Hubble Space Telescope WFC3 STAN - Issue 8, June 2011
WFC3 Space Telescope Analysis Newsletter - Issue 8, June 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. Phase II Update for Cycle 19 - Larry Petro
Several features that will benefit some science programs in the
operation of Wide Field Camera 3 are available for the first time in
Cycle 19. These new capabilities are available through Version 19.2
of Astronomer's Proposal Tool (APT), which must be used to prepare
Cycle 19 Phase II Proposals. These features are described in the
WFC3 Instrument Handbook, the Phase 2 Proposal Instructions, and below.
Your Contact Scientist can provide additional information and assistance
regarding these new features and the preparation of your Phase II proposal.
IR Grism Apertures
Exposures with the IR grisms now require the use of one of five
new, special-purpose apertures. Those five apertures provide
full-frame and subarray fields of view. They are GRISM1024
(full-frame), GRISM512, GRISM256, GRISM128, and GRISM64. These
apertures are also used for direct reference image exposures in a
bandpass filter. These apertures are described in Chapter 14 of the Phase
2 Proposal Instructions. The apertures used in Cycles 17 and 18 for
IR grism exposures are no longer supported and the apertures for UVIS
grism observations are unchanged.
UVIS Charge Injection
Observers may ameliorate the effects of CCD charge transfer
inefficiency by using charge injection to fill the detector traps.
This capability is described in the fourth article in this issue of
the STAN. To use this capability the observer sets the Phase 2 APT
Optional Parameter INJECT=YES.
Two new convenience patterns are provided for WFC3 in Cycle 19. The pattern
WFC3-UVIS-GAP-LINE spans the UVIS inter-chip gap and allows any of the three
WFC3-UVIS small dither patterns to be added as secondary patterns. The
pattern WFC3-IR-DITHER-BLOB spans features the size of the IR detector
"blobs" and allows any of the three WFC3-IR small dither patterns to be
added as secondary patterns. (See WFC3 ISR 2010-09). Other large WFC3
patterns now also allow the addition of the small dither patterns as
secondary patterns. All WFC3 patterns are described in Section 8.4.4 of
Phase 2 Proposal Instructions.
The SPATIAL SCAN Special Requirement is offered in Cycle 19 to allow
the telescope to scan the line of sight during an exposure. This
capability enables photometry and time-series photometry of bright
sources. Section 7.3.3 of the Phase 2 Proposal Instructions
documents the use of this Special Requirement.
Increased Limits for Buffer Dumps
Data are dumped to the Solid State Recorder if either the volume of
science data, or the number of exposures fill the WFC3 data or
header buffers. In previous Cycles, a dump was required when the
number of exposures reached 100. In Cycle 19 that limit is 304.
For IR exposures, each readout counts as an exposure, and so this
increase in the limit will particularly benefit IR exposures with
small subarrays. See article 5 of this STAN for more information.
3. WFC3 Grism Support - Barry Rothberg
Recently we contacted the PIs and Co-Is of WFC3 grism programs who awarded time in Cycle 17 and 18
to learn what additional resources we can provide to improve the grism experience.
Based on requests from users, we are making available a
Frequently Asked Question webpage which covers topics ranging from Phase 1 proposals to
data reduction and analysis of grism data. This page provides users with
solutions to often asked questions, particularly those which have been submitted to our Help
Desk (firstname.lastname@example.org). The webpage can be reached directly here:
and will be continually updated with additional information (so check back often).
In addition, we are introducing a WFC3 Spectroscopy Resource Page:
where we will consolidate information for the WFC3 grisms. This page is designed to provide
users with a single gateway for obtaining information about all aspects of the WFC3 grisms.
The page contains pointers to all relevant documentation, calibration and reference files, as well
as a list of ISRs (Instrument Science Reports) relevant to the WFC3 grisms,
including short summaries of the ISRs listed. It also
includes links to conferences and publications which focus on the WFC3 grisms.
