WFC3 Space Telescope Analysis Newsletter - Issue 19, Ocotober 2014
- 1 WFC3 Celebrates its 5 Year Anniversary
- 2 Newly Improved and Updated WFC3/UVIS Distortion Reference Files
- 3 Using the Quad Filters with WFC3/UVIS
- 4 WFC3 Talks and Posters Presented at 2014 Calibration Workshop
- 5 New Documentation
WFC3 Celebrates its 5 Year Anniversary
The WFC3 team is pleased to take note of a major milestone as the instrument has passed five years in operation. WFC3 was developed with a design lifetime using components tested and certified for five years of operation in space. At this point, the instrument is still operating using its primary set of electronics (the redundant boxes have not been needed) and there are no significant liens affecting its science operations. We want to express our thanks and admiration to the hundreds of dedicated individuals at NASA's Goddard Space Flight Center, Ball Aerospace, e2v, Teledyne Imaging Systems, and the many other organizations who designed, built, and tested WFC3 as well as the crew of STS-125 who successfully installed it into HST in 2009.
Newly Improved and Updated WFC3/UVIS Distortion Reference Files
V. Kozhurina-Platais, H. Gunning
All HST WFC3/UVIS images require corrections for geometric distortion due to the non-linear mapping of the sky onto the detector, known as the optical field-angle distortion (OFAD). This geometric distortion is presented by a relatively low-order polynomial (up to 5th order) in the form of the reference file Instrument Distortion Coefficients Table (IDCTAB).
During the calibration of the WFC3/UVIS geometric distortion we were able to find the tiny low-scale distortions coming from irregularities in the pixel grid of the UVIS detector itself and in the various UVIS filters used for observations. The irregularities in the pixel grid are due to the lithographic mask pattern clearly embedded into the supposedly regular grid of pixels during the manufacturing process. On top of these irregularities, there are also random "swirls" spawned by small variations in the optical properties in virtually all filters. Such distortions are smaller than 0.1 pixel (equivalent to 4 milli-arc-seconds on the sky) over small extent of 50-100 consecutive pixels acting as highest-order polynomial, but large enough to be noticed while mosaicing images from sets of exposures with a range of dithers and telescope roll angles. To maximize the scientific output of WFC3/UVIS, these distortions should be calibrated and corrected for. The corrections for such low-scale distortions were formalized by constructing two-dimensional look-up tables fully covering each of the two WFC3/UVIS chips. These corrections now are presented in two reference files: D2IMFILE (Detector to Image) and NPOLFILE (non-polynomial filter-dependent part of distortion) which are bi-linearly interpolated by the STSDAS software DrizzlePac/Astrodrizzle in the STScI On-The-Fly Reprocessing (OTFR). Thus, a complete set of all geometric distortions corrections is formalized by the three reference files: IDCTAB, D2IMFILE and NPOLFILEs. These three WFC3/UVIS reference file types were recently improved and updated as described in WFC3-ISR-2014-12 by Kozhurina-Platais et al. Newly improved reference files were delivered to the HST Calibration Reference Data System (CRDS) with OPUS build 2014.3.
As of now, a complete correction of geometric distortions is available for 14 filters: 1) new polynomial coefficients of geometric distortion for 14 calibrated UVIS filters in the form of the IDCTAB file are improved to account for the lithographic-mask pattern in the WFC3/UVIS detector and filter-dependent distortion; 2) new look-up tables in the form of a D2IMFILE and NPOLFILEs significantly improve by 50-90% (depending on the filter) the fine-scale structure in the WFC3/UVIS geometric distortion. New improved IDCTAB coupled with the D2IMFILE and NPOLFILE can now successfully correct the WFC3/UVIS drizzled images to the precision level of 0.02 pixel (about 1 milli-arc-second). Ultimately, the precision level in WFC3/UVIS distortion mainly depends on how accurate is the centering technique for measuring the X and Y positions of under-sampled Point-Spread Function on drizzled WFC3/UVIS images.
The next step in improving the correction for UVIS geometric distortions will be calibration of all remaining band-passes (filters) of the UVIS channel.
