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WFC3 STAN - Issue 16, December 2013

WFC3 Space Telescope Analysis Newsletter - Issue 16, December 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 21 Calibration Plan

E. Sabbi

The Cycle 21 Calibration Program started on November 1st, 2013 and will continue until the end of October 2014. The program was formulated with the actual usage of WFC3 in mind. Details of the Calibration Plan are outlined below. Extensive information about the Calibration Plan can also be found at the WFC3 Calibration Web Page.

The WFC3 Cycle 21 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 33 different programs and can be divided in 8 categories: UVIS and IR Detector Monitors, Photometry Performances, Spectroscopy, Astrometry, Flatfields Quality, IR Traps Characterization, CTE Characterization and Calibration.

Monitor programs have been designed to monitor the health of the UVIS and IR channels. For the UVIS channel, monitor activities include a monthly anneal of the CCDs to repair hot pixels, a hysteresis (bowtie) monitor to remove QE offsets, the acquisition of bias, dark and flatfields to monitor the main properties of the instrument and reference files. These programs are continuations of the corresponding Cycle 17, 18 and 19 programs. In addition, we are executing programs to support post-flash observations and monitor the stability of the LED lamps with time. For the IR channel, we continue to monitor and update calibration files for dark current, non-linearity and gain.

Photometry programs include periodical measurements of the WFC3 throughput in a series of key filters, and check the zero-points for all the WFC filters.

Spectroscopy programs are designed to monitor the flux and wavelength calibration of the three WFC3 grisms, and characterize the two-dimensional structure of the IR background.

Flatfield programs are designed monitor and validate the WFC3 flatfields using the spatial scan of bright stars, and by observing a spectrophotometric standard in a number of positions across the UVIS detectors in a series of UV filters. Observations of the bright Earth are used to improve the IR inflight flats and monitor the health of the channel selecting mechanism (CSM), while UVIS and IR internal flats are used to monitor high spatial frequency variations.

Astrometric programs are intended to monitor the stability of the geometric distortion solutions for both UVIS and IR channels. In addition, spatial scan is used to improve our knowledge of internal distortion of WFC3/UVIS and measure the WFC3-FGS calibration.

On the UVIS side, particular attention is devoted to the characterization and correction of the CTE degradation of the CCDs. On the IR side, the main effort is to improve the model of the persistence caused by previous observations of bright sources. Particular effort is devoted to the characterization of the effect of traps in exosolar planet studies.

A total of 98 external and 1907 internal orbits have been allocated for the Cycle 21 calibration program.

New Tools Available in WFC3TOOLS

M. Sosey

Two new IR data analysis tools were added to the SSBX release of the WFCTOOLS package in STSCI_PYTHON. They are python replacements for the older IRAF tasks PSTAT and PSTACK.

wfc3tools.pstat plots the statistics for a specified image section up the stack of an IR MultiAccum image (the *_ima.fits files produced during pipeline reduction). Sections from any of the SCI, ERR, or DQ image extensions may be plotted. A choice of mean, median, mode, standard deviation, minimum and maximum statistics is available. The total number of samples is determined from the primary header keyword NSAMP and all samples (excluding the zero-read) are plotted using matplotlib. The SCI, ERR, and DQ statistics are plotted as a function of sample time. The sample times are read from the SAMPTIMe keyword in the SCI header for each readout. In addition to displaying the resulting plot, the data arrays are returned to the user for each axis. If no image section is specified, statistics for the entire image will be returned. Users can also turn off the default plot production and the tool will just return the data arrays.

Usage:

pstat(filename,extname="sci",units="counts",stat="midpt",title=None,xlabel=None,ylabel=None,plot=True)

>>> python
>>> from wfc3tools import pstat
>>> pstat.pstat(inputFilename,extname="sci",units="counts",stat="midpt",title="",xlabel="",ylabel="" )

You can then do anything you like with the data arrays, but if you wish to just save the plot which was displayed to the screen you can issue these commands:

>>> from matplotlib import pylab as plot
>>> plot.savefig('pstat_plot.pdf') #you can also save as a jpg by just changing the name of the file

Example PSTAT results

wfc3tools.pstack plots the stack of MultiAccum sample values for a specified pixel in an IR MultiAccum image. Pixels from any of the SCI, ERR , DQ or TIME image extensions may be plotted. The total number of samples is determined from the primary header keyword NSAMP and all samples, excluding the zeroth read, are plotted. The SCI, ERR, and DQ values are plotted as a function of sample time, while the TIME values are plotted as a function of sample number. The sample times are read from the SAMPTIME keyword in the SCI header for each readout. If any of the ERR, DQ, SAMP or TIME extensions have null data arrays, the value of the PIXVALUE extension header keyword is substituted for the pixel values. The plotted data values can be saved to an output text table or printed to the terminal.

