DASH (Drift And SHift)
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.
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.
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.
Illustration of the “drift and shift” (DASH) method of restoring unguided HST images. The top panel and inset show the standard data product (the FLT file) of an unguided, gyro-controlled exposure. The objects are smeared due to the lack of fine guidance sensor corrections. The middle panels show the twelve individual samples that comprise the exposure, created from the non-destructive reads. The smearing is small in each individual sample. The bottom panels show the reconstructed image after shifting the twelve samples to a common frame and adding them. From Momcheva et al. (2017).
Gabriel Brammer, Marijn Franx, Joel Leja, John MacKenty, Ivelina Momcheva, Lamiy Mowla, Adam Muzzin, Erica Nelson, Arjen van der Wel, Pieter van Dokkum, Katherine Whitaker
We are performing a survey using this technique, called COSMOS-DASH (GO-14114, PI: van Dokkum). This program observed 0.6 square degrees (456 pointings) of the COSMOS field in only 57 orbits, to a depth of H(F160W) = 25. We plan to release the full mosaic early in 2018.
Weight map of the COSMOS-DASH mosaic, produced by Gabe Brammer. The mosaic includes all H(F160W) data that have been taken in the COSMOS field. The white rectangle is the CANDELS COSMOS field. White squares are various individual pointings that had been obtained in COSMOS for a range of programs. The blue stripes are the COSMOS-DASH coverage. COSMOS-DASH overlaps with the UltraVISTA deep stripes, with excellent ground-based Y, J, H, and K data.