WFC3 Space Telescope Analysis Newsletter - Issue 10, March 2012
- 1. IR persistence images now available in the MAST archive
- 2. WFC3/UVIS Crosstalk and Crosstalk Correction
- 3. New zeropoints for all UVIS and IR filters
- 4. WFC3 UVIS Channel Flat Fields
- 5. New WFC3 Instrument Distortion Correction Table (IDCTAB)
- 6. New Documentation
The images obtained with the IR detector on WFC3 can contain spurious features in situations where earlier exposures have saturated portions of the detector. These after-images arise because of traps in the diodes of the array which are exposed to free charge as a result of changing voltage levels within the diode. The general phenomenon is known as persistence and is seen in essentially all IR arrays. A general discussion of persistence as it applies to WFC3 can be found here.
We have developed a model that describes the amount of persistence that one can expect in a science image. We are using this model to produce images that contain these estimates for all WFC3 IR images. These images, along with considerable ancillary information, can be retrieved from the MAST archive via a special persistence interface.
We encourage all observers with IR data to use this interface to check whether their data is affected by persistence. Typically, one would first search the data base by target name, proposal ID, dataset name, visit number or some combination thereof. The results of the search contain links to the ancillary information for each dataset (by following the link associated with the dataset name) and to the persistence data products (by following the link associated with the visit). The ancillary information should be sufficient to evaluate whether persistence is an issue for any observation. The data products include an image which contains the model as well as a "persistence-subtracted" flt file for each exposure which is available as long as the data is public or if the data belongs to an individual archive user. In some cases, the "persistence-subtracted" flt file provides a "better" data product for downstream analysis. However, more commonly, we encourage users to employ the persistence model image as an indicator of pixels to be excluded from further analysis. More information about the model is contained here.
The WFC3 group is continuing to work to improve its understanding of persistence as it applies to the WFC3 IR data, as the current model is known to be deficient in some respects. Please send any questions, comments, or suggestions you might have about this topic to email@example.com.
In the WFC3 UVIS channel, electronic crosstalk between the amplifiers during readout induces faint, negative, mirror-symmetric ghost images in the other quadrant of the same CCD. A recent analysis of on-orbit data confirmed that crosstalk inflight behaves as it did on the ground, namely: the crosstalk ghost depends linearly on the strength of the offending signal and there is no crosstalk between the two CCDs. Levels are relatively low, about -2*10-4 for crosstalk arising in quadrants B and D, and about -0.7*10-4 for crosstalk arising in quadrants A and C. For signal levels at full-well, the resulting crosstalk features can reach -15e-/pix in quadrants B and D, and -5 e-/pix in quadrants A and C. Due to its mirror-symmetric behavior, crosstalk features will not co-add when combining dithered data acquired with sufficiently large offsets since with each shift in pointing, the crosstalk features shift in the direction opposite to the target shift. However, for observers working with small dither-step data or with non-dithered data, there is now a crosstalk correction procedure. The algorithm successfully removes crosstalk features and restores the pixels to a mean which is well within 1 sigma of the mean of the surrounding pixels. The standalone code is intended for use on the 'flt' file; currently written in IDL, it is available for use and can be downloaded along with the ISR here. For further details, please see ISR 2012-02: WFC3/UVIS Crosstalk and Crosstalk Correction by A.Suchkov & S. Baggett.
The current WFC3 photometric calibration is based on a limited set of observations of three spectrophotometric standards, observed in SMOV4. We have generated new photometric zeropoints for all WFC3/UVIS and IR filters, now based on a new reduction of all SMOV4, Cycle 17, and Cycle 18 observations of GD153, G191B2B, GD71, and P330E. The independent calibrations from the four stars agree to within 1% in most filters, and the photometric zeropoint is set to the average of the measurements. This is a substantial improvement over the previous calibration, where a filter-specific solution was not calculated (i.e., a smooth curve was across all wavelengths, ignoring small departures). For the first time, the new data sets have been processed using the new AstroDrizzle software and with the newly released WFC3 flat fields. For the UVIS camera, these flats are substantially better than the previous generation (i.e., more spatially stable given the removal of the "flare").
The results from this analysis will replace the existing calibration of the WFC3 photometric sensitivity in the file headers, and also define new throughput tables for synphot (e.g., the ETC) and CDBS. Changes in the zeropoints are less than 2% for most filters. Two new zeropoint tables (one for the UVIS and one for IR) are available at WFC3 webpage, for an infinite aperture. Another email will be sent to the STAN distribution list when the throughput files have been ingested into CDBS later this week.
Improved flat fields for the complete set of 42 UVIS full-frame filters are now available from MAST. These solutions were delivered in two stages: on August 8, 2011 for seven popular broadband filters (F336W, F390W, F438W, F555W, F606W, F775W & F814W) and on December 14, 2011 for the remaining 35 filters. For the four month period between August and December, users may need to re-retrieve their observations to ensure that the latest inflight flats were used for calibration. These new reference files typically correct photometry by 0.6 to 1.8%, with a maximum change of ~3 to 6%, depending on filter. The flat fields previously used in the pipeline were obtained during ground testing and contained a large internal reflection (or flare) affecting ~40% of the detector. A simplified geometric model has been used to remove this artifact from the new flat fields. Next, residual low-frequency structures caused by differences in the ground-based and in-flight optical paths were computed from photometry of Omega Centauri, observed at various roll angles and with large dithered steps. Furthermore, photometry in a range of apertures was used to study the UVIS PSF in detail. For radii smaller than 0.4" (10 pixels) the PSF is strongly dependent on both the detector position and on the telescope focus at the time of observation. The new flat fields have therefore been normalized to "infinite" aperture by applying local aperture corrections to 10 pixels, making them more generally applicable for calibration. Initial validation suggests that the photometric response for a given star is now the same to ~1% for any position on the UVIS detector. An ISR describing the calibration will soon will be available. Until then, please see the WFC3 flats webpage for more information.
A new Instrument Distortion Correction Table (IDCTAB) has been installed in the HST CDBS. This table is updated with the newly calibrated UVIS F475W, F350LP and F390M filters from calibration program CAL 12353. These new filters are in addition to the 10 WFC3 UVIS filters from earlier calibration programs, as reported in the April 2011 issue of the WFC3 STAN.
These new ISRs have been published since the last STAN (September 2012):
- ISR 2012-02 WFC3/UVIS Crosstalk Correction (ISR only) - A. Suchkov and S. Baggett
- ISR 2012-01 Proposal 11923-UVIS Filter Wedge Check - E. Sabbi
- ISR 2011-19 IR Intra-pixel Sensitivity Variance - Pavlovsky, McCullough & Baggett
- ISR 2011-18 First Results from Contamination Monitoring with the WFC3 UVIS G280 Grism - Rothberg et al.
- ISR 2011-17 WFC3/UVIS CTE-EPER Measurement: Cycle 17 & 18 - Kozhurina-Platais et al.
The complete WFC3 ISR archive is at:/hst/wfc3/documents/ISRs/
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