Hubble Space Telescope WFC3 STAN - Issue 7, April 2011
WFC3 Space Telescope Analysis Newsletter - Issue 7, April 2011
For new information about WFC3 visit the "New in the Last 45 Days"
and "Late Breaking News" sections of the WFC3 website at http://www.stsci.edu/hst/wfc3.
1. New Alpha-Release Flat-fields for the WFC3/UVIS detector - Jennifer Mack
A subset of new flat-fields for filters F438W, F555W, F606W, F775W and F814W is now available from the WFC3 webpage, with other filters soon to follow.
The flat-fields currently used by the calibration pipeline were produced during ground testing at Goddard Space Flight Center. Soon after launch,
in-flight tests using aperture photometry of stars in 47 Tucanae and Omega-Centauri showed that the apparent magnitude of the same star varies by ~5%,
depending on detector position. In August 2010, a preliminary set of alpha-release flats were posted to the WFC3 webpage for off-line reprocessing by
users. These flats were computed using the same software and methodology for the ACS L-flat corrections. While the ACS residuals were well-constrained
via low-order polynomial solutions, the WFC3 UVIS flats proved to be more complex. Further in-flight calibration observations have given a more clear
picture of the interplay between three separate corrections which are required to improve the ground-based flat-fields: 1.) a wedge-shaped flare extending
diagonally from the center of quadrant D to quadrant A caused by light reflecting between the tilted UVIS focal plane and the detector's two windows,
2.) a unique gain value adjustment for each amplifier, and 3.) low-frequency residuals in detector sensitivity due to slight changes between the ground
and the in-flight optical path.
The maximum error from the flat-field calibration is now reduced from ~5% to ~1%. These new flat-fields have been independently verified by computing
aperture photometry on reprocessed data using both drizzled images and pipeline 'FLT products' corrected for varying pixel area. These data include
stellar observations obtained with large dithers and at various roll angles which place the same sources in different regions of the detector, and the
photometric residuals verify that the new flats are now accurate to better than 1% and show no spatially dependent signature. Once a more complete set
of flat-fields has been produced and further validation of the solutions has been achieved, these new reference files will be delivered to CDBS for use
in the calibration pipeline.
WFC3/UVIS users interested in applying the new flat-fields can manually recalibrate their raw data using CALWF3. Instructions are posted on the WFC3
alpha-release webpage, along with several figures to highlight the changes. News updates and additional flat-fields will be posted to this page as
they become available. http://www.stsci.edu/hst/wfc3/analysis/uvis_flats
2. An Error in the Orientation of the WFC3 IDCTAB Coordinate Frame - Larry Petro
The WFC3 reference file for geometric image distortion (the
IDCTAB) contains calibration data that serves two purposes. The
first, as the name suggests, is to remove geometrical distortion
from images. The second is to transform the image to a calibrated
tangent-plane coordinate system. Regarding the transformation onto
a standard coordinate system, recent tests have verified an error in
the UVIS and IR IDCTABs presently employed in the OTFR pipeline
(respectively uab1537bi_idc.fits and uab1537ci_idc.fits). Those
IDCTABs erroneously impart an approximately 0.09-degree rotation of
WFC3 images relative to the World Coordinate System. This error
will be corrected in a future release of the WFC3 UVIS and IR
IDCTABs that is expected by the end of April 2011.
Most users of the WFC3 IDCTABs will be unaffected by this
rotational error. In particular, those that use MultiDrizzle to
combine WFC3 exposures will be unaffected. The reasons are as
follows. First, the rectification of geometric image distortion is
independent of this rotational error. Second, the combination of
exposures in the same, or different observing visits will be
unaffected because drizzling and cosmic ray rejection depend only on
the relative alignment of the exposures, which is unchanged by this
Users that measure astrometric positions in the World Coordinate
System, or users that compare WFC3 images with those from other
Scientific Instruments (e.g., ACS) could be affected. Astrometric
positions in rectified images (e.g., as found in *_drz.fits files)
may be in error by as much as 1, or 4 pixels for IR and UVIS images,
respectively. The relative positions of objects in rectified WFC3
and, for example, ACS images could differ by similar amounts if the
standard practice of drizzling with a shift file is not followed.
