images. We are currently working on updating aXe to accommodate the use of new Astrodrizzle products, and maintain back-compatibility with Multidrizzle.  We anticipate that an Astrodrizzle compatible version will be ready in February, at which time an update will be issued and the cookbook and manual will be revised.  Information on how to access the updated software will be reported on the WFC3 grism webpage: http://www.stsci.edu/hst/wfc3/analysis/grism_obs/wfc3-grism-resources.html.

IR Grism Calibrations Updates

N. Pirzkal, J. Lee, B. Hilbert

New trace and wavelength calibrations have been derived for the WFC3 IR G102 and G141 grisms, and will be released by March on the WFC3 grism webpage: http://www.stsci.edu/hst/wfc3/analysis/grism_obs/wfc3-grism-resources.html.  The previous set of published trace and wavelength calibrations were based on Servicing Mission Orbital Verification data taken in 2009 (ISR 2009-17 and ISR 2009-18).

Both trace and wavelength calibrations for the +1 order are now based on a larger number of positions on the detector, enabling finer sampling of the possible field dependence of the grism dispersion solutions.  Significantly improved calibrations for the -1 order have also been derived.  Details will be made available in a forthcoming Instrument Science Report, which will posted here: http://www.stsci.edu/hst/wfc3/documents/ISRs.

Flux sensitivity monitoring of the +1 order for both the G102 and G141 grisms spanning from summer 2009 to winter 2011 has shown that the flux calibrations have had excellent stability with temporal variations constrained to be less than 1%.  Details can be found in ISR 2012-06.

Using the Tiny Tim PSF model for drizzled WFC3 data

J. Mack, M. Lallo


Tiny Tim is a PSF modeling tool which may be run via a web interface or downloaded as a stand-alone application.  The tool creates model images of an 'undistorted' PSF (tiny 2 output) and a 'distorted' PSF (tiny3 output).  This particular wording sounds as if the output products are intended for use with calibrated 'flt.fits' and 'drz.fits' images respectively.  In fact, only the 'distorted' model PSF is recommended for use with calibrated 'flt.fits' images.  

While tiny2 models the SI-independent OTA PSF, tiny3 adds the SI-specific sampling, distortion, and detector effects.  The tiny2 PSF is oversampled to 1.3 times better than the Nyquist sampling for the shortest wavelength in the passband (allowing for better interpolation) and is intended strictly for use by tiny3Tiny3 convolves this PSF with the charge diffusion kernel, introduces the observed geometric distortion at the specific detector position and then resamples the PSF to the WFC3 plate scale.  It represents what the PSF would look like in the 'flt.fits' image at the proper scale and including distortion.  

To derive an undistorted PSF model for use with drizzled archival data products, the output from tiny3 will need to be drizzled using the latest software, such as AstroDrizzle.  Unfortunately, the tiny3 output model does not contain the appropriate header information required for AstroDrizzle to work.  At this point, the model image (pixels only) may be copied into of one of the 'flt.fits' images (at the proper detector location) and then AstroDrizzle may be used to correct for the distortion, applying the same kernel function used to redistribute flux in creating the drizzled pipeline data products. 

Note that, while Tiny Tim modeling is available for the WFC3 detectors, it has not been optimized to reproduce the observed PSFs.  Progress has been made in understanding the short-comings in the model implemented for the WFC3/IR detector (ISR 2012-13).

2013 Calibration Workshop and AstroDrizzle Mini-Workshop

D. Hines, J. Mack


STScI will host the 2013 Calibration Workshop on April 8-11, 2013. Our goal is to foster the sharing of information and techniques between observers, instrument support teams, and instrument developers. The workshop will emphasize calibrations that are required to maximally exploit the capabilities of both HST and JWST and to assess any additional calibrations needed to ensure that the current HST and future JWST archives are robust and as scientifically valuable as possible.  The deadline for on-line registration is March 8, 2012.
A free AstroDrizzle mini-workshop will be offered the day after the main workshop (Friday April 12, 9am-12pm).  As of June 2012, AstroDrizzle has replaced MultiDrizzle as the supported tool for aligning and combining HST images.  This three hour, hands-on session will include drizzling tutorials and personalized assistance with your specific datasets.  While not formally part of the Calibration Workshop, attendees of the Calibration Workshop are invited to attend. For more information, please send an email to cal2013@stsci.edu or to the local organizer, Jennifer Mack mack@stsci.edu.

New Documentation

ISR 2012-15 The WFC3 IR "Blobs" Monitoring - N. Pirzkal, B. Hilbert
ISR 2013-01 WFC3/IR Spatial Sensitivity Test - T. Dahlen
ISR 2013-02 WFC3 Cycle 19 Proposal: 12690: UVIS Gain - H. Gunning, C. Pavlovsky, S. Baggett

The complete WFC3 ISR archive is at:/hst/wfc3/documents/ISRs/

Need help? /hst/wfc3/help.html

 


 

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