We have developed a new observing strategy for wide and shallow observations using the WFC3/IR camera on the Hubble Space Telescope, improving its efficiency for covering large areas by a factor of 4-8.

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The duration of a guide star acquisition by HST is typically approximately 10 minutes, limiting guided observations to no more than two pointings per orbit (with a large part of the orbit lost to the guide star acquisitions). This limitation can be circumvented by skipping the guide star acquisitions, turning off the guide star corrections, and guiding with gyros alone. This is not done routinely as the lack of guide star corrections causes the telescope to drift, which results in unacceptable image degradation in typical exposures of several minutes or more.

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This drift can be corrected, by making use of the particular way that IR cameras take data. The WFC3/IR camera performs non-destructive reads during the exposure ("up the ramp sampling"), and the time between these reads is much shorter than the total exposure time. The drift is only about 1/4 WFC3/IR pixels in the 25 seconds between consecutive, non-destructive reads of unguided exposures. The individual reads can be shifted relative to one another and co-added, restoring the full resolution of WFC3. This observing technique is illustrated below, and described in Momcheva et al. (2016). It is available as an approved mode since the Cycle 24 call for proposals.