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A new technique for wide-field near-IR imaging with WFC3

A new observing strategy has been developed to allow for wide shallow observations using the WFC3/IR camera on the Hubble Space Telescope. The length of a guide star acquisition by HST limits observations to no more than two pointings per orbit. This limitation can be circumvented by guiding with gyros alone, which is possible as long as the telescope has three functional gyros. Since HST drifts only by ∼ 1/4 WFC3/IR pixel 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. We have used this tiling technique to observe four orbits (32 pointings) of a pilot survey program (GO-14114, PI: van Dokkum). Both the telescope behavior and the data quality are within the expected range, confirming that this is a viable technique. By following the observing strategy of GO-14114, programs can cover 1 square degree in 100 orbits to a depth of H(F160W) = 25.0. While this is the most efficient observing strategy, other variations are possible, depending on the scientific goals of the program. This new observing technique is now available as part of the Cycle 24 call for proposals.

A more detailed description of this technique is available in Momcheva et al. (2016) as well as this document. Both in our paper and in the description we focus on the application in the "big" regime: covering a wide field in a "reasonable" number of orbits. However, another, and perhaps more common, application is to do small programs more efficiently such as doing an 8-point mosaic of a galaxy cluster in a single orbit rather than 8, or mosaics of two star forming complexes in 2 orbits rather than 16. This new observing technique is now available as part of the Cycle 24 call for proposals. Questions about it should be addressed to

A pilot program using this technique is being executed in Cycle 23, covering 0.6 square degrees of the COSMOS field in 57 orbits. The layout of the 456 pointings of GO-14114 (PI: van Dokkum) is shown here. The background image is a composite of the UltraVISTA deep H-band imaging and the CANDELS H(F160W) WFC3 imaging (the "hole" in the mosaic above the center of the middle stripe). The pilot data from this program covers the top area of the center stripe. Small gaps between pointings are deliberate: they coincide with bright sources that would cause persistence problems.