Usage:

pstack(filename,column=0,row=0,extname="sci",units="counts",title=None,xlabel=None,ylabel=None,plot=True):

>>> python
>>> from wfc3tools import pstack
>>> xdata,ydata=pstack.pstack('ibvwh2zeq_ima.fits',column=340,row=210,extname="sci",units="counts",
   title="Example Plot")

New WFC3/IR Full-frame and Subarray Dark Calibration Files Released

M. Dulude, B. Hilbert, S. Baggett

New WFC3/IR dark reference files for use in the calibration pipeline are now available for the full-frame and subarray observing modes tabulated below.

The new superdarks were generated from a much larger number of input ramps taken over a longer timespan than the previous darks. The larger number of input ramps significantly improves signal to noise and reduces error values. For most modes, the signal to noise in the new reference files is a factor of 5-8 higher than in the previous reference files for the full-frame darks, even higher for some subarray darks. In addition, these new superdarks incorporate a non-linearity correction and were processed using the persistence masks now available from MAST (http://archive.stsci.edu/prepds/persist/search.php; more information on persistence is available at http://www.stsci.edu/hst/wfc3/ins_performance/persistence/).

The pipeline (OTFR) began using the SPARS200/full-frame dark reference file on May 16 2013 15:55:19 GMT, the rest of the full-frame dark reference files on Oct 16 2013 20:34:13 GMT, and the subarray dark reference files on Dec 12 2013 21:32:31 GMT. To apply the new dark reference files to observations retrieved before the dates listed, users can either download the appropriate dark reference file(s) listed below and manually reprocess their observations, or simply re-retrieve the science data from the archive.

Observing ModeDark Reference File Name Observing ModeDark Reference File Name
RAPID Full-Frame xag19292i_drk.fits RAPID SQ512SUB xcc20397i_drk.fits
SPARS10 Full-Frame xag19294i_drk.fits SPARS25 SQ512SUB xcc2039ci_drk.fits
SPARS25 Full-Frame xag19295i_drk.fits STEP25 SQ512SUB xcc2039di_drk.fits
SPARS50 Full-Frame xag19296i_drk.fits RAPID SQ256SUB
(For data taken before
2011-01-09 00:00:00 GMT)

xcc20395i_drk.fits
SPARS100 Full-Frame xag19293i_drk.fits RAPID SQ256SUB
(For data taken after
2011-01-09 00:00:00 GMT)

xcc20396i_drk.fits
SPARS200 Full-Frame x5g1509ki_drk.fits SPARS10 SQ256SUB xcc2039ai_drk.fits
STEP25 Full-Frame xag19299i_drk.fits SPARS25 SQ256SUB xcc2039bi_drk.fits
STEP50 Full-Frame xag1929ai_drk.fits RAPID SQ128SUB xcc20394i_drk.fits
STEP100 Full-Frame xag19297i_drk.fits SPARS10/SQ128SUB xcc20399i_drk.fits
STEP200 Full-Frame xag19298i_drk.fits RAPID SQ64SUB xcc20398i_drk.fits

WFC3/UVIS CTE Correction Code Update

J. Anderson

An updated version of the WFC3/UVIS CTE pixel-based correction is available. Overall, the correction appears to be working very well on both full-frame and subarray data. A frequent comment concerning version 1 had been how long it takes to run. To address that issue, I have made use of the fortran OpenMP libraries to allow the program to perform the most time-consuming steps in parallel. This successfully speeds up execution time by up to a factor of up to 20 (for machines that have more than 20 cores).

To take advantage of these improvements, please download the parallelized version of the software from the WFC3 CTE Tools page (http://www.stsci.edu/hst/wfc3/tools/cte_tools) and compile it with:

gfortran wfc3uv_ctereverse_parallel.F -o wfc3uv_ctereverse_parallel.e -fopenmp

Omitting the "-fopenmp" flag will compile the routine in single-thread mode. Note that the g77 compiler is unable to access the OpenMP libraries, so you must use gfortran in order to experience the run-time improvement.

In closing, please note that there are some Cycle 21 calibration programs (CAL-13568 and CAL-13567) that are designed to allow us to recalibrate the parameters of the CTE model, so that we will have the most up-to-date model as we contemplate making the pixel-based correction part of the UVIS pipeline. If you would like to be notified by email when updates to the CTE correction code are made, send an e-mail to majordomo@stsci.edu with "subscribe update_wfc3uv_cte" in the body.

WFC3 Posters to be Displayed at 223rd American Astronomical Society Meeting

M. Bourque

The 223rd American Astronomical Society Meeting will be held from January 5-9, 2014 in Washington, D.C. Four WFC3 posters, with the abstracts shown below, will be displayed at the meeting. If you are attending, please consider visiting with the WFC3 presenters and ask any questions you may have! Electronic copies of the posters will be available at http://www.stsci.edu/hst/wfc3/documents/meeting_posters/ after the conclusion of the meeting.

