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WFC3 STAN - Issue 9, September 2011

WFC3 Space Telescope Analysis Newsletter - Issue 9, September 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. WFC3 UVIS Channel Flat Fields
2. aXe Grism Extraction Software Updated
3. MultiDrizzle Bugs Reported
4. WFC3 Cycle 19 Calibration Plan
5. New Documentation

1. WFC3 UVIS Channel Flat Fields - Jennifer Mack, Elena Sabbi, Tomas Dahlen

Improved flat fields for seven UVIS filters (F336W, F390W, F438W, F555W, F606W, F775W, and F814W) are now available from MAST. Observations in these filters retrieved after August 08, 2011 will be calibrated using the new reference files. The total change peak-to-peak with respect to the previous pipeline flats (obtained during ground calibration) ranges from 3.6% to 5.6%, increasing with wavelength. Calibration observations were also obtained for the filters F225W, F275W, and F850LP, and flats for these filters will be released in the near future. Flat fields for the remaining UVIS filters will be computed via wavelength interpolation of these 10 broadband filters. For more information, please see the following WFC3 webpage:

The flats for WFC3 obtain during ground testing contained a reflection ghost that affected ~40% of the field, and these were a major source of error in the flat-field calibration. Because of the tilted UVIS focal plane, light is reflected multiple times between the detector and the two chamber windows. This ghost or "flare" has imprinted a wedge-shaped feature in the ground-based flats. A simplified geometric model of the light reflections has been used to remove the flare from the ground flats, however the wavelength dependence of the reflectance, based on the refractive index of fused silica (SiO2), is still not well-constrained. Further calibrations are planned to better characterize this effect.

Once the flare is removed, residual low-frequency structures caused by differences in the ground-based and in-flight optical paths are computed. Photometry of the globular cluster Omega Centauri, observed at various roll angles and with large dithered steps, has been used to quantify magnitude differences between the same stars as they fall on different regions on the detector. The alpha-release flats, which were available from the WFC3 webpage from March-Aug 2011 (and described in the April 2011 STAN), include the same flare correction as the new flats; however, the low-frequency corrections were derived using a small 0.2" (5 pixel) aperture to minimize contamination from neighbors in the crowded calibration field. Since that time, photometry in a range of apertures has been used to study the UVIS PSF in more detail. For radii smaller than 0.36" (9 pixels) the PSF is strongly dependent on both the detector position and on the telescope focus at the time of observation. Beyond 0.4" (10 pixels) the PSF is extremely stable. While the alpha-release flats are accurate for photometry performed with a 5 pixel aperture, the new pipeline flat-fields have been normalized to an "infinite" aperture by applying local aperture corrections to 10 pixels, making them more generally applicable. When using these new flat fields, users performing photometry with apertures less than 10 pixels are strongly urged to compute local aperture corrections to 0.4".

2. aXe Grism Extraction Software Updated - H. Bushouse & N. Pirzkal

The public release of STSDAS v3.14 in August 2011 included aXe v2.3. Detailed release notes for aXe v2.3 can be found at The updates included several bug fixes in the drzprep task and upgrades to the axeprep, axedrizzle, and fcubeprep tasks. In particular, many calls to IRAF tasks from the axeprep and axedrizzle tasks have been replaced with equivalent Python and PyFITS file utilities and numpy array arithmetic. This not only removes dependencies on IRAF, but also increases the execution speed of many modules and avoids the problems that some users were having with these tasks when running under the 64-bit version of IRAF v2.15.

An updated version of the aXe manual is also now available (in pdf form) at

3. MultiDrizzle Bugs Reported - Warren Hack & Larry Petro

Recent testing of MultiDrizzle version 3.3.8 has revealed a few bugs that affect some operations for combining some kinds of WFC3 images. A description of each bug and, where available, workarounds may be found on the STSDAS web page, That page also documents bugs previously identified that affect images taken with each of the HST Scientific Instruments.

4. WFC3 Cycle 19 Calibration Plan - Elena Sabbi

The Cycle 19 calibration program is about to begin. It was formulated with the actual usage of WFC3 in mind. During Cycle 19, the UVIS and IR channels will both be heavily utilized: 56% of the WFC3 exposures will be acquired with the IR channel and the remaining 44% with UVIS, with 77% of the data acquired as direct imaging. During the coming Cycle, 42 of the 62 UVIS filters will be used, with more than 50% of the UVIS exposures collected in the UV filters and ~100 exposures with the grism G280. In the IR channel 12 of the 15 filters will be used, and 8 of these filters will acquire more than 200 exposures each.

The WFC3 Cycle 19 Calibration Program will run from October 2011 through September 2012 and it 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 28 different programs and can be divided in 6 categories: Monitor, Photometry, Spectroscopy, Detectors, Flatfields and Image Quality.

Monitor Programs have been designed to monitor the health of the UVIS and IR channels. The monitor activities include a monthly anneal of the UVIS CCDs to repair hot pixels, a UVIS 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 and 18 programs.

Photometry Programs include periodical measurements of the WFC3 throughput in a series of key filters, establish a calibration flux ladder from V=5 down to V=14 and characterize the count rate non-linearity down to H=14-17.

Spectroscopy Programs are designed to derive the flux calibration and refine the wavelength calibration of the three WFC3 grisms as a function of the position on the detectors.

Detector Programs On the UVIS side a particular attention will be devoted to the characterization and correction of the Charge Transfer Efficiency (CTE) of the CCDs, while for the IR channel the effort will be to model and remove persistence due to previous visits to less then 0.01 electrons per seconds.

Flatfield Programs will validate the spatial and wavelength stability of the WFC3 flatfields using the spatial scan of a bright star and by observing a spectrophotometric standard in a number of positions across the WFC3 detectors in a series of key filters.

Image Quality Programs will check for image stability by measuring the encircle energy of the PSF as a function of radius and position, and will monitor the secular changes of both the UVIS and IR plate scales.

A total of 125 external and 1497 internal orbits will be used. Further information about the Cycle 19 Calibration Plan can be found at the WFC3 web page

5. New Documentation - Cheryl Pavlovsky

These new ISRs have been published since the last STAN (June 2011):

ISR 2011-16 Geometric model of UVIS window ghosts in WFC3 - P. McCullough

ISR 2011-15 An Independent Determination of WFC3-IR Zeropoints and Count Rate Non-Linearity from 2MASS Asterisms - A. G. Riess

ISR 2011-14 WFC3 Cycle 18 Calibration Program - S. Deustua

The complete WFC3 ISR archive is at:

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