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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

This and previous issues of the STAN can be found at

1. Phase II Update for Cycle 19
2. New Instrument Distortion Correction Tables
3. WFC3 Grism Support
4. WFC3/UVIS Charge Injection in Cycle 19 as Charge Transfer Inefficiency Mitigation
5. CALWF3 Image Combination Limit
6. New Documentation

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.

Convenience Patterns
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.

Spatial Scans
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.

2. New Instrument Distortion Correction Tables - Larry Petro, Vera Kozhurina-Platais, Michael Dulude

New Instrument Distortion Correction Tables (IDCTABs) for the UVIS and IR channels were installed in CDBS on May 27, 2011 for use with all on-orbit exposures. They replace those in use since October 11, 2010 in the OPUS pipeline. These new tables correct a 0.09-degree rotation of the coordinate reference frame, as described in the April 2011 issue of the WFC3 STAN. The content of the files are the same as described in the October 2010 issue of the WFC3 STAN.

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 ( 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 ( 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".

6. New Documentation - Cheryl Pavlovsky

These new ISRs have been published since the last STAN (April 2010):
ISR 2011-13 WFC3 Cycle 17 Proposal 11906: UVIS Gain - T. Borders, C. Pavlovsky, S. Baggett

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:

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