ST-4000XCM, 1800s total exposure 7/14/12 Monte Bello Open Space Preserve, CA
This galaxy cluster, thought to be about 1 billion light years from us (z=0.072), is the most distant target we have imaged so far. If you zoom in on the central region of the image, most of the objects you see are galaxies. Comparing with the charts for Abell 2065 in Hoffman and Crane, Astrophys. J. 215 , 379 (1977), it appears that the faintest galaxies visible are about 19th magnitude.
SBIG ST-4K, 1200s total exposure 6/24/17 Monte Bello, CA
Another attempt at the Corona cluster, not quite as sharp.