Notes from the WFC3 Team

John W. MacKenty, mackenty@stsci.edu

The Wide Field Camera 3 (WFC3) continues to operate nominally and produce lots of science. The WFC3 team is pleased to report that the total number of in-flight exposures (including external and internal science, and calibration exposures) passed 100,000 as of 0814UT on April 23, 2013. The milestone image of CLJ1226+3332 was taken as part of Marc Postman’s CLASH program. Fittingly, this was a F275W ultraviolet image.

Last summer, we commissioned a capability to perform a post-flash to reduce the impact of accumulated radiation damage in the charge-coupled device (CCD) sensors in the ultraviolet-visible (UVIS) channel. This post-flash has now been widely adopted by observers. It is meant to compensate for the radiation-induced decline in charge-transfer efficiency (CTE), which is most significant for exposures with low backgrounds (i.e., with ultraviolet or narrow filters). Due to the very low readout noise in the UVIS detectors, CTE decline had become a limiting factor for many observations. For observations with low background, inclusion of a post-flash to elevate the background signal level to ~12 electrons produces less trailing of charge and, most importantly, improves the detection of very faint sources.

In mid-March 2013, the Institute released a preliminary version of a tool in the post-observation software to further correct for charge trailing in CCD images. While it is most effective when used in combination with a post-flash, this tool is also suitable for correcting any UVIS observation. More information is available on the WFC3 web page under “Performance/CTE” and “Software Tools/CTE Tools.” The WFC3 team would particularly appreciate user feedback on this tool. We plan to continue to experiment and evolve the algorithms and calibration for the new tool to correct charge trailing. Our goal is to incorporate it into the calibration pipeline in 2014. We expect to implement an architecture similar to the one in place for the Advanced Camera for Surveys, with products available that are both corrected and uncorrected for CTE decline.

Other interesting news from WFC3 includes the increasing use of spatial scans. In this technique, the telescope is moved during the observation, so the light from each source is spread over many pixels in an organized fashion. These spatial scans enable new classes of observations that were previously impossible. The applications include: recording the transits of exoplanets, precision photometry of very bright sources, relative astrometry with precision approaching 30 micro-arcseconds, and efficient validation and improvement of flat-field calibrations.