Currently you can find information on our 2010 Slitless Spectroscopy Workshop,
as well as technical and white papers on the instrument. In the future, we will
feature refereed publications and links to Press
Releases highlighting the scientific achievements of the WFC3 grisms. We encourage
users to provide us with feedback on any additional information they would like to see
made available on our new resource page.
4. WFC3/UVIS Charge Injection in Cycle 19 as Charge Transfer Inefficiency
Mitigation - Kai Noeske
Like all CCD detectors in low-Earth orbit, the WFC3/UVIS CCDs
experience a degradation of their Charge Transfer Efficiency (CTE)
over time, introduced by their exposure to energetic radiation. As of
March 2011, moderately faint stars (500 to 2000 electrons) far from the
read-out amplifiers experience flux losses approaching 10% (r=3 pixel
apertures) at close to zero background. ***Losses are lower for brighter
targets and/or images with higher sky backgrounds: for a background level
of 20-30 electrons per pixel, the aforementioned stars (500 to 2000
electrons) show significantly lower CTE losses of only ~2%. Stars with
2000 to 4000 (8000 to 16000) electrons show, on essentially zero background
images, CTE losses of ~4% (2%). [Corrected on June 30, 2011]***
While CTE losses can be mitigated by placing the target close to an
amplifier, this is in practice not always possible. Corrections can
be applied after the data are acquired , e.g., photometric formulae
for aperture photometry or a pixel-based image reconstruction. However,
these options have disadvantages as well.
Beginning in Cycle 19, we offer an alternative method of mitigating
CTE effects, detector Charge Injection. Injecting every N-th detector
row with a defined charge partially fills charge traps during readout.
In Cycle 19, the available mode will be LINE17, i.e. every 17th row
will be charge injected. This mode will be supported and calibrated.
For detailed and updated information on CTE, Charge Injection, and
considerations for observers, please refer to the WFC3 UVIS CTE
Website (revisions are currently in progress, this page will be fully available June 30, 2011):
Further information is available from the STScI Helpdesk (
email@example.com) or your Contact Scientist.
5. CALWF3 Image Combination Limit - Howard Bushouse
WFC3 observations that include multiple exposures taken at the same
sky position (e.g. CR-SPLIT and REPEAT-OBS sets) will go through the
"wf3rej" step of the calwf3 calibration program, which produces a
combined, CR-rejected, image from the multiple exposures. The wf3rej
task can currently handle a maximum of 120 exposures to be combined,
due to programming details associated with file pointers. Work is
underway to modify the software to allow for larger numbers of images
to be combined.
In the meantime, GOs who take advantage of the increased limit
on the number of images that can be stored in the WFC3 instrument buffer,
which will now allow up to 304 images (see article 1 in this STAN), may run into this wf3rej
processing limit if there are more than 120 images at the same sky
pointing to be combined. The latest version of calwf3, version 2.4,
at least handles this situation gracefully by warning the user that the
wf3rej processing can not be completed, and still then performs full
calibration processing on all of the individual exposures.
Sets of exposures that exceed the wf3rej 120 image limit can always
be combined using another program like "multidrizzle" or "imcombine".
ISR 2011-12 Sunlight Glinting from Water: not a hazard for WFC3 IR - P. McCullough
ISR 2011-11 Sky Flats: Generating Improved WFC3 IR Flat-fields - N. Pirzkal, J. Mack, T. Dahlen, E. Sabbi
ISR 2011-10 Interpixel Capacitance in the IR Channel: Measurements Made On Orbit - B. Hilbert and P. McCullough
ISR 2011-09 IR Detector Timing and Persistence - K. S. Long, T. Wheeler, H. Bushouse
The complete WFC3 ISR archive is at:http://www.stsci.edu/hst/wfc3/documents/ISRs/
Need help? http://www.stsci.edu/hst/wfc3/help.html
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