WFC3 Space Telescope Analysis Newsletter - Issue 14, June 2013
- 1 Postflash Added to the WFC3/UVIS Exposure Time Calculator
- 2 aXe Software Updates
- 3 WFC3/UVIS Pixel-Based Correction For Some Subarrays
- 4 Improved Astrometric Corrections which includes the WFC3/UVIS Detector Lithographic Pattern
- 5 Documentation of CTE, Postflash, and Spatial Scanning
- 6 New Documentation
Postflash Added to the WFC3/UVIS Exposure Time Calculator
With the current release of the ETC, v21.2.1 .(June 2013), postflash is now included as an option in SNR and exposure time calculations. Prior versions of the ETC DID NOT INCLUDE postflash, which may have resulted in optimistic signal to noise calculations. Postflash has been available for use with the WFC3/UVIS channel since the fall of 2012. Its use is recommended for observers concerned about effects of CTE losses on their data, especially for faint sources and/or low backgrounds. Version 21.2.1 of the Exposure Time Calculator (ETC) enables WFC3 users to calculate signal to noise (SNR) and exposure times for imaging data when adding postflash electrons. Postflash noise is time-independent, similar to readnoise, but is added to the background. In practice, postflash adds between 0.5 and 5 seconds to an individual exposure, depending on the number of postflash electrons added. An exposure in this sense refers to each time the shutter opens and closes, and the CCD is read out.
Postflash electrons are entered into the appropriately labeled box under “Specify Additional CCD Parameters”. The default value is 0 electrons/pixel. For WFC3/UVIS, users can enter integer numbers between 0 and 25 electrons/pixel. We recommend first running calculations without postflash, i.e., the ETC default value. If the total background level is less than the recommended threshold of 12 electrons/pixel, the ETC issues a warning message along with the estimate of the background. Note that the ETC predicted backgrounds will vary depending upon the zodiacal light, earthshine, and airglow levels selected in Section 5 (“Specify the expected background levels”) of the WFC3/UVIS ETC.
For advice in choosing the optimum flash level, users can consult the report by Anderson et al, The Efficacy of Post-Flashing for Mitigating CTE-Losses in WFC3/UVIS Images and the 2012 CTE white paper by MacKenty and Smith. The WFC3 CTE webpages can be found at http://www.stsci.edu/hst/wfc3/ins_performance/CTE/.
For comparison to ETC-predicted backgrounds, the table below summarizes for each filter the approximate exposure time (in seconds) required to reach 12 electrons/pixel background (based on archival data evaluated in Baggett & Anderson, ISR 2012-12, WFC3/UVIS Sky Backgrounds). As the table illustrates, exposures acquired with the visible wide filters typically achieve the desired 12 electrons/pixel background in 500-1000 sec while the UV and narrowband filters as well as some medium bands will require a post-flash to attain the desired threshold.
aXe Software Updates
J. Lee, M. Sosey, N. Pirzkal, B. Hilbert, M. Dulude
aXe is a software package designed for the extraction, calibration, visualization, and simulation of spectra from slitless spectroscopic instruments. The software and documentation was originally developed by the Space Telescope - European Coordinating Facility (ST-ECF), a unit of the European Space Agency (ESA). Since January 1, 2011, STScI has assumed responsibility for support and distribution of the software. The aXe web documentation has now been re-organized and updated, and can be found at http://axe-info.stsci.edu/. Please update your bookmarks.
Compatibility with AstroDrizzle
aXe was originally designed to support MultiDrizzle style astrometric information, and relied on MultiDrizzle output to transform object pixel coordinates between user supplied "drizzled" direct images and corresponding grism spectral images. All versions of aXe bundled with STSDAS, including the most recent version released in 2013 May (v3.16), only support processing with Multidrizzled direct images, and should NOT be used with AstroDrizzled products. This update supercedes the aXe-related information given in the STSDAS v3.16 release notes and e-mail announcement.
We have begun to implement and test changes to the software which will make it fully compatible with AstroDrizzle products, while still maintaining back-compatability with MultiDrizzle products. Versions of this updated software are available through IRAFX (http://stsdas.stsci.edu/irafx/), the STSDAS development and testing environment, in releases dated 2013-03-28 and later. The aXe tasks that are affected are "iolprep" and "fcubeprep." AstroDrizzle updates to "iolprep" have been fully implemented. However, updates to "fcubeprep" are still in progress, and processing with AstroDrizzle can only be performed correctly if the "dimension_info" parameter is set to "[0,0,0,0]." More information on aXe software access, bug reports, and version information can be found at http://axe-info.stsci.edu/extract_calibrate. The WFC3/IR grism reduction cookbook, available at http://www.stsci.edu/hst/wfc3/analysis/grism_obs/cookbook.html, has been updated to reflect the changes to aXe implemented in IRAFX. Updates have not yet propagated to the aXe manual http://www.stsci.edu/institute/software_hardware/stsdas/axe/extract_calibrate/axe_manual, but an announcement will be made at http://axe-info.stsci.edu/ under "aXe News" when a revised manual becomes available.
WFC3/UVIS Pixel-Based Correction For Some Subarrays
We have received several requests to have the standalone pixel-based CTE correction routine work on subarrays. Since the readout cadence for subarrays is different from that of full-chip readout (even for full-chip subarrays), it is not clear that the CTE trails will be properly modeled for subarrays. Nevertheless, it can still be useful to have some estimate of the impact of CTE.
Most subarrays have pre-scan reference pixels available, and we can use them to zero-point the raw exposures and thus estimate how many electrons are in the background. We were able to get the routine to work on the following subarrays: UVIS1-2K4-SUB, UVIS1-2K2A-SUB, UVIS1-2K2B-SUB,UVIS1-2K2C-SUB, UVIS1-2K2D-SUB, UVIS2-C1K1C-SUB, and UVIS2-C512C-SUB. Users can download a version of the pixel-based correction that allows for subarrays at:
It is run in a similar manner to the regular routine, except that it does not output an "rac" file. It works only in "FLC+" mode. The results look good in a qualitative sense, but we have not done any quantitative tests.
Unfortunately, the subarrays in the middle of the detector (ie, M1K1C) do not record any reference pixels, and that makes it very difficult to determine the background. Furthermore, since these subarrays are not adjacent to the readout register, they have to be shuffled very quickly in the vertical direction before they are read out. It is unknown what impact this will have on CTE losses. For all these reasons, we have not adapted the code to work with these subarrays.
Improved Astrometric Corrections which includes the WFC3/UVIS Detector Lithographic Pattern
V. Kozhurina-Platais, D. Hammer, N. Dencheva, W. Hack, H. Gunning, M. Dulude
- Set the D2IMFILE keyword in the primary header (add if necessary) to the local directory of the new look-up table, which may be downloaded here.
- Update the image astrometry via AstroDrizzle with the parameter "updatewcs" set to Yes/True. We do not recommend using the stand-alone version of "updatewcs" located in the STWCS python module.
Documentation of CTE, Postflash, and Spatial Scanning
WFC3/IR Internal Flat Fields - T. Dahlen
ISR 2013-05 WFC3 Cycle 20 Calibration Program - E. Sabbi & The WFC3 Team
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