131F Flat-Field5" Apo Refractor, Good for Imaging and Visual Description Askar's new 131F Flat Field quad refractor offers sharp images and versatility at a reasonable price. Like its smaller siblings, the 131F's optics cover full frame imagers, without the need for a field flattener. Backfocus distance doesn't need to be precise, making astrophotography simpler when using filter drawers, OAGs, and other accessories. The 131F's 983mm focal length gives you a 2. 1