https://phys.org/news/2019-01-hubble-deepest-images-deeper.html

The HUDF is the result of combining hundreds of images taken with the Wide Field Camera 3 (WFC3) of the HST during over 230 hours of observation which, in 2012, yielded the deepest image of the universe taken until then. But the method of combining the individual images was not ideally suited to detect faint extended objects. Borlaff says, "What we have done is to go back to the archive of the original images taken by the HST, and improve the process of combination, aiming at the best image quality not only for the more distant smaller galaxies, but also for the extended regions of the largest galaxies.

The WFC3 was installed by astronauts in May 2009, when the Hubble had already been in space for 19 years. This presented a major challenge for the researchers because the complete instrument (telescope and camera) could not be tested on the ground, which made calibration more difficult. To overcome the problems, they analysed several thousand images of regions across the sky with the aim of improving the calibration of the telescope on orbit.