Featuring an image circle big enough for medium-format photography, the RedCat 91 WIFD 91mm f/4.9 Apochromatic Refractor Telescope from William Optics is a photo-only optical tube assembly built for making gorgeous deep-sky images. The RedCat 91 features a fully multicoated five-element lens with super ED, ED, and Lanthanum elements in a field-flattening Petzval design that delivers edge-to-edge sharpness and eliminates complex back-focus calculations. The William Optics Internal Focus Design does away with rear focusers that sag under heavy cameras and accessories; instead, a rigid and balanced 1:10 rack-and-pinion focuser in the middle of the OTA prevents tube tilt and resulting vignetting.