William Optics UltraCat 56mm Apochromatic Refractor

The William Optics Ultra-Cat 56 is a compact apochromatic astrograph designed specifically for astrophotography. It combines a 56 mm aperture, 269 mm focal length and fast f/4.8 focal ratio, making it a wide-field imaging telescope suited to large and medium-sized deep-sky objects.

Its five-element Advanced Petzval optical system uses two Super ED elements, one ED element and lanthanum glass to provide well-corrected, colour-accurate images. The Petzval design incorporates field correction into the telescope itself, so no separate field flattener is required. The large 50 mm image circle supports Full Frame, APS-C and Micro Four Thirds cameras, with William Optics also listing Medium Format compatibility.

The 269 mm focal length gives the Ultra-Cat 56 more image scale than shorter wide-field astrographs such as the MiniCat 51, while retaining a broad field of view. This makes it particularly well suited to large nebulae, star fields, open clusters and other extended deep-sky targets. Larger galaxies and globular clusters can also be photographed, although smaller galaxies and compact targets will generally benefit from a longer focal length and larger aperture.

The telescope uses William Optics’ WIFD internal rack-and-pinion focuser, with 30 mm of travel and a 1:10 fine adjustment. The internal focusing mechanism is designed for astrophotography and is compatible with electronic automatic focusers. William Optics recommends approximately 55 ± 3 mm of back spacing for astrophotography.

For camera alignment and framing, the Ultra-Cat 56 includes the Sensor Tilt Xterminator and a camera angle rotator. The sensor tilt adjustment allows the camera’s alignment to be fine-tuned, while the rotator makes it possible to change framing without rotating the telescope itself.

An integrated Bahtinov mask is incorporated into the front cap, providing a practical aid for achieving accurate focus on a bright star. The telescope also includes an M48/M54 camera adapter and a dual-sided mounting plate compatible with both Vixen-style and Arca-Swiss systems.

An important point for prospective owners is that the Ultra-Cat 56 is designed for astrophotography only and is not intended for visual observation. It should therefore be considered a dedicated imaging astrograph rather than a general-purpose telescope.

With its combination of a compact form, fast f/4.8 optics, advanced Petzval design and astrophotography-focused mechanical features, the Ultra-Cat 56 is designed for photographers who want a portable wide-field imaging system with more aperture and image scale than the smaller Cat-series models.