149.04
SPACE
TELESCOPE
SCIENCE
INSTITUTE
Heather Gunning, M. Sosey, J. Anderson, J. Lee, N. Pirzkal, J. MacKenty, V. Platais, S. Deustua,
D. Hammer, T. Dahlen, E. Sabbi, J. Mack, S. Baggett, and the WFC3 Team
Operated for NASA by AURA
Detector-to-image Distortion Corrections
Abstract
After Correction
Y (pix)
Flat Fields and Chip-Dependent Zero points
In August 2013, the WFC3 Team
delivered a new type of reference
file (D2IMFILE) which corrects a
lithographic-mask
pattern
(a
manufacturing artifact). Any data
retrieved after September 10,
2013:
X (pix)
Before Correction
X (pix)
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 zero points, 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CALWF3 is no longer being updated.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).
Y (pix)
The figures above are 2-D residual maps, magnified by a factor of 2000,
between two F336W drizzled images. The figure on the left has no defect
correction, while the figure on the right is corrected for the lithographic
pattern. The corrected image has ~70% reduction in residuals.
• Chip-independent flat fields to be replaced with chip-dependent fields
• With updated encircled energy tables and flat fields, UVIS zero points are anticipated to have <1% change
• will have 4 new extensions
• 2
extensions
per
chip
covering both x and y
dimensions
• has 30-60% improvement (on
average)
in
astrometric
residuals.
• will require Drizzlepac v1.1.10
for reprocessing
aXe version 2.4.3
Why we’re upgrading
The UVIS CCDs were manufactured separately; hence, it is likely that each chip will age differently.
In-flight corrections to the UVIS ground flats were delivered to the HST archive in December 2011. These new
products improve the accuracy of point source photometry by 0.6-1.8% (rms), with maximum differences of 36% depending on filter. The photometric response for any given star using the revised flat fields is accurate to
±1% over the detector for most UVIS filters.
For all UV filters bluer than ~300 nm, the ground flats were obtained under ambient conditions (-49C vs -82C),
and mid-frequency sensitivity residuals of ± 2-3% remain. These correlate with a "cross-hatch" pattern in the
detector structure, where dark regions have lower total throughput in the UV. Several calibration programs are
underway to model and correct this residual UV structure.
• Fixes bug in special case background subtraction
• Improves support for multiple extension FITS files
• Supports excess (padding) pixels when using fcubeprep in combination with
AstroDrizzle inputs.
New versions of aXe are available through http://stsdas.stsci.edu/irafx/
Post-Flash Calibrations
Flat Field Accuracy
The Post-Flash Image File (FLSFILE) is used to correct
exposures that take advantage of the post-flash procedure for
mitigating CTE (Charge Transfer Efficiency) effects.
• ~5% difference between shutter
blades A and B
F225W flat field, with measured
Ratio of illumination between blades
positions of a photometric
standard. Sources falling on the
dark
'crosshatch'
pattern
produce photometry which is 23% too low in the UV.
Flat accuracy versus wavelength using
photometric standards stepped across
the detector. Red lines indicate ± 1%
residuals.
HSTCAL and CALWF3
.
Install and use CALWF3 through HSTCAL available for download at:
http://www.stsci.edu/institute/software_hardware/stsdas/download-stsdas
Execute CALWF3 in Python or PyRAF using WFC3TOOLS
In Python:
>>> from wfc3tools import calwf3
>>> calwf3.calwf3(filename)
In PyRAF:
>>> import wfc3tools
>>> epar calwf3
From the command line:
> calwf3.e filename
Contents of WFC3TOOLS
•
•
•
•
•
•
•
CALWF3
WF32D
WF3CCD
WF3IR
WF3REJ
PSTACK
PSTAT
Post-Flash illumination image
CALWF3 multiplies the normalized
Illumination image (shown to the
right) by the FLASHDUR header
keyword value and divides by the
gain before subtracting from the
science exposures. Each shutter
position (SHUTRPOS) has its own
FLSFILE reference image. The
ratio of these FLSFILEs is shown
to the left.
• ±20% variation across entire
FOV
• Monitoring of post-flash LED shows no evidence of longterm variations at ~0.1% level
References
Astrometric Correction for WFC3/UVIS Lithographic-Mask Pattern, ISR 2013-14, WFC3
http://www.stsci.edu/hst/wfc3/documents/ISRs/WFC3-2013-14.pdf
aXe Info
http://axe-info.stsci.edu
In-flight Corrections to WFC3/UVIS Flat Fields, ISR 2013-10,WFC3
http://www.stsci.edu/hst/wfc3/documents/ISRs/WFC3-2013-10.pdf
STSDAS Download Page
http://www.stsci.edu/institute/software_hardware/stsdas/download-stsdas
WFC3 Reference Files
http://www.stsci.edu/hst/observatory/crds/SIfileInfo/WFC3/description.html
WFC3TOOLS Documentation
http://ssb.stsci.edu/doc/stsci_python_2.14/wfc3tools-1.1.doc/html/index.html