WFC3 Space Telescope Analysis Newsletter - Issue 15, October 2013
New File Structure for UVIS Calibrated Data Products
J.Mack, B. Hilbert, J. Lee
Beginning September 10, 2013, UVIS data retrieved from MAST includes an improved distortion correction which accounts for shifts induced by the lithographic-mask pattern imprinted on the detector during the manufacturing process (see STAN Issue 14 from June 2013 for details). As a result, the raw.fits and flt.fits image headers have an additional keyword (D2IMFILE) which points to the new distortion reference file. MAST calls an updated version of the DrizzlePac software (v1.1.10) to interpret the new D2IMFILE reference file containing these corrections, and it appends four extensions of type 'D2IMARR' (two for each chip and dimension) to the flt.fits image, as shown below. Note that users who retrieve observations after September 10 and wish to manually reprocess using AstroDrizzle must obtain the latest development version (available via IRAFX) to correctly interpret and apply the D2IMFILE corrections. See the DrizzlePac website for details on obtaining software updates.
Running AstroDrizzle with UVIS CTE-corrected Images
J.Mack, B. Hilbert, W. Hack, J. Anderson, J. Lee
The UVIS pixel-based CTE correction software corrects calibrated flt.fits data products for CTE losses during readout. This fortran code writes flc.fits data products which may then be used as input to AstroDrizzle for distortion correction and image combination. Currently, this software strips the new D2IMARR extensions from flc.fits data products, and this will cause AstroDrizzle (and other DrizzlePac tools) to crash during manual reprocessing. As a temporary workaround, users may simply delete the D2IMDIS keywords from the science image header and then turn on the 'updatewcs' parameter when running TweakReg or AstroDrizzle for the first time. This will restore the proper format of the image header (with a total of 11 extensions) such that the full distortion solution is properly interpreted by the software.
The PyRAF command to remove the D2IMDIS keywords is:
hedit *flc.fits d2imdis* delete+
Alternately, this may be done in python using the following commands:
flc = pyfits.open('icdm60ipq_flc.fits',mode='update')
The CTE correction software is now being updated so that flc.fits products are compatible with latest DrizzlePac tools, and a notice will be posted on the WFC3 CTE webpage when the new version is available. This new version of the software will also allow for the use of multiple processors (when available) to improve speed.
N. Pirzkal, M. Sosey, J. Lee, G. Brammer
An updated version of aXe, the software package designed for the extraction, calibration, visualization, and simulation of spectra from slitless spectroscopic instruments, has been released. This new version (aXe 2.4.3) is available to users as part of IRAFX (http://stsdas.stsci.edu/irafx/), the STSDAS development and testing environment, released on Sept 25th 2013. aXe 2.4.3 includes several bug fixes related to background subtraction in cases where too few pixels are available. It also provides increased support for multiple extension FITS files (MEF). Finally, aXe 2.4.3 adds support for excess (padding) pixels when using fcubeprep in combination with astrodrizzle input products.
WFC3 Instrument Handbook for Cycle 22
The WFC3 Instrument Handbook is currently being revised for Cycle 22. If you have suggestions to help us correct, clarify, or add material to the handbook, please send them to firstname.lastname@example.org. Suggestions received by Oct 17 will be given priority for consideration in the current revision.
Choosing the Shutter Blade Side for WFC3 UVIS Short Exposures
K. Sahu, L. Dressel
WFC3 UVIS uses a shutter blade with two sides, A and B, which are used alternately in the exposures (for a description of the shutter mechanism, see WFC3 ISR 2008-44). These two sides are referred to as SHUTRPOS A and B in the header of the science data. The current position of the shutter determines which side will be used next, so a series of exposures can have the sequence ABA... or BAB...
Stellar PSFs in short-exposure images taken with side B of the blade show larger FWHM and lower sharpness compared to those taken with side A, due to shutter vibrations (see WFC3 ISR 2009-20 for comparisons). The difference in the PSF widths in consecutive exposures taken with the two sides can be as much as ~20% for the shortest (0.5 sec) exposures. The difference decreases as the exposure time increases and is negligible for exposure times of 15 seconds or larger.
There are GO programs with short exposures where it is critical to the science goals to have a sharp PSF. Such programs may now be able to make short exposures with less vibration using the exposure-level option BLADE=A in APT (version 21.2.2). Since this option causes additional movement of the shutter mechanism, its use will be allowed only as an available mode when sufficiently justified. The interested PI should send a short scientific justification for using this mode for exposures with less than a specified exposure time to the Contact Scientist or the Program Coordinator, who will forward it to the appropriate instrument scientist for consideration.
ISR 2013-06 Characterizing Persistence in the WFC3 IR Channel: Finite Trapping Times - K. Long, S. Baggett, J. MacKenty
ISR 2013-07 Characterizing Persistence in the WFC3 IR Channel: Observations of Omega Cen - K. Long, S. Baggett, J. MacKenty
ISR 2013-09 WFC3/UVIS Bowtie Monitor - M. Bourque, S. Baggett
ISR 2013-10 In-flight Corrections to the WFC3 UVIS Flat Fields - J. Mack, E. Sabbi, T. Dahlen
ISR 2013-11 UVIS PSF Spatial & Temporal Variations - E. Sabbi, A. Bellini
ISR 2013-12 WFC3 Post-Flash Calibration - J. Biretta, S. Baggett
ISR 2013-13 Evaluation and Comparison of Deep UVIS PSFs Observed at Three Epochs - L. Dressel
ISR 2013-14 Astrometric Correction for WFC3/UVIS Lithographic-Mask Pattern - V. Kozhurina-Platais, D. Hammer, N. Dencheva, W. Hack
The complete WFC3 ISR archive is at: /hst/wfc3/documents/ISRs/
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