William Optics UltraCat 76mm Apochromatic Refractor

The William Optics Ultra-Cat 76 is a dedicated apochromatic astrograph designed for astrophotography. With a 76 mm aperture, 365 mm focal length and fast f/4.8 focal ratio, it combines relatively high light-gathering capability with the broad field of view expected from the Ultra-Cat series.

Its five-element Advanced Petzval optical design combines two Super ED elements, one ED element, lanthanum glass and a proprietary X-Element. William Optics states that the design provides a flat field across a 50 mm image circle, while the Petzval configuration means that no separate field flattener is required. The telescope is designed for Full Frame, APS-C and Micro Four Thirds cameras and is also specified as compatible with Medium Format cameras.

The 365 mm focal length gives the Ultra-Cat 76 considerably more image scale than the smaller Ultra-Cat 56, while its f/4.8 focal ratio keeps it firmly in the fast wide-field imaging category. This combination makes it particularly well suited to large nebulae, extended deep-sky objects, star fields and open clusters. William Optics’ own example gallery includes the Rosette-region IC 1396, the Heart Nebula region, the Jellyfish Nebula, M24, the North America Nebula and Orion, as well as comet C/2025 A6 Lemmon.

The telescope uses William Optics’ WIFD internal rack-and-pinion focuser, with 20 mm of specified travel and a 1:10 fine-adjustment mechanism. The internal focusing design places the focusing mechanism within the telescope rather than using a conventional rear drawtube. William Optics says this helps eliminate tube tilt caused by the weight of cameras and accessories while maintaining an unobstructed optical path. The focuser is also compatible with electronic automatic focusers.

For astrophotography, William Optics recommends approximately 55 ± 3 mm of back spacing. The company notes that this helps limit drawtube extension and reduces the risk of light leakage.

The Sensor Tilt Xterminator (STX) provides accessible adjustment of the camera sensor’s alignment without removing the camera. The integrated camera angle rotator allows the framing to be changed without rotating the entire telescope. These features are particularly useful when working with large camera sensors, where even small alignment errors can affect star shapes across the image.

Focusing is assisted by an integrated Bahtinov mask in the front cap. The telescope also has a retractable 52 mm dew shield, which helps protect the objective from stray light and moisture while allowing the telescope to be shortened for transport.

The Ultra-Cat 76 is explicitly designed for astrophotography only and William Optics states that its optical design does not support visual observation. It should therefore be regarded as a dedicated imaging instrument rather than a general-purpose telescope for eyepiece observing.

With its 76 mm aperture and 365 mm focal length, the Ultra-Cat 76 occupies an interesting position within the Ultra-Cat range: it retains the fast, wide-field character of the smaller model while providing more light-gathering capability and greater image scale. It is therefore particularly attractive for photographers who want to move towards somewhat smaller deep-sky targets without leaving the wide-field astrograph category.