William Optics UltraCat 108mm Apochromatic Refractor

The William Optics Ultra-Cat 108 is a 108mm apochromatic astrograph designed specifically for astrophotography. It combines a 518mm focal length with a fast f/4.8 focal ratio, providing a wide field of view while the larger aperture delivers greater light-gathering capacity and more image scale than the smaller Ultra-Cat 56 and Ultra-Cat 76.

Its five-element Advanced Petzval optical system uses two Super ED elements, one ED element, one lanthanum element and a proprietary X-Element. The Petzval design provides an inherently flat imaging field, so William Optics states that no separate field flattener is required. The 60mm image circle is specified for Medium Format, Full Frame, APS-C and Micro Four Thirds cameras.

The 518mm focal length places the Ultra-Cat 108 further toward detailed deep-sky imaging while retaining the fast f/4.8 design. William Optics’ own example images include large nebulae such as the North America Nebula, Rosette Nebula and Cygnus Loop, as well as objects such as the Andromeda Galaxy and Markarian’s Chain, demonstrating the range of deep-sky subjects for which the optical system is intended.

Focusing is handled by William Optics’ WIFD internal rack-and-pinion focuser, with 30mm of stated travel and electronic focuser compatibility. The design keeps the focuser mechanism internal, while the included Sensor Tilt Xterminator provides adjustment for sensor alignment. A camera rotator and M48, M54 and M68 camera adapters are also included.

The Ultra-Cat 108 incorporates a built-in Bahtinov focusing mask and William Optics’ Anti-Shrink Lens Cell Technology, which is designed to minimise the effects of thermal expansion and contraction on optical alignment. The telescope is supplied with mounting rings and handle bar, a Vixen-style mounting plate, soft carry case and Allen key set.

William Optics specifies the Ultra-Cat 108 for astrophotography only and states that its optical design does not support visual observation. It should therefore be treated as a dedicated imaging telescope rather than a general-purpose visual telescope.