Hubble Space Telescope 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 http://www.stsci.edu/hst/wfc3.
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: http://www.stsci.edu/hst/wfc3/analysis/uvis_flats
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
http://axe-info.stsci.edu/extract_calibrate/releasenotes#axe_2_3. 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.
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
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.
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:http://www.stsci.edu/hst/wfc3/documents/ISRs/
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