WFC3: Status and Advice for Cycle 22 Proposers - Poster 149.02 - January 6th, 5:30-6:30 PM
John W. MacKenty1, Sylvia M. Baggett1, Susana E. Deustua1, Derek Hammer1, Janice C. Lee1, Peter R. McCullough1, Norbert Pirzkal1, Vera Kozhurina-Platais1, Adam G. Riess1

The Hubble Space Telescope's Wide Field Camera 3 provides observers with powerful imaging and slitless spectroscopic capabilities from 200 to 1700 nm. In this paper we present a summary of WFC3's current status and performance characteristics together with highlights of key new information for astronomers developing proposals for future science investigations. Over the past couple of years, observers have made increasing use of WFC3's ability to obtain high precision astrometric and photometric observations. We discuss improvements to the general astrometric calibration and recent advances in techniques for obtaining specialized observations with an astrometric precision better than 30 micro arc seconds. We also report on the photometric recalibration of the UVIS channel which incorporates independent solutions for the two CCD detectors resulting in improved zero points and color terms in the near ultraviolet and on measurements which demonstrate the excellent astrometric and photometric stability of this instrument. Finally, we provide advice for observers to better understand and predict astronomical backgrounds with the aim of improving the sensitivity of deep observations.


WFC3: Understanding and Mitigating UVIS Charge Transfer Efficiency Losses and IR Persistence Effects - Poster 149.03 - January 6th, 5:30-6:30 PM
Sylvia M. Baggett1, Jay Anderson1, Knox S. Long1, John W. MacKenty1, Kai Noeske1, John A. Biretta1

A panchromatic instrument, Wide Field Camera 3 (WFC3) contains a UVIS channel with a 4096x4096 pixel e2v CCD array as well as an IR channel with a 1014x1014 Rockwell Scientific HgCdTe focal plane array (FPA). Both detectors have been performing well on-orbit since the installation of the instrument in the Hubble Space Telescope (HST) in May 2009. However, as expected, the harsh low-earth orbit environment has been damaging the UVIS CCDs, resulting in a progressive loss of charge transfer efficiency (CTE) over time. We summarize the magnitude of the CTE losses, the effect on science data, and the pre- and post-observation mitigation options available. The IR FPA does not suffer from accumulating radiation damage but it does exhibit persistence i.e. an after-glow from sources in previous exposures, an anomaly commonly seen in these types of IR arrays. We summarize the characteristics of persistence in WFC3, suggest methods for reducing the effects during observation planning, and describe the calibration products which are available via the Mikulski Archive for Space Telescopes (MAST) for addressing persistence in IR science data.


WFC3: Improved WFC3 Calibration Products - Poster 149.04 - January 6th, 5:30-6:30 PM
Heather C. Gunning1, Megan L. Sosey1, Jay Anderson1, Janice C. Lee1, Norbert Pirzkal1, John W. MacKenty1, Vera Kozhurina-Platais1, Susana E. Deustua1, Derek Hammer1, Tomas Dahlen1, Elena Sabbi1, Jennifer Mack1, Sylvia M. Baggett1

The Wide Field Camera 3 (WFC3) is a fourth-generation UV/visible and IR imaging instrument on the Hubble Space Telescope (HST). Installed in May 2009, during HST servicing mission 4, both channels have been performing very well on-orbit. To provide optimum calibrated data, the WFC3 team routinely updates and refines the calibration software and associated files, designated as calibration products. We present some of the recently improved calibration products that will be of interest to current and future users of WFC3, including information on the chip-dependent zeropoints and flat fields, post-flash calibrations, and detector-to-image distortion corrections. The latter results in four new extensions (two per chip and dimension), in all UVIS FLTs retrieved from MAST after September 10, 2013. The D2IMFILE contains astrometric corrections for shifts of the raw X and Y positions induced by the lithographic-mask pattern. We discuss the migration of CALWF3 from the STSDAS package to HSTCAL, a package independent of IRAF; as a consequence, the IRAF/STSDAS version of CALWF3 is no longer being updated. Finally, we summarize recent improvements to aXe, a PyRAF/IRAF software package that enables automated extraction of spectra from WFC3 slitless spectral (grism) images. Updated versions of aXe are made available as part of the STSDAS testing environment (SSBX).


WFC3: Precision Infrared Spectrophotometry with Spatial Scans of HD 189733b and Vega - Poster 347.21 - January 8th, 5:30-6:30 PM
Peter R. McCullough1, Nicolas Crouzet1, Drake Deming3, Nikku Madhusudhan2, Susana E. Deustua1

The Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) now routinely provides near-infrared spectroscopy of transiting extrasolar planet atmospheres with better than ~50 ppm precision per 0.05-micron resolution bin per transit, for sufficiently bright host stars. Two improvements of WFC3 (the detector) and HST (the spatial scanning technique) have made transiting planet spectra more sensitive and more repeatable than was feasible with NICMOS. In addition, the data analysis is much simpler with WFC3 than with NICMOS. We present time-series spectra of HD 189733b from 1.1 to 1.7 microns in transit and eclipse with fidelity similar to that of the WFC3 transit spectrum of HD 209458b (Deming et al. 2013). In a separate program, we obtained scanned infrared spectra of the bright star, Vega, thereby extending the dynamic range of WFC3 to ~26 magnitudes! Analysis of these data will affect the absolute spectrophotometric calibration of the WFC3, placing it on an SI traceable scale.

1STScI, Baltimore, MD, United States
2Yale, New Haven, CT, United States
3University of Maryland, College Park, MD, United States

 

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