That standard practice will determine the relative rotation of
images rectified with different IDCTABs with the Pyraf task
tweakshifts. The images may be accurately aligned, then, with the
shift file that contains the value of that relative rotation. For
details on the use of tweakshifts and shift files, please see the
MultiDrizzle Handbook, section 188.8.131.52. For a step-by-step example
on deriving a shift file by hand, see section 6.2.3.
3. CALWF3 v2.3 Release - Howard Bushouse
A problem was recently discovered in the IR zeroth-read correction (zsigcorr)
step of the WFC3 calibration program "calwf3", when applied to IR sub-array images.
The zsigcorr step is used to estimate the amount of signal in each pixel at the
time of the zeroth-read (the readout that occurs immediately after the detector
reset at the beginning of each exposure). This information is used in later
stages of calwf3 processing, after the zeroth-read has been subtracted from all
other readouts. In particular, it is used in the non-linearity correction
(nlincorr) step to account for this "lost" signal when checking for saturation
and applying the non-linearity correction factor.
The algorithm that had been in use in the zsigcorr step had relied on
knowledge of the effective exposure time of the zeroth-read, which is the time
that elapses between the reset and the zeroth-read for each pixel, which is
stored as the value of the "sampzero" keyword in WFC3 IR image headers.
Unfortunately, this time is not accurate for sub-array exposures, because the
elapsed time between the reset and the zeroth-read for sub-array exposures
varies across the field, reaching 2-4x the value of sampzero at the outer
edges of a sub-array image. Thus the algorithm was underestimating the true
signal in the zeroth-read. This in turn lead to pixels not being flagged as
saturated when they should have been and also to under correction for
non-linearity. The effect was particularly severe for very bright targets
(i.e. those that reach >20% full-well already in the zeroth-read).
Calwf3 v2.3 implements a new algorithm in the zsigcorr step that does not rely on
the effective exposure time of the zeroth-read, thus yielding accurate
estimates of the zeroth-read signal for all pixels in sub-array images.
We anticipate having this version of calwf3 installed in the STScI OTFR
pipeline on or about April 6, 2011. It will also be available in a patch
release of STSDAS, which can be obtained from the STSDAS
web site at http://www.stsci.edu/institute/software_hardware/stsdas. Anyone
working with IR sub-array exposures containing very bright sources is
encouraged to either retrieve their data again from the HST archive - which
will recalibrate the images using the latest calwf3 - or recalibrate the data
themselves after installing the STSDAS patch release.
This new UVIS bias has a useafter of Oct 03, 2010 for use with quad filter and corner subarrays.
The new IR flat fields are a combination of the existing ground-based flatfield and a new low-frequency correction (L-flat) derived from a large number (>2000) of long exposure (>300s) science images
that have been combined after masking out objects, effectively equating the L-flat with a high S/N sky image.
5. New Documentation - Cheryl Pavlovsky
These new ISRs have been published since the last STAN (October 2010):
ISR 2011-08 The Photometric Performance of WFC3/IR: Temporal Stability Through Year 1 -- J. S. Kalirai, S. Deustua, A. Rajan, and A. Riess ISR 2011-07 High Contrast Imaging using WFC3/IR -- Rajan et al. ISR 2011-06 WFC3/UVIS-Cycle 17: CTE External Monitoring-NGC 6791 -- V. Kozhurina-Platais, R. Gilliland, & S. Baggett ISR 2011-05 Revised Flux Calibration of the WFC3 G102 and G141 grisms -- H. Kuntschner et al. ISR 2011-04 WFC3/IR Banding -- M. J. Dulude, S. Baggett, H. Bushouse, & B. Hilbert
The complete WFC3 ISR archive is at:http://www.stsci.edu/hst/wfc3/documents/ISRs/
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