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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

This and previous issues of the STAN can be found at

1. New Alpha-Release Flat-fields for the WFC3/UVIS detector
2. An Error in the Orientation of the WFC3 IDCTAB Coordinate Frame
3. CALWF3 v2.3 Release
4. New Reference Files
5. New Documentation

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.

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 error.

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 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 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.

4. New Reference Files- Tiffany Borders

UVIS Darks
New UVIS darks span useafter dates from Aug 20 - Oct 13 2010.

UVIS Bias This new UVIS bias has a useafter of Oct 03, 2010 for use with quad filter and corner subarrays.

IR Flats 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.

All reference files can be found at:

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:

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