Hubble Space Telescope WFC3 STAN Issue 5, October 2010
WFC3 Space Telescope Analysis Newsletter - Issue 5, October 2010
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.
Count Rate Non-linearity on WFC3-IR - Adam Riess
An initial measurement of this effect was made by comparing the photometry of star clusters observed
over a wide dynamic range and at overlapping wavelengths in WFC3-IR and NICMOS and/or ACS-WFC. We found
a significant detection of a non-linearity in WFC3-IR photometry which is in the same direction but a few
times smaller than that of NICMOS. From the stars we measured a non-linearity of WFC3-IR of ~1% per dex
over a range of 10 magnitudes (4 dex) which was independent of wavelength. The impact of this non-linearity
is that photometry of faint (i.e., sky dominated) sources calibrated with WFC3-IR zeropoints will appear
0.04 +/-0.01 mag too faint. Because this is a difficult measurement we are still in the process of refining
it and we will be analyzing other data sets to test this initial calibration.
The Temporal Stability of WFC3/UVIS - Jason Kalirai
WFC3/UVIS has imaged bright, isolated spectrophotometric standards over dozens of epochs
during its first year of science operations. These stars are used as calibration sources
for the photometric zero point measurements and UVIS contamination monitor programs.
Photometry of the S/N >> 100 detections at each epoch provides a measurement of the temporal
stability of the instrument over a 400 day period. We analyze three different standards (two
white dwarfs and a solar analog), all imaged in small subarrays at the four corners of the
WFC3/UVIS detector. For a given location on the detector, our measurements indicate that
the instrument is photometrically stable with a measured standard deviation of 0.4%,
over a time span extending from the installation of WFC3 in Hubble in May 2009 to the
present day. The overall stability of WFC3 is the combination of this temporal stability
and the spatial stability over the detector, which is currently being measured from on-orbit
flat field analysis. These results will soon appear in an ISR.
New Instrument Distortion Correction Tables - Larry Petro
Instrument Distortion Correction Tables (IDCTABs) for the UVIS and IR channels have been installed in CDBS and replace
those in use since January 2010 in the OPUS pipeline. The new tables contain the same data as the alpha-release
tables posted on the WFC3 website (see the Late Breaking News Archive for
July 26, 2010 (UVIS)
and August 24, 2010 (IR)).
The UVIS table contains unique polynomial distortion coefficients for ten UVIS filters (F225W, F275W, F336W, F390W, F438W,
F555W, F606W, F775W, F814W, and F850LP). For all other UVIS filters, the values of the polynomial coefficients are from the
F606W polynomials. The IR table contains unique polynomial distortion coefficients for five IR filters (F098M, F110W,
F125W, F139M, and F160W). For all other IR filters, the values of the polynomial coefficients are from the F160W
polynomials. Image shifts caused by filter wedge (of order 0.5 pixel) are not calibrated in this release.
For the 15 newly calibrated filters, displacements between rectified images less than 0.1 pixel (0.06 pixel r.m.s.)
can be obtained with MultiDrizzle and a user-determined shift file (e.g., determined with the IRAF task GEOMAP) that
represents differential shift, scale, and rotation between exposures. (Revised 13 Oct 2010)
WFC3 Data Processing Updates - Howard Bushouse
As reported in the previous STAN, WFC3 images that went through
ground processing and calibration before April 19, 2010 did not have
the velocity aberration correction factor computed and stored in the
image header keyword "VAFACTOR". This correction factor is used within
MultiDrizzle processing to compensate for changes in plate scale that
result from HST's orbital motion. The STScI data management team has
reprocessed all WFC3 exposures obtained before April 19, 2010, so
that the VAFACTOR value is now correct for all WFC3 datasets in the
DADS archive catalog.
There have not been any updates to the calwf3 and MultiDrizzle processing
software since the last STAN. Calwf3 v2.1 and MultiDrizzle v3.3.7 have
been in use in the OPUS pipeline since August 25, 2010. As always, you
can get further info about any software version by checking the release
notes on the web at:
http://www.stsci.edu/hst/wfc3/pipeline/CALWF3ReleaseNotes.html/
For those observers who use WFC3 grism data, note that all of the grism
user and software support that has been provided in past years by the
ST-ECF will be transitioning to the WFC3 team at STScI at the end
of 2010, due to the impending closure of the ST-ECF. At that time, the
STScI will take over all support, distribution, and maintenance of
the aXe software used to extract and calibrate WFC3 grism spectra.
The WFC3 team has delivered a set of 7 bias files for the UVIS channel, covering November 8, 2009 through July 12, 2010. Each bias file in this collection provides a measure of the bias signal on the UVIS detector over a 5 to 6 week period within the dates listed above.
UVIS Dark Current Files - Oct 16 - July 18
We have also added to the library of on-orbit dark current files since the publication of the previous STAN. We have delivered a collection of 72 dark current files that provide a measure of the UVIS channel's dark current behavior between October 16, 2009 and July 18, 2010. Each file represents a measurement of the dark current in a 4-5 day period within the range of dates above.
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