Using the Quad Filters with WFC3/UVIS
When using the WFC3/UVIS quad filters, users should be aware of which photometry keywords are applied in the image headers. APT provides three aperture options: (1) UVIS-QUAD, (2) UVIS-QUAD-FIX and (3) UVIS-QUAD-SUB when the Spectral Element selected is a quad filter (e.g. FQ889N). The first two options are full frame images (4096 x 4096 pixels), while the third will be a sub-array (2049 x 2050 or 2047 x 2050 pixel depending on the quadrant) corresponding to the quadrant of the selected filter. When either UVIS-QUAD or UVIS-QUAD-FIX is selected, the values of the photometry keywords in the image header (PHOTFLAM, PHOTPLAM, PHOTBW) correspond to that of the filter given in the FILTER keyword. In this situation, the filter value in the FILTER keyword is that of the first filter in the wheel (which may not necessarily be the commanded filter), and the photometry keyword values will correspond this listed filter. The science target will be in the correct quadrant for the selected quad filter, however. Therefore, users are reminded to check the keyword values for the appropriate quad filter and quadrant before applying zeropoints to their data. If needed, the photometry values can be obtained from the WFC3 webpages.
For example, if the request in APT is Filter=FQ492N, Aperture=UVIS-QUAD, the keywords in the image header will be FILTER=FQ387N, and the photometry keyword values will correspond to FQ387N, NOT FQ492N, as such:
PHOTMODE= 'WFC3 UVIS2 FQ387N CAL' / observation con
PHOTFLAM= 1.98755875E-17 / inverse sensitivity, ergs/cm2/Ang/electron
PHOTFNU = 9.94810405000000E-06 / inverse sensitivity, Jy*sec/electron
PHOTZPT = -21.1 / ST magnitude zero point
PHOTPLAM= 3873.65175 / Pivot wavelength (Angstroms)
PHOTBW = 15.0304235 / RMS bandwidth of filter plus detector
To flux calibrate the FQ492N data, users need to obtain the correct values of PHOTFLAM, PHOTFNU, PHOTPLAM and PHOTBW from the WFC3 Photometric Zeropoints website.
If the UVIS-QUAD-SUB aperture was selected, the FILTER and photometry keyword values correspond to the commanded filter.
WFC3 Talks and Posters Presented at 2014 Calibration Workshop
The 2014 Calibration Workshop was held on August 11-13, 2014 at Space Telescope Science Institute in Baltimore, Maryland. Several WFC3-related talks and posters were presented and are listed below. Talk and poster abstracts can be found on the Talk Abstracts and Poster Presentation webpages. Video webcasts of the talks are also available in the STScI Webcast Archive, and the posters are available on the WFC3 Calibration Workshop webpage. Please feel free to send any questions that you have to firstname.lastname@example.org.
- The Status of HST/WFC3 (J. MacKenty)
- Grism Spectroscopy (I. Momcheva)
- Persistence in Near-IR Detector Arrays (K. Long)
- WFC3 Spatial Scanning (Exoplanets) (P. McCullough)
- WFC3 Spatial Scanning (Astrometry) (S. Casertano, A. Riess)
- WFC3 UVIS Shutter Blade (K. Sahu)
- Elevated Sky Backgrounds in WFC3 from He 10830 (G. Brammer)
- WFC3 Time Variable Background Correction (B. Hilbert, M. Roberto, R. Lucas)
- WFC3/UVIS Charge Transfer Efficiency Losses: Mitigation and Correction (J. Anderson)
- The WFC3/UVIS Dark Calibration and CCD Monitor (M. Bourque)
- The WFC3 Two Chip Solution: New Zeropoints and Flatfields for WFC3/UVIS (S. Deustua)
- A New Generation of WFC3/IR Dark Calibration Files (M. Dulude)
- Longterm Photometric Trends in WFC3/UVIS (C. Gosmeyer)
- The WFC3/UVIS Gain Monitor and Low Sensitivity Pixel Population (H. Gunning)
- WFC3/UVIS Flat Field Accuracy & Improved Solutions for UV Filters (J. Mack)
- Charge Transfer Efficiency in WFC3/UVIS: Monitoring and Corrections from Star Clusters (K. Noeske)
- The IR Background as seen by WFC3 (N. Pirzkal)
ISR 2014-16: WFC3/UVIS Photometric Transformations - K. Sahu, S. Deustua, E. Sabbi
ISR 2014-17: Updated Non-Linearity Calibration Method for WFC3/IR - B. Hilbert
ISR 2014-18: Pixel-to-Pixel Flat Field Changes in WFC3/UVIS - H. Gunning, S. Baggett, J. MacKenty
ISR 2014-19: Sink Pixels and CTE in the WFC3/UVIS Detector - J. Anderson, S. Baggett
ISR 2014-20: Update on the WFC3/UVIS Stability and Contamination Monitor - C. Gosmeyer, S. Baggett, S. Deustua, D. Hammer
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